Updated Thinking on This Weekend’s Lake Erie Lake Effect Snow

11-10 les

Updated thinking on the lake effect tonight through early Sunday off of Lake Erie…

A band will ramp up over the lake this evening by 7 or 8 PM as the first of a few troughs pushes across the lake.  Winds ahead of the trough are WSW if not SW, so this initial band will likely set up parallel to the shore and come inland into Buffalo and its immediate southern suburbs.  Temperatures ahead of the trough are cold enough for plenty of graupel given the developing instability, but don’t quite get cold enough for a change to all snow and potential accumulations until after it passes.  This band will likely push inland a bit and shift south of the Buffalo area by 11 PM or Midnight as the trough passes.  Winds behind this trough briefly come around to about 270 (due west) over the lake and remain firmly south of west overland.  This will likely push the trough into Erie County PA within 10 miles of the lake (as well as Chautauqua/northern Cattaraugus NY)…it will probably be able to graze Lake//northern Geauga/Ashtabula Ohio but will not push far inland.  As the trough pushes through temperatures will quickly fall to levels that typically support accumulation with intense, early-season bands so the precip will change to all snow/graupel.  This initial push will not last long and temps will only become supportive as it passes, so I’m guessing that other than perhaps some car toppings of graupel there won’t be any accums this evening in Buffalo proper…but could be a quick inch or two inland over southern Erie Co NY, and also where the band impacts in northern Erie Co PA and in Chautauqua/Cattaraugus.  There may be dustings in extreme NE OH if the band makes it on land there.

The winds quickly start backing again after 2 AM ahead of the next trough.  This will likely cause a more consolidated band to quickly re-develop that starts shifting up the shoreline.  As this occurs, extreme northern Ashtabula Co could get a dusting late tonight.  Parts of Erie Co PA will also likely get an hour or so of snow as the band lifts back north during the pre-dawn, and given its likely intensity that will cause some quick accumulations.  The winds by morning get to be WSW over water and nearly due SW over land.  This should still lift the band north of the vast majority of Erie Co PA…though it could graze the extreme NE portion.  Despite models taking the band into the Erie PA area into Chautauqua County in this time period, I simply don’t see how it can push inland that far south given the winds…this fits with the known bias of modelling being too far south with these bands.  I suspect it will sit near the lakeshore between Northeast, PA and Dunkirk, NY and then actually come inland near or a bit north of Dunkirk and affect extreme NE Chautauqua Co, the NW tip of Cattaraugus, and southern Erie Co NY, probably getting back into the southern Buffalo suburbs.  Winds will support the band in this general area for several hours from before sunrise Saturday through 1 or 2 PM.

The character of the band and whether it actually stalls or keeps slowly moving will determine how much snow it can dump.  Extreme instability (lake to 850 diffs of 25C, lake to 700 diffs of 35C, and lake to 500mb diffs of 45 to briefly almost 50C) resulting in boundless inversion heights of 15-20k feet, deep moisture to above 10k feet, little directional shear and good synoptic support as a 500mb low moves just north of the lake suggest very heavy snow under the band.  Snow ratios won’t be much higher than 10:1, especially near the lake where they’ll be lower, but it will be puking thundersnow and will still pile up quickly, if there is an organized/persistent band.  The wind speeds in the boundary layer over the lake will be 35 to 40 knots which is stronger than preferable, but with a full-fetch of the lake, the flow favoring good shoreline convergence into SW NY, and broad synoptic convergence ahead of the approaching trough all argue for a persistent band.  Given the strong shoreline convergence in this area, I suspect the band will drift around in the same general area for 6-8 hours Saturday morning/early afternoon before the trough passage pushes it south.  This still isn’t a high confidence forecast, but given the likely rates if an organized band occurred the potential still remains for locally up to a foot of snow in a short time by early Saturday afternoon in SW NY, including southern Erie County NY and the southern Buffalo suburbs.  Given the wind speeds and short window, I will say that I’m not as confident as I would like in a focused area of persistent enough snow to get that, but I think the potential very easily exists with multiple factors still arguing for it.

As the trough pushes through the band will likely intensify back to the central basin, especially with some Lake Michigan pre-seeding, and then swing southeast into much of the Snowbelt from the western suburbs points east.  Winds on land are still not expected to veer north of WSW Saturday afternoon/evening over Ohio or NW PA, which is reasonable since the upper trough rapidly pulls east by that point and surface ridging builds in from the SW.  But, winds over the lake come around to 280-290 degrees (WNW) for several hours late Saturday afternoon through the entire evening.  This push over the lake should still allow for shoreline convergence over Cuyahoga County and perhaps even NE Lorain County for a time, allowing for the band to get into the immediate Cleveland area and western lakeshore, then moving inland near or north of 322 into the eastern suburbs and Geauga County.  It should push into central and southern Ashtabula County for a time, and though the inland penetration in NW PA will face similar issues to OH, their farther north location will likely allow the snow to briefly get as far south as northern Crawford County.  Instability is still moderate to borderline extreme by later Saturday afternoon as the band affects more of the NE OH and NW PA Snowbelt with inversion heights of over 10K feet, which along with strong convergence caused by the trough interacting with the shore should still allow for moderate to heavy snow.  Temps begin moderating some by later in the day so the lakeshore will likely be snow under the heaviest band but then may struggle once the intensity drops…so I’m thinking maybe a quick coating to 2” along the lake from Cleveland up through Ashtabula County, but not more with this band.  Inland it could be more like a quick 1-3” or so, especially if the band stalls for a few hours in the early-evening.

Overnight the low-levels remain very unstable over the lake and winds will remain well-aligned, with convergence continuing over the Snowbelt as surface high pressure continuing building in from the south.  Inversion heights and moisture depth decrease, but strong low-level instability, lack of shear, and convergence should still allow a moderate band or two to continuing through the night.  It will slowly lift north through the NE OH Snowbelt and into Erie County PA overnight.  Because the bands won’t be as intense and will slowly be moving, I’m lowering my expectations to 1-3” of additional overnight accumulations inland from the lake in the NE OH Snowbelt north of 322 and in the half of Erie County PA closer to the lake…suspect southern Erie County dries out overnight.  The remnant band will get into SW NY Sunday morning, but will be quickly shifting by then and the high will really begin choking it off, so likely no additional accums with it in NY Sunday morning.  Given the moderating airmass overnight, the bands may struggle to accumulate much additional snow near the shore.

For the map and changes to it, focused the 6-12” corridor in NY where they’ll see the best potential snow tonight and also where the band is likely to affect Saturday morning.  Removed the 4-8” from anywhere in NE OH as I don’t think they’ll see much if any tonight, and I’m not quite as impressed with the Saturday night snowfall there…but the higher terrain north of 322 from NE Cuyahoga into southern Lake/northern Geauga should still get a good few inches Saturday afternoon and evening.  Did expand the 1” into more of northern Cuyahoga County given potential for a relatively intense band to affect that area for a time late Saturday afternoon/early evening.  Reduced the area of 4-8” in Erie County PA for similar reasons as in NE OH, and mainly focused it on the NE corner where better banding will graze late tonight through early Saturday afternoon…that’s not to say that someone can’t squeak out locally more than 6” in central or western Erie County PA if a more persistent band sits there later Saturday afternoon or evening, but I don’t have enough evidence to support it in any given spot.  Did bump Erie proper into 3-6” since I think they see a brief burst late tonight, get grazed Saturday morning, and get another good burst early to mid-afternoon as the trough pushes through…though that’s admittedly low confidence, and I feel more confident farther up the shore.  1-3” amounts were extended farther SE overall in NW PA and SW NY on the southern edge to give a nod to most guidance, though didn’t go as far as some models due to likely issues with banding getting too far inland.

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Quick Thought’s on This Weekend’s Lake Effect Snow

Forgive the crude map…gets the point across:

11-10 les

With the synoptic precip Friday morning temp profiles are close to supporting snow…there will be some dynamic cooling as lift is pretty good and the precip will be moderate, so the higher terrain in NE OH and NW PA will probably try to flip to snow for an hour or two…it may be enough to try to start lightly coating grassy surfaces and such…lakeshore probably just sees rain.  NW OH a bit colder and better timing so will probably be some grassy accums there to start Friday.

For the lake effect over the weekend, it’s honestly more impressive than I initially thought parameter wise, but some questions about the duration and wind direction make it hard to be confident about a lot of snow falling in any given spot.  My impression based on the parameters alone is that someone will hit the 6″/12 hour or 8″/24 hour warning criteria, but a band will need to lock in somewhere for that to happen.

The band will start taking shape late Friday evening as winds line up out of the WSW.  With an approaching upper-low and extreme instability developing, it will likely be intense…it may graze the Lake, Ashtabula, and Erie lakeshore for a time, but at some point between 1 AM and 5 AM Saturday will lift out over the lake and come inland south of BUF as winds go more SW.  The band will likely be mixed before midnight, but after midnight as 850mb temps crash to -10 to -12C by daybreak the band will be all snow/graupel down to the shoreline given its likely intensity.  Winds lock in for a few hours Saturday morning, which could allow the band to slow enough to drop substantial amounts on parts of SW NY along the lakeshore and inland into the southern Buffalo suburbs.  Instability will be extreme, with equilibrium heights of near 20k feet thanks to hilarious lake-to-500mb differentials of -45C (approaching -48C briefly).  Wind speeds will be strong, but when the band is completely parallel to the long axis of the lake that should allow it to be organized enough.  Given the instability, large-scale lift with the upper-low, full-lake fetch and strong convergence the band into SW NY will have a lot of lightning and likely contain very high rates…despite the wet snow, over 2″ per hour will be possible.  Even though the duration is only a few hours, the possibility exists for up to a foot in SW NY if the band locks in Saturday morning.  This could graze northeastern Erie County but I suspect if it locks in it does so just up the shore from Erie proper…though they could get grazed for a time late Friday night/early Saturday and see some snow.

The band will swing into most of the primary NW PA and NE OH Snowbelt Saturday afternoon as a surface trough pushes through and brings the winds around to the WNW over the lake.  It probably won’t sit over the lakeshore for more than an hour or two as the wind shift is fairly sharp and abrupt, but given the likely instability and forcing it could drop a quick inch or two of snow/graupel near the shore.  As for how far south the band can push and what kind of amounts it can drop inland, winds on land don’t really back from WSW as the surface high starts building in quickly Saturday evening.  With WNW winds over the lake the band can probably get down to about Downtown and then push east across NE Cuyahoga and into northern Geauga, north of 322.  It should get into central and southern Ashtabula as well as most of Erie County, but likely struggles to push into Crawford PA.  It may maintain this southern extent for a few hours before drifting back north Saturday evening.  The upper support quickly departs late Saturday afternoon and the inversion height lowers substantially, so the band intensity will likely be more moderate when it reaches its southernmost point…it could drop a light/slushy accum on the Cuyahoga lakeshore from Downtown points NE and perhaps a quick 1-3” inland in a few hours.

Overnight Saturday night instability does remain sufficient as the ridge builds in and slowly backs winds over land and pushes the lake effect back up the shoreline.  With lightening wind speeds and lake to 850mb diffs remaining easily over 20C through the night, it wouldn’t surprise me if a moderate band or two does reorganize from parts of Lake County and perhaps the chimney of Geauga northeast across northern/central Ashtabula County and northern/central Erie County PA, perhaps into Chautauqua County NY as well later in the night.  This band won’t be as intense, but will have light winds and good convergence so should be organized and likely won’t move quickly, so it could drop another 2-6” overnight Saturday night into early Sunday where it sets up.

Overall the potential for heaviest snow will be Saturday morning in SW NY and perhaps extreme NE Erie County, and I’m thinking a quick 6-12” with that where the band sets up…shouldn’t have issues accumulating along the lake given temps aloft and the expected intensity.  Expecting a quick 1-4” across the primary Snowbelt as the band swings south Saturday afternoon/early evening before lifting back north…in Cuyahoga and Geauga Counties this is likely confined to along/north of 322 as it looks now.  I expect another few inches in the northern Snowbelt overnight Saturday night into early Sunday as bands re-organize and slowly drift north as the surface ridge builds in and as enough instability hangs on.

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September 10th, 2018: Large & Powerful Florence Churns West-Northwest Towards a Likely Extreme-Impact Carolinas Landfall

Disclaimer: The thoughts in this post are only mine and don’t represent the thoughts of any other companies, groups, websites, or organizations.  They also shouldn’t be used for decision making in the impacted areas…

The majority of the satellite and model images in this post are courtesy of Tropical Tidbits and Weathermodels.com…  Any poorly drawn illustrations on the images are my own.

This will likely be my last post on Florence, and preparations should already be well-underway in the impacted areas.  If you have interests in the affected areas, you should seek official info and follow orders from local officials.

Now the post…

Main takeaways:

  1. Florence is currently a category 4 hurricane, and there appears to be some room for additional intensification between now and Wednesday evening.
  2. Florence is still likely to make landfall in the Carolinas on Thursday, perhaps just east of the South Carolina/North Carolina boarder. There remains some uncertainty, and anything from a direct hit on South Carolina to a stall just southeast or east of North Carolina without a direct landfall remain somewhat on the table.
  3. Florence is already a large hurricane, and its wind field is expected to expand further before impacting the Southeast later this week.
  4. Although Florence will not be at peak intensity when it makes landfall, it is still likely to be a major hurricane with a large wind field. This will bring a substantial wind damage and storm surge risk to the coast.
  5. Florence’s large size will likely allow strong winds that can down trees and powerlines to make it well inland.
  6. Florence’s large size and expected slow motion after landfall will bring a risk for severe freshwater flooding from rainfall inland from the coast. This flooding rain may make it north into the Mid Atlantic by Friday or the weekend.  This often is the hazard that results in the most deaths from tropical cyclones, along with storm surge flooding closer to the coast.
  7. Although a lower-impact solution without a direct landfall is still remotely possible, a direct landfall is likely with weather going downhill before the actual center arrives. Given this, preparations should continue to be underway and earnest across the Southeast.
  8. It’s really too early to say where exactly Florence drifts after its expected landfall. I suspect it’ll slowly drift northwest and then north, and get kicked east next week, but there may be some unforeseen wobbles along the way that won’t be modeled well at this range.
9-10 Florence vis

Visible satellite imagery of Florence valid Monday afternoon

One of the better rapid-intensity forecasts ever made has unfortunately panned out from Florence, as the storm has intensified from a tropical storm Sunday morning to a category 4 hurricane by Monday afternoon.  The satellite images of Florence are as you’d expect quite impressive this afternoon…with a closed eyewall with vigorous convection completely enclosing a well-defined eye with mesovorticies evident at times on GOES-16 imagery.  Anti-cyclonic upper-level outflow is well-established in all quadrants, particularly in the western and northern portions of the storm.

The storm has continued to gradually intensify since its classification as a category 4 around 12:00PM EDT Monday.  At the moment, the eyewall features a solid ring of -65 to -70C cloud-top temperatures, with the eye temp of about 15C on IR imagery.  If we’re getting nit-picky, the eyewall cloud-temps could be colder and the eye temp could be warmer…so despite the nearly perfect structure, the convective intensity, at this time, just doesn’t suggest a further intensification to category 5 intensity in the immediate future.  (side bar, between starting to write this around 5PM and finishing around 9PM EDT, the structure has degraded a little bit, suggesting it is certainly not intensifying at this time).

9-10 Florence MW

Microwave imagery of Florence from Monday afternoon

A microwave pass from earlier this afternoon shows the well-organized inner core.  A secondary eyewall is not yet forming, so if the eyewall convection becomes a little bit more intense then this core could support additional intensification.  The outer rainband that’s evident will probably eventually congeal/contract enough to initiate an eyewall cycle, but that doesn’t appear anywhere near imminent.

 

9-10 Florence SST

Current sea surface temperatures (C) across the western Atlantic

 

Florence is not yet over the warmest SSTs of its journey towards the Southeast, as water temps and heat content will increase noticeably as early as tonight.  Just on this alone, given the current structure, it’s possible we see convective intensity increase further with tonight’s convective max and support further strengthening of the hurricane.  The kinematic environment around the hurricane is also expected to remain as favorable as or perhaps even become more favorable than it is right now…

9-10 Florence GFS sounding

GFS model sounding for the environment around Florence Monday afternoon.  Wind in the barbs on the right, temperature and humidity in the middle.

This sounding from the 18z GFS analysis…essentially the current conditions this afternoon…show that Florence is in a weak vertical shear environment, with a uni-directional flow and minimal speed shear below 200mb, and a mean RH of around 70%.  This could be a little bit higher, but with little shear and a well-organized inner core, this small amount of dry air is not negatively impacting the hurricane.

9-10 Florence ECM 48

Wind shear forecast from the Euro valid Wednesday morning

The Euro and GFS (Euro shown above for some variety) both show the upper-level anti-cyclonic flow remaining prominent over Florence through Thursday, with the flow expanding/intensifying gradually Tuesday into Wednesday.  Both models also suggest that the poleward outflow channel will be enhanced by a modest jet streak over the northwest Atlantic Tuesday night into Wednesday.  This all means that the shear will remain minimal through about Thursday morning, with outflow remaining expansive and likely becoming even stronger than it currently is.

9-10 Florence ECM 48 PWAT

Atmospheric moisture anomaly forecast valid Wednesday from the European model

In addition, dry air isn’t expected to be a potential issue through the expected landfall in the Carolinas, with much of the western Atlantic (and the eastern U.S. for that matter) mired in a more humid airmass than average, due to the same ridge pushing Florence west towards the Southeast coastline causing a lack of shortwaves or cold fronts with dry air that make it far enough south to possibly influence Florence over the next few days.

The combination of warmer waters, continuing low shear, little dry air, and strong outflow that will if anything become stronger strongly suggests that Florence has not yet reached its peak intensity.  It’s a possibility that Florence becomes a category 5 hurricane at some point between later tonight and Wednesday night…though there will likely be an eyewall cycle or two at some point that causes a weakening in the overall winds but further expansion of the already sizeable wind field.  My impression is that if we don’t see Florence reach a category 5 by some point on Tuesday or Tuesday night…before a likely eyewall cycle begins…that we probably don’t see it peak that high…as the window for the ERC to complete and allow for re-intensification before conditions become slightly less than completely optimal is probably quite small once the ERC starts.

9-10 Florence ECM 72

Forecast wind shear from the European model valid Thursday morning

This is due to what will be some likely increase on shear starting around Thursday as a weak shortwave moving through the Northeast likely imparts a strong enough S/SWrly upper-level flow to cause some shear on Florence.  The shear won’t be strong and won’t rip the hurricane apart, but a gradual decay starting by Thursday and continuing through expected landfall by Thursday night is likely.  Though there is less dry air than normal over the Continental United States as Florence approaches, its diminishing forward speed through landfall does also suggest that any dry air and also friction caused by land over parts of its circulation may also have some brief opportunity to start inducing weakening before likely landfall.

All told, I suspect we see Florence peak as a high-end category 4 or “low-end” category 5 hurricane at some time on Tuesday or Tuesday night, likely fluctuate as a category 3-4 Wednesday into Thursday due to a likely eyewall cycle, and then probably start trending down as Thursday wears on due to some increase in shear and the beginning of land interaction.  With that said, assuming the timing holds and we see a Thursday afternoon or evening landfall, Florence will likely make landfall as at least a category 3 hurricane.  Depending on the exact timing of the ERC, Florence’s exact intensity before and after the ERC, and its forward speed coming in and ultimately the timing of the landfall, a rare Carolinas category 4 landfall is still possibly in the cards…though I’d put my money on landfall intensity ultimately being in the category 3 range.

9-10 Florence current steering

Current steering currents across the Atlantic

The track forecast is relatively straight-forward through Wednesday, and then uncertainty increases around the time of likely landfall in the Carolinas.  A strong ridge over the western Atlantic will be the dominant feature steering Florence through Wednesday night, and will impart a west-northwest motion turning perhaps a bit more northwest with time.

9-10 Florence EPS 60

European ensemble mean 500mb pattern valid Wednesday evening

Through Wednesday evening the steering mechanisms remain fairly straight-forward.  The ridge to the north/northeast of Florence will remain dominant, at this point imparting a northwest motion on the hurricane.  A weaker lobe of the ridge over the southeast (with no true break between the two) will impart a west or even southwesterly motion.  The ridge to the east being dominant will likely result in a net WNW to NW motion at this point.

9-10 Florence EPS 72

European ensemble mean 500mb pattern valid Thursday morning

By Thursday morning the ridge to the northeast of Florence is beginning to weaken some, but there’s still no real break in the ridging in front of the storm, with a weaker ridge over the Appalachians still imparting if anything a modest west/southwest component of motion.  This should still result in a continued WNW to NW motion into Thursday, with perhaps the slowing beginning around this point late Wednesday night/Thursday morning as the ridge to the northeast starts weakening some.

 

9-10 Florence EPS 84

European ensemble mean 500mb pattern valid Thursday evening

 

By Thursday evening as Florence makes its likely Carolinas landfall, the ridge to the east/northeast continues to sag south and weaken a bit, with the ridge to the west/northwest of the hurricane remaining in place.  There are a couple of weak shortwave troughs in the flow, but they’re weak and aren’t close enough to the hurricane at this point on the European ensembles to disrupt the steering from the ridges in the vicinity of the eastern U.S. and western Atlantic.  The trough over Nova Scotia dropping S/SW at this point is something that could push Florence south/southeast away from the coast, but only if the hurricane ends up tracking on the right side of guidance towards or just east of the Outer Banks.

At this point, the Euro suite has Florence nearly equidistant to the two ridges, does not have a break in the ridging north of Florence, and is still somewhat stronger with the ridging to the east/northeast.  This would cause a somewhat slower but still WNW motion to continue through landfall, with the steering weakening further after landfall.

9-10 Florence GEFS 66.png

GFS ensemble mean 500mb pattern valid Thursday morning

The GFS suite remains to the “right” of the European suite in general today.  The GFS is weakening and shifting the western Atlantic ridge farther south quickly, and turns Florence north a bit quicker, allowing more of a northerly motion to be imparted before the model and its ensembles also collapse the steering currents later Thursday into Friday, causing it to stall in the vicinity of the NC coastline (perhaps just off/east).  This track also gives more of an opportunity for the small shortwave dropping in from the northeast (which doesn’t show up well on the GEFS mean image above) to give Florence a small nudge away from the coast, which causes many GEFS members to stall just offshore.

9-10 Florence EPS

European ensemble member forecast tracks

The European ensemble members, given their evolution of the pattern discussed above, remain generally set on a landfall in the Carolinas Thursday afternoon or evening.  A few members (it’s about 3/51) do manage to stall it just east of North Carolina, and several members are as far southwest as Georgia.  But as has been the case for a number of runs, the bulk are focusing on the Carolinas.

9-10 Florence GEFS

GFS ensemble member forecast tracks

The GFS members, due to the slightly different evolution discussed above, are generally right of the Euro ensembles.  Roughly half the members still plow it right into NC, though pretty much the other half stall it just offshore to the east or southeast.  From there the members are a mess due to the breakdown in steering currents, and many members eventually drift it back west this run after stalling it offshore.

9-10 Florence EPS 108

European ensemble mean 500mb pattern valid Friday evening

As we head into Friday and the weekend, the steering currents really do break down some more as the strong ridge to the northeast of Florence weakens considerably.  There will be a few varying areas of ridging steering Florence weakly, with a few other small, embedded shortwaves that won’t be modelled well at this range that may also have a considerable influence on Florence’s exact motion:

9-10 Florence ECM 108

European model forecast 500mb height and vorticity valid Friday evening

This all likely means that Florence’s motion will be slow and erratic from somewhere around Thursday night or Friday morning until a more substantial shortwave can kick it east.  It could drift towards the Mid Atlantic or Ohio Valley, or could meander around the Southeast.  It’s too early to say for sure.  I will say that with all of the unusual ridging north of the storm that I’d be careful to expect significant northward movement, though perhaps it eventually drifts up there.

9-10 Florence EPS 216.png

European ensemble mean 500mb pattern valid Wednesday 9/19

Unfortunately, the ridging doesn’t break down enough for a more substantial shortwave to come in and push Florence east until somewhere around Tuesday or Wednesday…so wherever it’s drifting, it will be doing so for a while.

My ultimate thoughts here are that the European suite has been leading the way the entire time with this storm…from missing the initial recurve opportunity last weekend to honing in on a US landfall while many other models were still taking it to the east…and the ensembles have been very insistent on a Carolinas hit for the last 3 days while other guidance continues to waiver.  The European ensemble is the guidance to beat historically, has the hot hand, and seems locked in.

Because the steering currents do fall apart right around the likely landfall…with some questions regarding the shortwave coming around the ridge from the northern Atlantic that could induce a “tug” away from the coast at the last second…a landfall in the Carolinas can’t truly be “locked in” with 100% certainty yet (but I’d say it’s about 90-95% right now).  There is still some level of uncertainty here, but not nearly enough to stop preparations for a major hurricane landfall.

With the dominant steering ridge to the north/northeast of Florence appearing to remain dominant through Thursday, even if it starts weakening some on Thursday, and no other strong mechanisms to steer Florence, the inertia argument seems to make sense to me…the storm may begin slowing on its approach, but it’ll ultimately keep going WNW to NW into the Carolinas Thursday afternoon or evening because there’s nothing to really stop it.  That’s what I’m expecting.  The one fly in the ointment that could result in an actual “push” away from land (as opposed to just a slowing of the forward speed) would be the shortwave to the northeast of the storm being close enough to actual have any influence…but that shortwave is going to be fairly weak, and unless the storm tracks farther to the right over the next 48 hours, it will probably be too far away to feel any influence from it.  It’s also important to note that Florence itself is intensifying that ridge to its north due to all of its anticyclonic outflow and latent heat release, which also kind of argues against a farther right track.  I tend to favor a landfall somewhere in North Carolina, probably just east of the South Carolina boarder, but a small left shift is all that would be needed to get a landfall in South Carolina, and it’s way too early to rule that out.

With Florence slowing considerably as it comes in and potentially drifting around for several days after landfall, severe freshwater flooding is a real concern.  Florence already has a large area of hurricane-force winds, and will likely increase in size further due to an eventual eyewall cycle.  This will cause damaging winds to affect a large area when it makes landfall with a significant storm surge along and to the right of the center as well.  Although Florence will spin down as it moves inland, its large size may allow strong enough wind gusts to topple trees and knock out power to occur well inland…especially if the Euro’s faster speed pans out and this gets all the way to the Appalachians…where the terrain would certainly cause locally enhanced winds and hence wind damage.  All told, this adds up to a severe impact to the Carolinas if it comes to fruition, near the upper-end of what they’ve seen before.  This is a serious situation, and hopefully people do what they can to protect their lives and get out of harm’s way.  Flooding rains could get as far north as parts of the Mid Atlantic and maybe even the upper Ohio Valley as well, though the evolution that far into the future is far from certain yet.

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September 8th, 2018: Florence Poses a Major Risk to the Southeast U.S. as it Churns Westward

Disclaimer: The thoughts in this post are only mine and don’t represent the thoughts of any other companies, groups, websites, or organizations.  They also shouldn’t be used for decision making in the impacted areas…

The majority of the satellite and model images in this post are courtesy of Tropical Tidbits.  Any poorly drawn illustrations on the images are my own.

Now the post…

Main takeaways:

  1. Florence’s weakening trend is over, and the storm will likely become a hurricane again by Sunday and a major hurricane again on Monday or Tuesday. Peak intensity as a category 4 or 5 hurricane over the western Atlantic is likely.
  2. Florence will accelerate to the west-northwest into the western Atlantic, south of Bermuda, over the next several days.
  3. The odds for landfall on the U.S. mainland on Thursday or Thursday night are increasing and are now more likely than a miss to the east. I suspect the Carolinas would be the most likely landfall location, though a direct impact on Georgia, northern Florida, and perhaps eastern Virginia can’t yet be ruled out.
  4. Florence would likely make landfall as a major hurricane if it hits the Southeast U.S., bringing a substantial wind damage and storm surge risk in the areas directly impacted.
  5. Florence would likely slow substantially and perhaps stall just inland if landfall occurred, bringing a substantial risk for severe fresh-water flooding as well. The risk for flooding could leak farther north into the Mid Atlantic by the end of the week or next weekend.
  6. A miss just to the east with no direct landfall still is possible at this juncture. If you’re not an idiot, you’ll root for that solution over the next couple of days, but it’s important to note that it unfortunately just is not the most likely outcome as of this writing.

Discussion:

 

9-8 florence vis.png

Satellite image of Florence Saturday PM

 

Florence’s days long battle with moderate to strong shear and dry air is coming to an end.  The shear has relaxed, allowing the circulation to become vertically aligned once again, with the low-level center under the deep convection.  It does appear there’s still a considerable amount of dry air entrained into the circulation as the convective structure is a bit bare-bones at the moment, however, there is a relatively intense and organized curved band that may have some developing inner-core structure.  The storm itself is quite large, with a rather low minimum pressure from a tropical storm (989mb) and already has expansive outflow in all quadrants.  Once the dry air is mixed out and deep convection fires more persistently, the storm appears to have the pieces in place to become a large and very powerful hurricane as it moves west-northwest.

 

9-8 florence shear

Current shear across the Atlantic

 

Shear analysis from Saturday afternoon shows Florence is already under a developing upper-level anti-cyclone, which is acting to fend off stronger winds from an upper-low to its north.  The map does also on its face appear to show a corridor of favorable, low-shear conditions ahead of Florence.

 

9-8 florence wv

Current water vapor across the Atlantic

 

There is an expansive corridor of weak upper-level lows that could in theory cause increased wind shear ahead for Florence, but unfortunately Florence’s size and intensity will likely allow it to dodge any weak punches the atmosphere will attempt to throw at it over the next few days.

 

9-8 Florence GFS 24

GFS model depiction of the upper-level lows valid Sunday

 

You can see this happen on the GFS…the model has an axis of upper-level lows/disturbances ahead of Florence in the short term, but…

 

9-8 Florence GFS 84

GFS depiction of the upper-level lows and ridges valid Tuesday

 

As Florence intensifies so does its upper-level anti-cyclone, and this helps the storm power through the mine-field of weak upper-level disturbances that could shear the storm.  It almost becomes a positive feedback, as the upper-disturbances act to help ventilate the storm at the top of the troposphere, strengthening the storm and its anti-cyclone even more.  The GFS has a habit of being a little too aggressive with this process, however, the Euro has consistently shown the same thing:

 

9-8 Florence Euro 78

European model forecast wind shear Tuesday PM

 

Slightly different plot, of the deep-layer shear, but the flow is clearly anti-cyclonic away from Florence with any stronger shear staying away from the storm.

 

9-8 Florence SSTs

Current water temperatures (C)

 

With the very favorable upper-level wind pattern expected and Florence moving over 29-30C SSTs (mid 80s) all the way up until the Southeast US coast, the ingredients appear to be in place for Florence to become a very powerful hurricane during the first half of this week.  The storm already appears to have the pieces in place once it can mix out the dry air.

 

9-8 Florence RH

GFS model forecast mid-level relative humidty valid Sunday

 

The environment ahead of Florence does have a modest amount of dry air in the mid-levels, nothing worse than normal for this part of the Atlantic, and certainly not the Saharan Air Layer the storm battled last week over the eastern Atlantic.  GFS storm-average forecast soundings show the mid-level RH rising above 70% on Sunday and about 80% for Monday and Tuesday, which is plenty high that when combined with little shear to tilt the circulation and entrain dry air in…and good outflow and warm waters promoted vigorous convection…that dry air intrusions shouldn’t be a hindrance to re-intensification over the next few days.

 

9-8 Florence sounding

GFS model forecast sounding valid Monday evening for the environment around Florence

 

Here is a look at a GFS forecast sounding valid Monday evening for the environment around Florence, showing minimal shear (less than 5 knots) and fairly limited dry air (mid-level RH of near 80%, which is favorable for intensification).

All told, Florence will certainly become a large major hurricane over the western Atlantic during the first half of this week.  The only real questions are does it mix out the dry air and start rapidly intensifying on Sunday or Monday, and does it max out as a category 4 or category 5 storm?  The difference in potential impacts down the road isn’t much as this is a substantial risk either way if it makes landfall as a major hurricane.

 

9-8 Florence Euro 108

European model forecast upper-level winds Wednesday evening

 

Unfortunately for the Southeast U.S., if Florence does make a direct landfall it appears that the unusually strong ridge pushing the storm west towards the coast will also keep the jet stream and any notable wind shear well north and west of the storm until after it made a possible landfall.  If Florence missed the Southeast and worked farther up the East Coast then it’s possible that southwesterly shear and cooler waters would have an impact…but if this makes landfall from North Carolina points south, the only things that appear to possibly weaken the storm are internal dynamics (eyewall cycles) and perhaps, perhaps some entrainment of continental dry air if the storm slows upon approach.  Essentially, I struggle to see how this doesn’t maintain at least major hurricane intensity if it goes into the Southeast.  If the storm slows significantly coming in, or goes farther north, then it’s possible it weakens further.

 

9-8 Florence current steering

Current steering pattern across the Atlantic Ocean

 

The current steering pattern near Florence shows what was, at one time long ago, supposed to be Florence’s opportunity to recurve harmlessly out to sea.  As has been discussed ad-nauseam by everyone, that isn’t happening as ridging is quickly building over the western Atlantic.  This ridge will become the dominant steering feature into the first half of this week for Florence, causing it to accelerate to the west and west-northwest through the middle of the week.  This will take Florence south of Bermuda and dangerously close to the Southeast U.S. coast.

 

9-8 Florence EPS 108

European ensemble forecast steering pattern valid Wednesday evening

 

The ridge will be very strong over the western Atlantic through much of the week, near record strong for this region this time of year, and this will cause Florence to maintain a west-northwest heading to dangerously near the Southeast coast.  This much is agreed upon.  It’s the exact location of the western edge of the ridge that is still somewhat in doubt, and is the only factor that suggests that a last second recurve is still not completely ruled out.

Note that on the above image that the European ensemble mean does have the heart of the ridge off the East Coast itself, so there is some opportunity for a last second turn…but also note that well-above average heights extend west into the Mid Atlantic, suggesting that the window to turn is probably not enough to avoid landfall unless the western edge of the ridge is weaker and farther east than the majority of the European ensemble members have.

 

9-8 Florence EPS 132

European ensemble forecast steering pattern valid Thursday evening

 

The European ensembles do eventually weaken the western Atlantic ridge…just a little bit…but by that point it’s too late, as Florence would have already made a Southeast U.S. landfall, and because an intensifying ridge over the Ohio Valley would probably prevent much if any north/northeast motion…if anything this would just cause Florence to slow considerably and essentially stall for an extended period of time until the ridging can break down and a shortwave can come in and kick the storm or whatever is left of it out…which, might be a while..

 

9-8 Florence EPS 240.png

European ensemble steering pattern forecast valid 9/18

 

This means that if Florence does not curve east and slow down/stall just offshore, that it would bring a lot of rain to parts of the Southeast and perhaps Mid Atlantic for quite a while before the eastern U.S. finally gets cleared out several days after potential landfall.

 

9-8 Florence GFS 96

GFS model steering pattern valid Wednesday morning

 

The Saturday 12z GFS run showed the alternative, much less impactful solution for Florence.  The model had a weak shortwave erode the western edge of the ridge just enough that Florence was able to turn north and end up east of North Carolina/the southern Mid Atlantic before ridging building over the Ohio Valley/Great Lakes forced a stall for a few days just offshore.

 

9-8 Florence GEFS tracks

Saturday afternoon GFS ensemble member forecast tracks

 

The GFS suite is generally more optimistic about the chance for a very close miss than the European suite this afternoon, but still, it’s at least half or a little more of the members that manage to make a landfall in the Carolinas…in addition…

 

9-8 Florence GEFS trend.png

Forecast trend for mid-level heights from the GFS ensemble

 

The trend over the last two days’ worth of GFS ensemble runs has been to intensify the ridging north of Florence that would need to weaken quicker to allow for a curve just east of the Carolinas.  This trend has slowed a bit in today’s run, but the more extended range GEFS runs were too weak with this ridging…not a good sign that the suite that’s been consistently more optimistic about curving Florence off the coast has gradually been trending away from that idea.

Let’s take a look at the last two cycles of the European ensemble and the member track forecasts…

 

9-8 Florence EPS 0z tracks.png

Friday night European ensemble member forecast tracks

 

A small cluster (slightly more than 10% of the members) misses the coast.  The rest hit somewhere.  The majority hit SC, GA, or FL, with another notable cluster hit NC and/or extreme SE VA.  This is not going into the Gulf.

 

9-8 Florence EPS 12z.png

Saturday afternoon European ensemble member forecast tracks

 

The 12z EPS generally shed some outliers on both side…though a few members hang on to a GFS-like solution.  The members that make landfall did cluster a bit more on SC/NC than the 0z run.  This was overall a modest right trend, but the number of recurves east of the U.S. did not increase.

At the end of the day, my impression is that the European suite has generally led the southwest trend with Florence over the last several days (with a nod to the UKMET at well that remains on the southern side of the envelope).  With ridging remaining strong until Florence is essentially just off the Carolina coast on Wednesday, and then perhaps only weakening briefly before more ridging builds in over the Ohio Valley/Great Lakes, and a large majority of EPS members continuing to show hits, a curve to the east of North Carolina while still possible just feels like a bit of a long-shot.  Unless we start seeing more substantial trends among the European suite away from a landfall idea in the next day or so, we can likely start “locking in” a Southeast landfall.  We aren’t quite there yet, but it’s getting close.

If landfall occurs, which at this point I’d say is probably about an 80% proposition, I do suspect that the brief weakness in ridging as it approaches pulls Florence north enough that a NC or SC hit is more favored than a GA or northern FL hit, though all options are still on the table so everyone in the Carolinas…Georgia…northern Florida…and even Virginia should make sure their hurricane plan is in place…while hoping that the increasingly long-shot recurve solution just to the east somehow happens to play out…we should be able to say by Monday if that has a shot of occurring still or not.

As discussed above, Florence if it made landfall would likely be a strong hurricane, probably a major hurricane.  This would obviously bring a damaging wind and storm surge risk to the coast.  However, with ridging building in north of the storm as it makes its possible landfall, the risk for a stall or prolonged slow motion just inland is high…which means the risk for someone in the Southeast or perhaps southern Mid Atlantic seeing very heavy rain and significant fresh-water flooding is quite high if Florence makes landfall.

If Florence makes landfall it would be a severe impact in the Southeast, with perhaps flooding rains getting north into the Mid Atlantic…the odds of landfall are becoming increasingly high but aren’t 100% yet, so we’ll continue watching for a window for it to curve just to the east of the U.S. coastline.  With landfall risk appearing to continue to increase however, those potentially in the path should make sure their hurricane plan is in place and ready to be put into action early this week if landfall continues to look likely.

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September 7th, 2018: Gordon Remnants to Bring Flooding Concerns to Ohio

rain 9-7 final

The remnants of Tropical Storm Gordon will move across the southern Great Lakes as a non-tropical low-pressure this weekend.  This will combine with a front, the jet stream, and lingering tropical moisture to wring out quite a bit of rain across a good portion of Ohio.  The potential exists for some areas to get heavy rain, which will cause flooding concerns.  Here’s the quick summary:

-There will be some rain today and tonight in southern and central Ohio, though except for perhaps localized issues close to the Indiana boarder this won’t be the heaviest portion of the event

-Rain will gradually become more widespread and heavy as Saturday afternoon wears on

-The heaviest rain will likely start in western Ohio late Saturday and gradually work east-northeast through the state through Sunday evening.  Northeast Ohio won’t get into the heavier rain potential until late Saturday night or Sunday morning

-Except for a few localized exceptions, much of the state hasn’t been that wet lately and can generally handle some rainfall without flooding.  The concern will be for prolonged tropical downpours that may cause flash flooding later this weekend where they occur, and also in areas that do see several inches of rain, which will likely be enough to cause river flooding concerns despite the recent dry conditions

-Embedded areas of heavy rain and thunder will ultimately drive the heavier totals.  This will be inherently a little bit inconsistent.  I expect most areas to get a good amount of rain, but only areas that get hardest will see the upper end of these totals

 

9-7 NAM 1.png

Modeled fronts, low to mid-level flow, and atmospheric moisture valid Sunday AM

 

The non-tropical area of low pressure associated with the remnants of Gordon will move northeast across the southern Great Lakes Sunday into early Monday, bringing periods of heavy rain to Ohio as the lift from that low pressure, the fronts, and the jet stream interact with the tropical moisture being drawn north.

I can’t rule out a few slow-moving downpours/storms with locally heavy rain mainly in far west-central or southwestern Ohio this Friday afternoon/evening, but the meat of this heavy rain event doesn’t really start taking shape until Saturday afternoon…

 

9-7 NAM 2.png

Model forecast mid-level winds and temperatures/humidity valid late Saturday PM

 

This will start occurring when the low to mid-level flow starts increasing across the Ohio Valley.  This will essentially cause moisture to get squeezed/wrung out by the front into central and southern Ohio.

 

9-7 NAM 3

Model forecast mid-level winds and temperature/humidity valid early Sunday

 

As the low intensifies and slowly moves northeast Saturday night into Sunday, the lift along the front in the low to mid-levels will also get stronger, making heavy rain more likely.  This process will only slowly shift north, so the concern is for some areas to get at least a few hours of moderate to heavy rain with decent rainfall rates Saturday night into Sunday as the warm frontal zone slowly lifts across the state.

The warm front likely gets close to Lake Erie by Sunday afternoon, so a round of heavier rain is likely well into northern Ohio, though it will probably take into late Saturday night or Sunday morning to get there.

Aside from the heavy rain from the warm frontal lift, the cold/occluded front will slowly move west to east across Ohio Sunday into Sunday evening, sparking more locally heavy rain/storms that may not be short-lived in some spots:

 

9-7 NAM 4.png

Modeled fronts, low to mid-level winds, and atmospheric moisture Sunday afternoon

 

The occluded front will slowly move across the state Sunday into Sunday night, and with the highest moisture being just ahead of this front and storm motions being nearly parallel to the front, some training rain/storms with heavy rainfall rates will be a concern Sunday into Sunday night as the occluded front moves slowly east across Ohio.

Some instability will gradually build north Saturday night into Sunday, and be maximized Sunday into Sunday night ahead of the occluded front the pushes east.  The instability won’t be that strong, but will enhanced the potential for embedded heavy downpours:

 

9-7 NAM 5

Model forecast instability valid overnight Saturday night

 

 

9-7 NAM 6

Modeled instability valid late Sunday afternoon

 

In addition, the upper-level winds will be in a favorable, diverging pattern both Saturday night into Sunday morning as the warm-frontal rain is lifting north, and Sunday evening as the occluded front pushes east:

 

9-7 NAM 7

Forecast upper-level winds valid late Saturday night/early Sunday morning

 

 

9-7 NAM 8.png

Forecast upper-level winds valid Sunday evening

 

This will also further increase the potential for widespread rain with embedded moderate to heavy rain…

Essentially, many factors are aligning for heavy rain later Saturday through Sunday night…

-Strong lift from warm front slowly lifting across state

-Additional lift moving across the state shortly thereafter as the occluded front pushes east

-Large-scale upward motion due to favorable jet stream dynamics as both above processes occur

-A warm, very moist airmass that will promote heavy precipitation rates

-Weak instability, which will enhanced the potential for embedded heavy rain.

Weak instability in a warm, very humid airmass with multiple sources of lift is about the best way to maximize potential rainfall rates.  Embedded heavier rain/thunder could produce rain rates in excess of 1” per hour at times, and much of the state will see multiple opportunities of at least a few hours for heavy rain.  The rain will be inherently a little inconsistent because it’s summer, but where the stars align there will be several inches of rain.

 

9-7 recent rain.png

Observed rainfall percent of normal over the last 14 days

 

Over the last two weeks, rain has generally been at or a bit below normal across most of Ohio.  A few big exceptions, and those areas will be more vulnerable this weekend, but in general rivers aren’t running that high going in and the ground isn’t too saturated…so we can take some rain.  However, prolonged heavy rates would still cause flash flooding, and several inches over a large area will overwhelm and flood some rivers, so the risk for flooding is certainly elevated this weekend, and those in flood prone areas should be prepared for the potential.  River flooding, flash flooding of streams and creeks, and roadway flooding will all be concerns given the potential for heavy rates and high amounts over a large area.

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September 6th, 2018: All Eyes Now on Florence

Disclaimer: The thoughts in this post are only mine and don’t represent the thoughts of any other companies, groups, websites, or organizations.  They also shouldn’t be used for decision making in the impacted areas…

The majority of the satellite and model images in this post are courtesy of Tropical Tidbits.  Any poorly drawn illustrations on the images are my own.

Now the post…

Key takeaways at this point:

  • Florence is unlikely to take the early recurve out to sea this weekend, meaning it will get into the western Atlantic and put Bermuda on guard during the first half of next week…and the East Coast on guard starting around Wednesday.
  • Whether or not Florence actually manages to hit the East Coast directly or recurves out to sea with much more minor, indirect impacts is impossible to determine at this point, as it will come down to finer-scale details that can’t be accurately forecasted at nearly a week out. At this point it should be stressed that less than half of all available, reliable guidance actually directly hits the coast, but that most data does at least get it close with a smaller portion actually hitting directly.  The threat is there, but nothing is a given this early.
  • Florence has weakened dramatically over the last 12-18 hours and further weakening is expected into the first half of the weekend.
  • Re-intensification, quite possibly back into a major hurricane for a time, is likely as we head into early next week as Florence tracks into the western Atlantic.

 

Florence IR

Current satellite imagery of Florence

 

Hurricane Florence basically did everything I said it wouldn’t do over the last couple of days.  It defied a marginal environment and intensified rapidly, to at one point a category 4 hurricane in an unusual location that far north in the eastern Atlantic, and also stayed on the northern side of the envelope of possible forecast tracks over the last couple of days.  Why it intensified despite expected moderate shear, dry air, and marginal water temps (SSTs) is a little bit beyond me…it’s possible the storm itself was able to create a small bubble of little shear in its immediate vicinity or that some sort of internal dynamics fought off the shear for a couple of days.

The above satellite image is much less impressive than what we were seeing yesterday…Florence is exhibiting a smaller area of deep convection (with warmer cloud tops), with no discernable eye or eyewall on convectional imagery.  The center is located near the southwestern edge of this deeper convection, with outflow still good in the northern and eastern quadrants but very restricted/missing in the southern and western quadrants.

 

Florence shear

Current analysis of wind shear across the Atlantic showing 30-40 knots of shear over Florence

 

We appear to be at the point where whatever mechanism that was fighting off the shear has finally been overwhelmed, as 30-40 knots of westerly/southwesterly shear have clearly had an effect overnight.  The storm will end up weakening from a category 4 to a category 2 in the matter of 18 hours by the time the 11AM EDT advisory comes out.  Florence will be tracking through this belt of strong west/southwest shear for another couple of days, and further weakening is likely.

 

Florence SAL

Current analysis of the “Saharan Air Layer” (dry air), with the layer showing up in yellows/reds

 

It does not help matters that Florence is still in proximity to some mid-level dry air courtesy of a Saharan Air Layer.  The west/southwest shear will tilt Florence’s circulation to the east/northeast with height, making it more prone to dry air entrainment than it’s been.  Shear won’t abate until somewhere around Saturday, and don’t be surprised at how much another full day or two of strong shear/dry air entrainment can weaken Florence (considering it’s already weakened quite a bit)…this very well could be a tropical storm by the time conditions start becoming more favorable for it to strengthen again over the weekend.

 

Florence GFS 90

GFS model forecast upper-level winds valid Sunday evening

 

Florence is expected to move north of the belt of inhospitable upper-level conditions by Sunday and into early next week.  Ironically, the weak upper-level disturbances shearing it now may end up acting as outflow channels that help intensify the hurricane into early next week as it tracks into the western Atlantic.  The jet stream to the north could also help with ventilating the hurricane in the upper-levels.

 

Florence SSTs.png

Current SSTs across the Atlantic basin

 

Florence will also be moving over increasingly warm SSTs over the next several days, considerably warmer than what it was over on Wednesday as a category 4.  This, combined with the expected favorable upper-level wind pattern, will likely foster re-intensification into early next week…likely back into a major hurricane.

 

Florence GFS sounding

GFS model forecast sounding for the environment surrounding Florence on Monday.  The plot shows winds with height on the right, and temperature/humidity in the middle.  The red line is temperature, green line is humidity.  The gap between the two represents some dry air.

 

This is still a kind of delicate situation, as forecast soundings show dry air continuing to be perhaps somewhat of a hindrance to intensification into early next week…500-850mb RH is 70% on this storm-averaged sounding from the GFS, which is on the margins of being unfavorable.  If shear is in fact weak (less than 10 knots) as the GFS and Euro both show, this won’t be a significant issue, but if shear fails to weaken below about 10 knots, then dry air intrusions would slow re-intensification.  At this point I suspect that given the favorable SSTs that once Florence moves north of the strongest shear over the weekend that the strong outflow will favor deep convection over the center that allows it to intensify pretty quickly…but, we’ll need to see exactly how much of a toll the next couple days of unfavorable conditions has on the storm.

Long story short on the intensity, expect continued weakening (potentially rapid) through Friday and into Saturday before re-intensification.  How quickly it re-intensifies is up in the air, but there appear to be enough “positives” for intensification that Florence will be a hurricane and likely at times a major hurricane again over the western Atlantic early next week.

As for the track forecast…

 

Florence historical tracks

Track of all tropical storms or hurricanes on record that have tracked within 100 nautical miles of Florence’s Thursday morning position

 

The first comment is that we should keep in mind the historic precedent…no storm that has tracked within 100 miles of Florence’s 5AM Thursday location as a tropical storm or hurricane has struck the US.  So, we’d need an unusual pattern to see that happen here…it does appear we’ll have that.

 

Florence current steering

Current steering flow across the Atlantic Ocean (Thursday morning)

 

In the short term, Florence will likely start slowing some to end the week and into the weekend and turning more “left” or back towards the WNW (instead of the recent NW heading) as ridging builds in to its north.  Florence will be weakening over the next day or two as mentioned above, and this will make it more prone to getting shunned to the left by the ridge to its north.

 

Florence EPS 72

European ensemble mean upper-level weeather pattern valid Saturday evening

 

As we head into the weekend Florence’s initial opportunity to recurve comes up on us for Saturday night-Sunday as a weakness in the ridging briefly opens up to the north of the storm.  For a number of reasons, a recurve at this juncture now looks less likely that it did a couple of days ago:

  • The trough to the north does not dig as far south, resulting in the ridging breaking down later, less, and building back in quicker
  • Florence will be moving rather slowly (note the random, and not particularly strong steering currents ahead of Florence), and may not gain much latitude before ridging builds back in
  • Despite Florence’s unexpected intensity this week, it’s expected to be near the pinnacle of its weakening trend when this weakness in the ridging briefly opens up. Florence being weak when its brief escape hatch is open decreases its chances of taking it.

 

Florence EPS 96

European ensemble mean upper-level weather pattern valid Sunday evening

 

The weakness in the ridging north of Florence closes quickly, within a day, as the trough over the north Atlantic quickly zips off to the east-northeast and ridging builds back in.  Given all of the reasons mentioned above, Florence likely doesn’t get to hit the weakness in the ridging before it closes, which would force it to turn back more to the west into the western Atlantic as what will likely be an intensifying hurricane.

 

Florence EPS 144

European model ensemble mean upper-level weather pattern valid Tuesday evening

 

As has been the fear all along here, if Florence misses the early recurve (now expected), the anomalously strong ridging over the western Atlantic would force Florence to move west to dangerously  near the US East Coast.  That is still expected, and Florence will in all likelihood make at least a dangerously close approach during the second half of next week.

Whether or not Florence gets close, but then turns away, or is able to make landfall is still unknown and may ultimately come down to finer details that just can’t be resolved at this range.

 

Florence GEFS 144

American model ensemble mean upper-level weather pattern valid Tuesday evening

 

For instance, the GFS ensemble (American model) has a little bit less riding over the western Atlantic, which would make the “close call” scenario more likely than the European solution, which has a stronger ridge and makes a direct landfall more likely.

 

Florence GEFS trend

Trends in the upper-level weather pattern among the last several runs of the “American” model ensemble

 

With that said, even the GFS ensemble has had some concerning trends over the last several runs…towards a stronger ridge over the western Atlantic, and perhaps a bit of a trough over the southern US, which could not only help intensify the ridge but also perhaps pull the storm west if located in the correct location…many East Coast hurricane hits have some sort of trough over the Ohio or Tennessee Valley that helps pull a storm more north-northwest instead of letting it just curve around a ridge.

 

Florence EPS tracks

Wednesday night’s European ensemble member track forecasts for Florence, showing a wide spread of possible solutions

 

Last night’s European ensemble has continued the trend of the last few cycles of going from just a small cluster of members getting Florence into the western Atlantic to nearly every member missing the early recurve and getting the storm into the western Atlantic.  A little less than half the members actually hit the US directly, but that’s still up from previous runs, and even most of the misses are just off the coast and close enough for at least indirect impacts (dangerous surf, beach erosion).

The big takeaways at this point are that Florence is likely getting into the western Atlantic, putting Bermuda and the East Coast on guard.  It’s still simply too early to tell if this will directly hit the coast or simply curve away just off the coast with more minimal impacts.

Factors that could make a hit more likely would be the upper-level trough over the southern US coming in a bit deeper, perhaps pulling the storm more west as it rounds the western edge of the ridge.  The ridge itself being stronger or located farther west would also inherently force Florence to track farther west, increasing the risk for an actual landfall.

A weaker ridge overall, or the ridge being located just a bit farther east, would make the “near miss” scenario more likely to pan out.

Ultimately it’s too early to know any of these details, at this point we just know that some sort of threat is there for the second half of next week for the US East Coast (and Bermuda during the first half of the week).  The ridge’s location would likely make a direct hit on the Long Island/New England area harder to come by…I think if this hits the US directly it more likely does so in the Mid Atlantic or Carolinas.

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September 2, 2018: Tropics heating up just in time for peak season

Disclaimer: The thoughts in this post are only mine and don’t represent the thoughts of any other companies, groups, websites, or organizations.  They also shouldn’t be used for decision making in the impacted areas…

The majority of the satellite and model images in this post are courtesy of Tropical Tidbits or weathermodels.com.  Any poorly drawn illustrations on the images are my own.

Now, the post…

The Atlantic hurricane season was rumored to be a quiet one in the spring and early summer…that’s largely been the case thus far.  With that said, even quiet seasons usually have some hurricanes and maybe a major hurricane or two, and it looks like this season’s window for more substantial activity is coming up over the next 2-3 weeks…right on schedule compared to the climatological peak of the Atlantic hurricane season:

 

9-2 climo

Number of Atlantic hurricanes and tropical storms by date, showing the typical peak of the season around September 10th

 

This expected burst of activity is due to a large-scale weather pattern favoring upward motion and storminess of the Atlantic coinciding with an active African Wave Train (the latter is pretty common for this time of year).

 

9-2 MJO ECM

Ensemble forecast for the MJO phase over the next two weeks from the European ensemble

 

The large-scale pattern is called the “Madden Julian Oscillation” or MJO, and is basically areas of enhanced lift and thunderstorms near the Equator the move around the globe and impact both tropical cyclone development and the jet stream in the mid-latitudes.  The MJO has been forecast consistently by our recent modelling to move into phases 8 and 1 in the week 1-2 timeframe (see above image), which climatologically correlates to upward motion over the Atlantic:

 

9-2 MJO 200

Composites of upper-level vertical motion for each MJO phase in the early fall

 

Blues indicate large-scale upward motion, favored over the Tropical Atlantic in a phase 8 or 1 MJO…forecast models also show this general upward motion over the next couple of weeks:

 

9-2 MJO GFS

Forecast upper-level vertical motion from the GFS model for the next two weeks

 

The lack of yellows/browns over the Atlantic is a general departure from most of this hurricane season so far, and indicates neutral or even upward motion in the greens compared to normal for this time of year.  This generally favorable regime for tropical development is coming with a couple of systems of note already over the Atlantic, and more lined up over Africa:

 

9-2 Atlantic IR

Current infrared satellite imagery across the Atlantic

 

 

9-2 Africa IR

Ditto for Africa

 

Out of the systems currently over the Atlantic, 1 won’t develop but is already producing some flash flooding along the Gulf Coast, 2 will likely develop into at least a tropical storm before making landfall along the northern Gulf Coast later this week, and 3 is Tropical Storm Florence, which won’t bother anyone for a while but could eventually…

 

9-2 7 vis

Visibile satellite imagery of Potential Tropical Cyclone 7

 

The system in the Bahamas (Potential Tropical Cyclone 7 or PTC 7) is the most immediate threat to the US as it moves across the Keys/southern Florida tonight into Monday and into the Gulf for Tuesday, before landfall somewhere along the northern Gulf Coast late Tuesday night or Wednesday.

This system is noticeably more organized today, with a clear mid-level circulation with convection that is still bursting a little bit rotating around it.  Some modest anti-cyclonic outflow has also become evident in most quadrants.  There’s no surface circulation yet so it’s not a tropical cyclone as of this afternoon, but there’s a pretty good chance one develops tonight or Monday with the next burst of convection given the current organization.

 

9-2 7 shear

Vertical wind shear analysis for the western Atlantic valid Sunday morning

 

PTC 7 is within an area of modest westerly wind shear; note how the outflow isn’t yet present in the western quadrant, likely due to this shear.  This shear is not strong enough to stop development from occurring in the near term.  There is a belt of stronger shear in front of the disturbance over the eastern Gulf of Mexico between an upper-low off the Carolinas and upper-level anticyclone near the Texas coast….however, the upper-level low is expected to drift northwest and the anticyclone to the west over the next couple of days, allowing PTC 7’s anticyclone and weaker shear to maintain as it moves into the eastern Gulf…

 

9-2 GFS 48

GFS model forecast upper-level winds valid Tuesday morning over the western Atlantic

 

This low-shear environment will combine with very warm water temperatures with moderate oceanic heat content to favor intensification through Tuesday evening while the system is over water:

 

9-2 w atl ssts

Current Sea Surface Temperatures in the western Atlantic (in Celcius)

 

The water temperatures starting tonight and early Monday as the system moves near the Keys and then through expected landfall Tuesday night in the vicinity of Louisiana are on either side of 30C (86F), which are easily warm enough to support robust tropical development.  Oceanic heat content is a little bit more moderate in the northern Gulf continental shelf waters, but still can support a stronger system than what PTC 7 will turn into…this isn’t a limiting factor.

 

9-2 7 sounding

GFS model forecast sounding for the environment around PTC 7 on Tuesday

 

If there is a limiting factor for PTC 7’s intensification over the Gulf (other than time), it appears to be some dry air.  Shear and water temps aren’t limiting factors, with less than 10 knots of shear on area-averaged forecast soundings around the cyclone…but, some dry air and perhaps a bit of asymmetry with the system may ultimately limit the potential for more rapid intensification.  PTC 7 should become a tropical storm (likely Gordon) at some point on Monday, giving it 24-36 hours to intensify further.  Intensification into a category 1 hurricane is easily within the realm of possibility here, but it would require the cyclone to mix out the dry air and become a bit more symmetrical.  Even in a generally favorable environment this can take a day or two…I suspect PTC 7 (likely Gordon) will be on an intensifying trend on the way in, but it’ll be making landfall right around when it may be able to become a hurricane.

 

9-2 EPS 48

European ensemble mean 500mb heights and anomalies valid Tuesday AM

 

PTC 7 will be steered by a ridge over the Mid-Atlantic towards the west-northwest through the middle of the week.  The ridge is expected to build back west near landfall , which may cause the system to move rather slowly just inland west through Louisiana and into Texas.  This slow movement will ultimately cause the greatest concern with PTC 7 to become flooding, even if it can become a hurricane and bring some damaging winds, storm surge, and power outages as well.

As for Florence…

 

9-2 Florence IR

Infrared satellite imagery of Florence

 

Florence is not currently a well-organized tropical storm.  Convection has been very bursty and asymmetric, with the center circulation near the southwestern edge of the bulk of the convection.  There is some anticyclonic outflow evident, but it’s non-existent in the western quadrants.  This is due to moderate westerly shear affecting the storm.  There’s also drier/stable air to the north and west of the cyclone (evidenced by the low stratus clouds visible on the satellite image above), with the shear entraining that into the circulation.  Florence is also moving over very marginal water temperatures.  Long story short, don’t expect much intensification in the short term.

 

9-2 ECM 84 shear.png

European model forecast deep-layer wind shear over the Atlantic valid late Wednesday

 

Florence will be moving headlong into a pretty solid belt of moderate to at times shear for the next few days (generally 15-30 kt over the storm for the next 4 days).  This, combined with the aforementioned dry air, and this:

 

9-2 SSTs ATL

Sea surface temperatues across the Atlantic (C)

 

Just don’t support much (if any) intensification through Wednesday or Thursday at least.  It’s probably not impossible we see Florence up into an open wave at some point as the Euro tries to show.  Once Florence gets north of the belt of stronger shear and over warmer SSTs towards Friday or the weekend, that’s when more substantial intensification could occur…though the status of the storm at that point is not quite certain, as the cyclone opening up into an open wave would complicate things.

 

9-2 GFS 700 48

GFS model mid-level winds, heights, and humidity vaild Tuesday

 

Over the next few days Florence will be steered towards the west-northwest by the low to mid-level flow south of ridging across the eastern Atlantic.  Florence is already fairly far north and will likely pass safely north of the Lesser Antilles around the end of this week.  The GFS, HWRF, GFS ensembles and Canadian models have a more northwesterly track over the next few days while the Euro and UKMET are farther south and more of a WNW heading.  Given questions over how much Florence can intensify, if at all, over the next few days, I’m inclined to think that the NHC forecast (which is a split between the two camps as per usual) will be too far north by days 4-5.

The track farther west is as always uncertain at this time range, with several different things that could happen…

 

9-2 EPS 168

European ensemble mean 500mb pattern valid September 9th

 

The first big fork in the road comes next weekend.  Florence will approach a weakness between ridging over the eastern Atlantic and strengthening ridging over the western Atlantic along the East Coast.  There will be a trough moving across the north Atlantic at this time, but it doesn’t penetrate very far south due to the building ridging over the western Atlantic.

Does Florence recurve at this point?  Does it kind of stall or slow, but ultimately keep going west as the ridge over the western Atlantic intensifies and becomes dominant?

Too early to say, unfortunately.  It will probably slow down considerably for 2-3 days starting around Friday.  A stronger storm prrrobably gains enough latitude here to eventually recurve, but a weaker storm may get trapped under the intensifying ridge to the west.  As mentioned above, it’s too early to speculate how intense the storm may be next weekend.  Conditions will probably become more favorable for intensification near the end of this week into next weekend, but if Florence is an open wave at that point it may not get strong enough quickly enough to recurve.  Normally this scenario would be a recurve with Florence already being so far north as far east as it currently is, with a break in ridging in the middle of the Atlantic, but the ridging intensifying to the west as Florence gets there along with Florence’s intensify not yet being known in 5-7 days adds at least a layer of intrigue.

 

9-2 MJO lagged

Lagged composite 200mb height anomalies for a phase 1 MJO in the fall

 

Interestingly, the all-important MJO even favors ridging along the US East Coast in phases 8-1 (phase 1 shown) in the August-September-October timeframe…though also note that a few days later in a phase 1 MJO that the ridge tends to be replaced by a trough over the east (blue colors on the day 4-5 lags).

 

9-2 EPS 216

European ensemble mean forecast 500mb heights valid September 11th

 

Essentially, all evidence suggests there will be an unusually strong ridge over the western Atlantic in the 7-10 day timeframe that would cause Florence to resume a WNW motion into the western Atlantic IF it doesn’t recurve through the weakness in the ridging in the central Atlantic around days 5-7.

 

9-2 EPS 240

Ditto valid a day later

 

Even then, IF Florence does make it into the western Atlantic, the ridging is progged to be centered off the East Coast with some hints at ripples in the jet stream that could perhaps erode the western side of the ridge.  This could potentially allow for a close brush instead of an actual hit on the East Coast (or the ridge could be a little stronger and allow for a hit, these are too fine of details to try to be guessing at 10 days out).

 

9-2 EPS Florence

European ensemble member forecast tracks for Florence

 

The European ensembles bare this out…some members do take the early recurve, though more do get farther west…but many then still manage to recurve out to sea, just closer to the East Coast.  Several members do have tracks that could eventually hit, so it’s worth watching, but right now that camp is decidedly smaller than the camp that finds a way to curve Florence out to sea before getting all that close to the US coastline.

Long story short on Florence, we don’t know if it will recurve over the central Atlantic in 6 or 7 days…recurve over the western Atlantic later…or miss all of those opportunities and actually threaten the US East Coast.  There are more ways this recurves than hits in my opinion, and that’s usually the case with storms coming from this far east, however, the unusually strong ridge over the western Atlantic and uncertainties with Florence’s intensity forecast don’t make a total miss a complete given yet.

Behind Florence…

There could be a couple of more waves to watch for development before this generally favorable pattern for tropical cyclone development over the Atlantic winds down after the middle of September.  The ridging over the Atlantic continues to look stronger than normal through the next two weeks, so we’ll have to keep an eye on anything that can develop…though as is always the case, it should be noted that the standard climo for African waves that manage to develop over the open Atlantic is for recurve to be favored until told otherwise…so though the pattern will continue to warrant some watching, let’s take each storm one at a time and not get too worked up this early on.

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