Winter 2019-2020 Forecast Thoughts, A Technical Discussion

These maps have been released publicly via video, so I’ll go ahead and post them here to go along with a more technical explanation and commentary on what I’m thinking and where I’m most worried:

Temperatures (December-February):

temps

Precipitation (December-February):

precip copy

Snowfall (first to last flake):

snow(2)

Since I’m using the maps I created at work, for clients on company time, I’ll “plug” our YouTube video…I take no responsibility for the extra glamor (the thumbnail and emojis), but the maps, anything in this post, or anything I verbally say in the video are all fair game to comment about.  Here’s the link to the video (the intended audience is the general public, so very minimal technical talk)… https://www.youtube.com/watch?v=ehVCUa24XCQ

Before getting any farther, a few of the odder acronyms used multiple times:

PMM: Pacific Meridional Mode (off-equator water temperatures)

IOD: Indian Ocean Dipole (a positive IOD has warmer waters in the western Indian Ocean and cooler waters closer to Australia and Indonesia)

QBO: Quasi Biennial Oscillation (a pattern of westerly and easterly winds in the stratosphere that impacts the polar vortex in winter, and also can interact with tropical thunderstorms by influencing the temperature at the tropopause and modulating instability)

“Low Pass” signal: The longer-duration tropical forcing pattern (standing waves as opposed to something that propagates like the MJO)

MJO: Madden Julian Oscillation, an eastward propagating pattern of enhanced thunderstorms that can influence the pattern globally.

As for the discussion, the general ideas used earlier in September to gather some analogs still largely hold…those ideas were:

  • ENSO: Neutral, though can allow for weak El Nino or weak La Nina if the year featured warmer waters near the dateline. Extra preference if coming off of a weak or moderate El Nino the prior winter-spring.
  • PDO: Neutral or positive (it’s positive right now though not strongly)
  • QBO: Positive trending negative, expecting the 30mb winds to flip to negative at some point in the early to mid-winter
  • Solar: Minimum
  • Indian Ocean: Positive IOD in the fall, can trend downwards during the winter
  • Off-equator Pacific SSTs: A positive to strong positive PMM (it’s very positive right now), with more weight given if it stays positive through winter
  • Atlantic: Neutral or positive AMO (it is positive right now though not strongly)
  • Tried to find matches based on similar tropical forcing in mid-late summer as this year, though not as strong of a weight.
  • Tried to find matches based on years with significant spring/summer high-latitude blocking, though was not as strong of a weight.

As a reminder, the analogs were: 1958-59, 1966-67, 1969-70, 1978-79, 1980-81, 1985-86, 1990-91, 1993-94, 1995-96*, 2003-04, 2004-05*, 2013-14*, 2014-15, 2017-18, with starred representing the highest scoring matches when considering all of the above and getting double weight.  That 500mb composite map for DJF looked like this:

Analog 500 sept

The big stand out features in the analogs were: significant potential for a –EPO, a somewhat +PNA favored with the Aleutian low displaced to the south, mixed NAO signal though at least some potential for a –NAO, and decent potential for a –AO over the course of the winter. The strong signal for a –EPO along with some signal for a –AO does imply increased potential for a cold winter in much of central and eastern North America, with the +PNA favoring a mild winter along the West Coast.

Just looking at the various drivers on their own…a neutral or weak ENSO (likely warm neutral with warmer waters near the Dateline), very +IOD this fall, +PMM, +PDO (that has seemingly trended more positive in September, we’ll see what the monthly value is), descending –QBO with a lingering +QBO in the lower stratosphere, solar min, and the “low pass/lower frequency” signal this summer looking somewhat Nino-ish still with the strongest chi anomalies over the central/eastern Pacific, you get an interesting picture. The lingering +QBO and strongly +IOD/Modoki-ish look, along with the ongoing signal in the tropics late summer/early fall, support the most active convection occurring outside of the West Pac warm pool (so, outside of phases 3-6 of the MJO) heading into winter…meaning it would be most favored/active in phases 8, 1, and 2. We already are seeing this now, with tropical forcing most active over Africa and the western Indian Ocean, with some of the longer range models (EPS and CFS) showing signs of life over the central Pacific in a few weeks. Here is a look at the “low pass” signal via velocity potential/chi anomalies since July 1st:

Jul-Sept ChiHere are the CPC’s 200mb height composite anomalies for phases 8 and 1 of the MJO in December-January-February:

MJO 200mb djf

During winter, phases 8/1 of the MJO are both cold. The analogs are overall somewhat chilly for December in the central/eastern U.S. (though in general, January and February are the coldest months compared to normal in the composites), and this is generally done via PNA/EPO ridging in December in the analogs with a +NAO for December in the mean. Interestingly, a Phase 8 MJO in DJF strongly favors a –NAO…given the lower frequency signal appears to involve over the western Indian Ocean, Africa, and the central/eastern Pacific, perhaps that’s why Paul Roundy’s low pass analogs show a strong –NAO signal (and somewhat of a -EPO signal) heading into December:

roundy low pass

Phase 1’s composite looks more like the analog composite for December, with any cold coming from PNA/EPO ridging more so than a –NAO. Given the propensity for a –NAO over the last several months, SSTs in the tropics potentially favoring convection in an area that teleconnects to a –NAO in winter, and current low-pass signal analogs also suggesting the same, it seems there are multiple signals pointing to the possibility for NAO help as early as later in November and December. The analogs and tropical SSTs, along with to some extent the same low-pass analog posted above, also suggest potential for a +PNA/-EPO as early as later November or December. Quick Siberian snow cover advance, favoring a stronger Siberian high and positive East Asian Mountain Torque events and subsequently an Aleutian low may also enhance the potential for a +PNA/-EPO to start winter.

Several other well-respected long range forecasters have mentioned the potential for winter to “start early” this year in the eastern U.S. (and if the NAO is involved, Europe) compared to most recent winters…if the tropical signal (driven by the strong +IOD, lingering Nino 4 warmth, and to some extent the +PMM and AMO) is not strongly disrupted at the wrong time by intra-seasonal variability (such as an amplified MJO passage through phases 3-6, or a negative East Asian Mountain Torque), then there appear to be multiple reasons for optimism as early as late November or December, with some signal for both +PNA/-EPO ridging and perhaps a –NAO. Assuming the lingering +QBO in the mid and lower stratosphere, strong +IOD, Nino 4 warmth/+PMM, and cooler waters near Indonesia persist over the next few months, the “ingredients appear to be in place” for less convection in the West Pac, suggesting a lessened risk for a destructive MJO passage. We’ll see how the tropics signal plays out as we head deeper into fall.

Heading through winter, a big potential source of uncertainty will be what happens as the –QBO continues to descend in a deep solar min and the +IOD presumably weakens (which is climo for northern hemisphere winter). Unless a stronger move back towards El Nino occurs this fall (still not ruled out with the warmth near/west of the Dateline, though if this occurred it’d very likely only be a weak El Nino), a weakening +IOD and any strat warming events (which may become favored during mid-late winter assuming the –QBO continues to descend when combined with the solar min) may increase the risk for a more amplified MJO in the “unfavorable phases.” So, there likely remains some warmer risk in the eastern U.S. in particular for mid-late winter.

A hat-tip to “Snowy Hibbo” on the 33andrain forum (website: https://longrangesnowcenter.net/) (twitter: @longrangesnow) for inspiring some further discussion on the current disconnect between the tropics and extra-tropics, and how this may tie into the cold December idea in the central/eastern U.S. and also the potential warmer risk later in winter…

gwo

The extra-tropics have been firmly in a more La Nina-like state since late June/early July…the above shows the Global Wind Oscillation since the end of June, and negative values are more characteristic of a La Nina atmosphere.  This contradicts the more El Nino like tropical signal (that is evidenced by the chi anomalies and other indices such as the persistent -SOI).

Anecdotally, La Ninas tend to be colder in December, milder in February in the eastern U.S., with El Ninos the opposite.  With all of the other factors above, driven by potentially favorable tropical forcing for cold occurring heading into early winter with a La Nina like extra-tropic base, does that add confidence to winter actually starting in later November/December in the east?  Also, as the +IOD presumably weakens in winter per climatology as discussed above, does the lingering El Nino state in the tropics also weaken, opening the door to enhanced convection in the West Pac during the mid-latter portions of winter?  This would bring that warmer risk to the eastern U.S., and may occur at a time when stratospheric warmings are more favored, which already increases that warmer risk at least temporarily before any SSW/vortex disruption downwells into the troposphere.  It seems the interplay between the Nina-like extra-tropics, and Nino-like tropics, and how long the latter hangs on, is a large point of uncertainty for winter forecasts.

It’s worth noting that La Ninas are, on average (with some exceptions to both cases), much colder as a whole across Canada and the northern U.S. than El Ninos in winter, and the forecast temperature map as is would probably fall closer to La Nina climatology than El Nino…so this La Nina element to the pattern may not be the worst thing for a lot of the U.S. if it continues into winter.  Many analogs had a similar interplay between a more La Nina-like pattern in the extra-tropics when coming off of an El Nino (or otherwise had Nino 4 warmth), and had cold winters, so this wrinkle isn’t necessarily bad if you want cold and snow in the U.S.

If the Nino base state holds on in the tropics through the winter, the warmer risk in the eastern U.S. may be mitigated a bit.  If a strat warming event occurred, there may be additional risk for –NAO blocking later in winter. It bears noting that the monthly analog composite mean has a +PNA/-EPO through March (strongest in January) along with a more –NAO in all three months (compared to December when the composite NAO is positive), with January also having the most –NAO out of JFM (February a close second).

Beginning to tie this into the maps…comments on the temperature: It seems very likely that the Great Lakes and Upper Midwest see a cold winter based on the analogs and presumed tropical signal favoring a –EPO and +PNA this winter. It conversely seems very likely that the West Coast is mild. The East Coast is a mixed bag…perhaps due to being snake bitten last winter, and remembering mid-January-February of 2018, I had a hard time bringing the below average temps for the winter too far southeast. I’m worried a somewhat brief (a couple-few week) period of very mild temperatures is possible as extensively discussed above. However, it can conversely be argued that barring an intra-seasonal very mild pattern that something can argue for colder weather at nearly all points in the winter (and early spring) in the eastern U.S…so if the cold signals trend stronger heading into fall, I may end up bringing below-average temperatures farther southeast. As for the Southeast U.S., due to similar reasoning I currently have somewhat mild temperatures for the winter as a whole, though there is a strongly implied risk for EPO-induced cold shots that can bring wintry precipitation into the Deep South even if the winter averages a bit warmer than average.

As for snow/precip…the analogs strongly suggest a wet/active winter from the Ohio/Tennessee Valleys into the northern Mid-Atlantic and New England. I couldn’t think of a good reason to deviate, though if things trend much colder in the east it would inherently trend drier. The analogs are also generally dry to very dry across much of the southern U.S., especially in Texas. A –EPO would suggest an active northern stream into the Plains, keeping precip from ending up too far below average and likely favoring above-average snow in the Upper Midwest, parts of the Plains and the eastern slopes of the Rockies. The analogs are very dry in the Pacific Northwest and couldn’t find good reason to deviate. Conversely, they lean dry for California and the Southwest but not as strongly as the Pac NW…with a lingering +QBO and El Nino influence to start the winter, along with a very +PMM, there is definitely potential for precipitation/mountain snow in California and the Southwest, especially through the first half of winter. For now went near normal precip/mountain snow in that area…may need to trim the below normal in the Dessert Southwest and perhaps consider above-average for parts of California if the lingering El Nino signal persists through the fall.

 

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Mid September, 2019 Winter Analog Notes

I posted this the second week of September on a couple of forums, just posting it here for the record/reference later…

Playing with some analogs right now…here are some analogs, discussion, and caveats, focused mainly on the U.S….FWIW, my temp map for work is not as cold as this analog composite would imply and allows some risk for a SE ridge to crop up should things trend too far in the wrong direction (read below).

Here are my key assumptions when looking for analogs right now…the analogs used to compile the attached 500mb map were found by subjectively rating every possible analog on each of the following criteria. Ultimately I give ENSO, PDO and QBO the most weight along with the solar cycle, though all of these are considered.
  • ENSO: Neutral, though can allow for weak El Nino or weak La Nina if the year featured warmer waters near the dateline. Extra preference if coming off of a weak or moderate El Nino the prior winter-spring.
  • PDO: Neutral or positive (it’s positive right now though not strongly)
  • QBO: Positive trending negative, expecting the 30mb winds to flip to negative at some point in the early to mid-winter
  • Solar: Minimum
  • Indian Ocean: Positive IOD in the fall, can trend downwards during the winter
  • Off-equator Pacific SSTs: A positive to strong positive PMM (it’s very positive right now), with more weight given if it stays positive through winter
  • Atlantic: Neutral or positive AMO (it is positive right now though not strongly)
  • Tried to find matches based on similar tropical forcing in mid-late summer as this year, though not as strong of a weight.
  • Tried to find matches based on years with significant spring/summer high-latitude blocking, though was not as strong of a weight.
The analogs I went with for this exercise are…1958-59, 1966-67, 1969-70, 1978-79, 1980-81, 1985-86, 1990-91, 1993-94, 1995-96*, 2003-04, 2004-05*, 2013-14*, 2014-15, 2017-18. Stars denote highest scoring matches when considering all of the above and double weighting.
ssts
Analog 500 sept
Obviously some of these are very cold winters for the eastern U.S. and the composite look is cold. There is a strong signal for Alaskan ridging and a -EPO, which is not a warm pattern for the central and eastern U.S., though where exactly the cold dives in can make a difference for the eastern U.S. with more mixed signals on an NAO. A neutral-ish ENSO and +QBO to start winter aren’t great signals for a -NAO, though the deep solar min is and the QBO will be improving through the winter. The SSTs up there support a -NAO, but aren’t a strong forcing mechanism on their own. There is not a strong correlation between negative summertime NAO and subsequent winter NAO and the forcing mechanisms are different, so the persistent -NAO this summer doesn’t really help or hurt.
In terms of what to watch for in the eastern U.S. in terms of swinging warmer or locking in cold…I’ll be watching to see if we hang on to the warm waters near the Dateline in the Equitorial Pacific and the +IOD as we head into fall. If we keep those we are more likely to see convection near the Dateline this winter which usually forces an Alaskan/western Canadian ridge. If we see a stronger push towards La Nina and lose the warmth near the Dateline, the risk for a more amped SE U.S.  ridge increases…we don’t need the Nino region 3.4 anomaly to be above 0C to have a cold winter in the east, but region 4 is pretty important and needs to stay warmer IMO.
Based on the persistent -SOI and forecast generally weak trades over the central and western Pacific over the next week or so, there won’t be a big La Nina push in the near-term, though the recent easterly trade surge did nudge things in that direction over the last couple of weeks and there’s still a lot of time for that to resume.  The waters near the Dateline and just west remain fairly warm both at and below the surface, and until that goes away some move back towards a weak Modoki El Nino also can’t be ruled out.  As we saw last winter, a SSW can really enhance tropical forcing/convection over the West Pac warm pool (which is usually warm for the eastern U.S.), so an initially +QBO and seemingly low risk for an early SSW may give some margin for error…but if the SST pattern becomes unfavorable between Australia and S. America for convection near the Dateline the pattern more likely supports eastern U.S. warmth this winter.
The analogs that have an Aleutian low in October generally had much colder subsequent winters than the ones that have an Aleutian high and subsequent trough over western Canada in October. I’m aware of what the longer range guidance hints at to start October up there, but wouldn’t lock it in yet. A continued drop of the QBO heading into the fall is also important for increased high-latitude blocking prospects as we head into winter and a lower risk for the Pacific jet to be too strong/zonal into the west coast, which would likely result in quite a bit of warmth for North America given how mild the entire Pacific is.
An additional caveat is the northern hemisphere water temperatures from the tropics up to the polar regions where there was another near-record sea ice melt are ON FIRE. Do older analogs break down as we continue to warm? If so, how do things change? The warming baseline gives less margin for error (you probably don’t luck into below-average or even average temps, you either have cold signals or you torch).
Lots of food for thought…I definitely don’t hate the prospects for a snowy winter from the Ohio Valley and Great Lakes into the Northeast, and the warm PMM and any remaining warm ENSO influence can bring some snow prospects to the mountains out west, but the pattern can turn warm quickly if we lose the warm equatorial waters closer to the Dateline.
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8/27/19: The Uncertain Case of Where Dorian Goes, how Strong it Gets

Dorian IR

Infrared satellite image of Dorian as of early Tuesday evening

(When dealing with tropical cyclones and other adverse weather, always follow the advice of local officials for your safety)

Dorian, a sputtering Tropical Storm between the Lesser Antilles and Puerto Rico over the eastern Caribbean that has its eyes set to the west-northwest towards Puerto Rico, the Dominican Republic, parts of the Bahamas and eventually parts of the Southeastern U.S.  A lot of uncertainty is involved in both the track and intensity forecast for Dorian over the next several days, meaning potential U.S. impacts range from very little to potentially significant, so let’s try to break it down.

The satellite appearance of Dorian continues to leave something to be desired, with intermittent bursts of deep convection that struggle to maintain long enough for substantial intensification due to very dry air surrounding the storm that continues to get entrained into the circulation.  Outflow is decent especially given the storm’s small size.

Dorian radar

Radar image of Dorian from early Tuesday evening

Dorian was terribly disorganized earlier Tuesday with a significant SW to NE tilt to its vortex…since then, aircraft reconnaissance and eastern Caribbean radar suggest the surface center re-formed north, closer to the more persistent convection.  Radar has shown some formative attempt at some inner core structure with a curved band wrapping partially around the circulation, though for now, the continued intermittent/bursting nature of the convection will likely keep short-term intensification from getting out of control, and current aircraft recon finds that Dorian has remained rather steady (perhaps modestly intensified) since the last plane was in there earlier this afternoon.

Dorian WV

Water vapor imagery from early Tuesday evening showing Dorian, dry air surrounding it, and an upper-low in front of the Tropical Storm

As for what lies ahead in the short term…dry air will continue to dominate the environment around Dorian until it gets north of the Greater Antilles, and shear will increase by Wednesday as Dorian encroaches on an upper-low and enhanced southerly upper-level winds to its west.  This upper-level dropsonde from around 5:30 PM EDT shows a notable amount of shear above 500mb along with a very dry environment near the inner core of Dorian…the shear will only increase more into Wednesday.

Dorian sonde

Upper-level dropsonde near Dorian, showing very dry air (separation between red [temperature] and green [dew point]) and some wind shear (barbs on the right-hand side)

Dorian’s increased organization today may make it slightly more resilient to these potential negative environmental influences, though it’s still a small, fairly weak storm, so there will likely be some weakening Wednesday into Thursday.

Dorian steering

Mean low-mid level flow as of early Tuesday evening

In the short term, Dorian will turn northwesterly as it approaches a fairly large weakness in the sub-tropical ridging to its north.  This northwest turn, along with Dorian’s center reforming farther north early this afternoon, essentially guarantees that Dorian will not track over or even that close to the Dominican Republic…however, a track over or extremely close to Puerto Rico seems very likely now.

Between Dorian’s small size/fairly weak nature…moderate shear and continued dry air intrusions over the next couple of days…and land interaction with Puerto Rico on Wednesday (and perhaps some indirect interaction with Hispaniola), there may very well be a net weakening through late Thursday.  Many global and dynamic hurricane models suggest this.  However, with much less interaction with the very mountainous Hispaniola, some increase in organization since Tuesday morning, and only 24-36 hours of markedly stronger shear, it seems considerably more likely than not (though still not 100% certain) that Dorian makes it north of the Greater Antilles as a tropical cyclone.

GFS 200 96

Forecast upper-level winds over the western Atlantic on the GFS model, valid Saturday morning

Should Dorian survive the next 48 hours, conditions will be considerably more favorable for intensification beginning Friday and through the weekend over the Bahamas.  The upper-low bringing increased shear over the next couple of days is expected to dissipate as an upper-level anti-cyclone develops over Dorian, bringing very light shear.  In addition, multiple outflow channels may develop, with a potential jet streak to the north being particularly concerning.  I find it worrisome that global models are developing this favorable environment before they really deepen Dorian, leading me to believe there’s a good chance this environment isn’t strongly/perhaps incorrectly influenced by Dorian on the models.

Dorian GFS 96 RH

Mid-level relative humidity forecast from the GFS model, valid Saturday morning

In addition to reduced shear, Dorian will find much less dry air north of the Greater Antilles this weekend.  While it may take some time to mix out the already-entrained dry air Friday-Saturday, the much moister ambient environment and lower shear, along with Dorian’s small size, should ensure that the process happens reasonably quickly.

Dorian SSTs

Water temperatures across the western Atlantic Ocean

The waters beneath Dorian will also be quite warm this weekend (and are warm to a sufficient depth).  The combination of a small storm, low shear, strong outflow, a moist environment and plenty warm waters could result in significant intensification starting Friday or Saturday as Dorian tracks northwest towards the northern Bahamas if the system can maintain a closed circulation through Thursday.  A high-end hurricane would be possible if all goes properly, and some recent modeling is pointing towards that solution.  There’s likely a middle ground solution between the (unlikely) total dissipation over the Greater Antilles and the much more troubling “holds together now, major hurricane later” that involves Dorian opening up into a trough over the next couple of days, but re-developing into a tropical cyclone this weekend.  That would likely put some sort of a cap on the future intensity of Dorian as it nears the northern Bahamas and threatens Florida, though still may have impacts if the storm hits land.

It’s ultimately still a little too early to know with much confidence how strong Dorian will be by Sunday or Monday, with that answer becoming clearer by Thursday when we see how Dorian fairs in a more hostile environment with some land interaction starting on Wednesday.  The environment and some modeling over the weekend is alarming, but we should not over-look the shorter-term questions and sound too many alarms yet…a heightened state of awareness in areas potentially affected is warranted though.

Dorian EPS 96

Large-scale weather pattern predicted by the European ensemble Saturday morning

Dorian will continually generally on a northwest track through the end of the work-week.  By Saturday, a trough over the Northeast U.S. will lift out and sub-tropical ridging will build back in over the Southeast, pushing Dorian more “left” and causing a bend back to closer to a westerly than northwesterly motion.

Dorian’s northwest motion in the meantime may take it largely north/east of the Turks and Caicos (perhaps grazing the eastern islands), though the turn back to a closer-to-west motion this weekend may bring it very close to or into the northern Bahamas.

The next item of interest is of course Florida, as the pattern as shown on Saturday above would push the storm towards the eastern Florida coastline as a strengthening storm.  This is where uncertainty increases…it is very likely Dorian at least approaches the Florida peninsula due to the ridge build in to its north to start the weekend, but does it keep going until hitting the coast?

Dorian EPS 144

Large-scale weather pattern predicted by the European ensemble Monday morning

There is still uncertainty regarding the final approach/potential impact to Florida, as a weak, progressive shortwave trough is expected to zip through the Northeast Sunday into Monday, eroding the ridging north of Dorian a little bit.  The exact speed and amplitude of this trough along with Dorian’s location, size, and intensity will determine if this shortwave has much/if any tug on Dorian and potentially allows it to recurve just east of the Florida coast.  The window for this occur is small as the trough zips east and ridging builds back in Tuesday into Wednesday next week.

Dorian EPS N Hem 144

Hemispheric forecast weather pattern from the European model valid Saturday morning

The hemispheric view shows the delicate/uncertain nature of the exact evolution of the fairly flat/progressive shortwave that may open up a weakness in the ridging Sunday/Monday north of Dorian.  The exact amplitude of that shortwave will be influenced heavily by a ridge over the western US (and to some extent by a large trough over eastern Canada).  That ridge out west is influenced by a number of features over the northern Pacific…so a lot can still change.  It’s a narrow window to recurve before hitting Florida, with a number of features causing a lot of uncertainty this far in advance.

Dorian GEFS trend

GFS ensemble pattern forecast “trends” over the last two days’ worth of runs, valid Sunday morning. Reds indicate a trend towards more ridging, blues towards more troughing

The GFS ensembles don’t show an encouraging trend in that regard, with the ensembles trending towards the trough hitting the Pacific NW earlier/stronger, raising heights over much of the northern/central U.S. and likely dampening out our shortwave even more.

So, long story short, Dorian if it holds together the next two days will likely get very close to or hit Florida Sunday or Monday as what could be an intense hurricane.  However, uncertainty over its fate over the next couple of days, along with a very delicate forecast for the pattern steering Dorian near the Florida coast, makes it way too early to make any sort of proclamation about how this story ends.  Needless to say, Florida has a storm to definitely watch closely, with nearby areas such as the Southeast (should it turn north and just miss Florida) and the Gulf Coast (should it not turn north and continue west) also needing to closely monitor Dorian.

Dorian EPS track

European ensemble forecast tracks for Dorian through the next 10 days

The European ensemble tracks show the risk to many areas (and huge uncertainty) well…the storm will make a WNW “turn” towards Florida this weekend (assuming it holds together)…does it keep going west-northwest into the peninsula (and possibly into the Gulf), does it slow down and turn just offshore (possible threatening Georgia or the Carolinas), or is something else on the table?  And how strong is it?  Lots of questions…as is often the case at this distance out not many answers, but plenty of reason to closely watch this one.

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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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