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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