Tough Road Ahead for Tropical Storm Erika; Still “Something to Watch” for Bahamas, Florida, Southeast and Gulf Coast

General Discussion:

Remember to consult the National Hurricane Center, your local National Weather Service Office, and heed advice from local emergency management officials when making decisions regarding Tropical Storms/Hurricanes.

National Hurricane Center: www.nhc.noaa.gov

National Weather Service: www.weather.gov

Erika NHC

Tropical Storm Erika is currently located a few hundred miles east of the Leeward Islands and is rapidly moving to slightly north of due west at a brisk speed of 18MPH. Erika is currently poorly organized and is a minimal Tropical Storm with sustained wind speeds of up to 40MPH. Due to marginal to at times unfavorably strong wind shear and dry air surrounding Erika, minimal intensification is expected through the remainder of the work-week…however, Erika may pass close to or over the Leeward Islands, the US Virgin Islands, Puerto Rico and Hispaniola between Wednesday and Friday as a Tropical Storm packing heavy rains and breezy conditions as per the National Hurricane Center forecast above.

Forecast uncertainty increases dramatically by Friday and into this weekend, however it appears as though there is high potential for Erika to pass near or over Hispaniola/Cuba or into the general vicinity of the Bahamas this weekend. The intensity of Erika by later this weekend is highly uncertain, but some strengthening is possible. There is some implied threat to Florida based on this forecast scenario, however it needs to be stressed that the forecast track and intensity of Erika this weekend into early next week are highly uncertain. Interests in the Bahamas, Florida, Gulf Coast and the Southeast are encouraged to monitor the progress of Erika this week and into this weekend.

Meteorological Discussion:

Erika s-w ir

Recent satellite loops of Tropical Storm Erika feature a swirl of low clouds associated with the low-level center of circulation, with this swirl of low clouds suggesting that the center is partially exposed to the northwest of the convection associated with the storm. In addition, convection has really been “pulsing” (IE, not sustained, large bursts) and hasn’t been very strong. These are all signs that Erika is battling wind shear and a somewhat dry/stable environment.

Erika microwave

A reasonably recent microwave pass confirms what convectional satellite imagery already suggested; convection with Erika is not very robust/well organized, consisting of one decent convective cell removed a good distance from the low-level center. The convection consistently firing well south/east of the surface circulation associated with Erika suggests the circulation is tilted with height due to the wind shear. Although it is not impossible for tilted/sheared tropical cyclones to intensify, the current organization of Erika suggests that it is very likely that any intensification in the near-term will be slow if any occurs at all.

Erika WV

A water vapor loop of much of the Tropical Atlantic suggests that the environment ahead of Erika likely won’t improve particularly quickly; a large and persistent TUTT (Tropical Upper Troposphere Trough) is located to the north-northeast of Erika over the central Atlantic, with a clear axis of shear (note how moisture on the loop appears to be moving west-east from Danny’s remnants over PR/Hispaniola towards the TUTT) located north of Erika. The shear axis associated with the TUTT is still north of Erika and may even be thinning/weakening some, however, upper level winds derived from satellite data show very light upper level winds ahead of Erika due to a fairly robust upper level anti-cyclone:

Erika UL winds

This may suggest on first glance that Erika will move into an area of weaker shear soon; however, Erika is embedded within a fairly strong surge of easterly trade winds (and is hence moving at 20MPH to the west); if you have strong easterly surface winds and calm upper level winds, there is still effective westerly shear affecting the storm, and the vortex will still likely be tilted as it is:

Erika LL winds

In addition, although Danny did erode some of the very dry air previously present in the path of Erika, there is still a reasonable amount of dry air ahead of Erika. The amount of dry air on its own likely wouldn’t prohibit intensification, however wind shear resulting in a tilted vortex makes dry air entrainment more likely into Erika, meaning its convection will likely continue to pulse and be generally removed from its center for until it can possibly move into an area of weaker shear in a few days.

Erika GFS 30

As for when Erika may move into weaker shear, that’s a tough call. By Wednesday evening, the Monday 18z GFS run shows Erika (the ball of 850mb vorticity approaching the Leewards) still under the upper level anti-cyclone per the 250mb wind barbs; however, a look at the lower level winds still shows Erika embedded in a rather strong surge of easterly trades:

Erika GFS 30 850

This suggests that the lower portions of Erika will still be attempting to outrun the mid-upper level portions of the vortex, meaning the vortex will likely still be tilted and the storm won’t be much if any more organized that it is now.

Erika GFS 54

Unfortunately for hurricane fanatics, a similar setup continues through the day Thursday, with weak upper level winds over Erika per the GFS (over or just north of PR on the model Thursday evening) thanks to the (shrinking) upper level anticyclone, but continued strong easterly low level winds, resulting in continued moderate to strong shear over the storm:

Erika GFS 54 shear

Monday’s 12z ECM run is in fair agreement with the GFS in terms of track of Erika through the next 2-3 days (across the northern Leewards towards PR and possibly Hispaniola) and environment; by Friday morning, the Euro has (what’s left of) Erika approaching eastern Hispaniola, with moderate shear over and in front of the storm:

Erika ECM 72

The GFS and Euro do however agree in an improving environment in the general vicinity of Erika in the day 4-5 timeframe as it moves through the general vicinity of Cuba or the Bahamas:

Erika ECM 96

By Saturday morning, the Euro has Erika moving over or just north of Cuba; the environment surrounding it isn’t particularly moist, however, the easterly low level winds will be weaker in this region by the time Erika gets there, and the storm (or what’s left of it) will be completely through the shear caused by the TUTT axis. The GFS has Erika in a very similar position by Saturday morning, and is beginning to weaken the shear as well:

Erika GFS 90

However, it is important to note the 18z GFS only has ONE loosely closed isobar (1014mb) and the 12z Euro has NO closed isobars with Erika as it passes near or over Cuba on the models on Saturday; both models open Erika up into an open wave by Friday at the latest, similar to what occurred with Danny between the Leewards and PR:

Erika GFS 90 surface

Before we start speculating further about future intensity and looking into the track of Erika more, let’s discuss what I think will or may happen with Erika’s intensity over the next 2-3 days as it tracks across the Leewards and towards PR/Hispaniola:

  • The environment ahead of Erika as it moves across the far NE Caribbean will be slightly less hostile than what was ahead of Danny, but not by much. The dry air in the atmosphere may have been worked out a little bit by Danny’s small moisture pouch as it moved by, the TUTT axis may slowly be weakening/shifting east, and the fairly robust upper level anti-cyclone over Erika may shield the storm some from the TUTT’s westerly shear.
  • However, there is good reason to believe that moderate shear (at least 20 knots) and fairly dry/stable air will plague Erika through Friday before potentially relaxing Saturday; that is another 72 hours of rather hostile conditions.

Erika TPW loop

-With that said, Erika has a larger circulation and a larger moisture pouch than Danny. This may make it less likely to open up into a trough of low pressure over the next few days of hostile conditions.

  • At their current longitude, per the TPW loop above, Erika has a more northerly component of motion than Danny did. This may make it less likely to get caught up in the Greater Antilles, but I will examine track more in a minute.
  • Erika is in my opinion a very marginal tropical cyclone at this time. Much more degeneration could result in the NHC declassifying it due to a lack of sustained convection near the center, or possibly cause the center to open up. Given the less than favorable environment, this could happen at any time over the next 72 hours.

Overall, Erika likely has a better chance of surviving the hostile next 3-4 days and emerging in a potentially better environment than Danny did due to a slightly less hostile environment and larger circulation. I’d put the odds of Erika surviving till Saturday as a tropical cyclone at 60% at this time. I’m going with a little better than 50/50 because the environment ahead of Erika doesn’t get much worse than it is now, and because the current environment while not fostering intensification has at least maintained Erika as a tropical cyclone today. One potentially valid concern is Erika getting caught up in the mountainous terrain of the Great Antilles this weekend, which I’ll discuss below.

More Specific Track Forecast and Longer Term Speculation:

Erika current steering

Currently Erika is being steering by sub-tropical ridging across the central and eastern Atlantic. There is a bit of a weakness in the ridging just north of Erika and a strong ridge centered east-northeast of the storm, allowing for some northerly component of motion despite a ridge being centered north-northwest of Erika. The ridge NNW of Erika appears to be slowly moving west, which will likely allow Erika’s current motion to continue in the near term. This motion continuing indefinitely would actually take Erika just north of Hispaniola around Friday:

Erika motion

Given I expect Erika to at best maintain status quo intensity for the next 72 hours, we can use the low level height/wind forecasts on the models to estimate its motion to see if it will change from the magical track above that would keep Erika just barely north of mountainous death:

Erika GFS GIF

When looking at the GFS, which opens Erika up into a trough of low pressure fairly quickly and does track it over the spine of Hispaniola and Cuba or close to it, the steering currents don’t appear to change too drastically over the next 3 days, and then appear to begin favoring a more northerly motion by the end of day 3. Although I can’t post images of it here, the Euro steering currents aren’t much different. Although Erika moves WNW towards a ridge, a cold front dropping south across the central and eastern Atlantic forces the ridging westward, meaning Erika won’t actually be running right into it…in addition, a trough slowly drifting east over the eastern US begins to impinge upon the ridge by Friday, allowing the ridge to take on a more NW-SE orientation, giving Erika more room to gain a more northerly component of motion.

Most track models and GFS ensemble members agree with the motion remaining close to its present motion over the next 3 days:

Erika GEFS

Erika ATCF

Overall, I’m conservatively giving the nod to Erika staying just north of Hispaniola and Cuba. It could pass over PR, although PR isn’t large or mountainous enough to disrupt a tropical cyclone’s circulation. The ATCF track models (second image of tracks) have actually shifted Erika a little north over the course of today’s runs, however it is important to note that the GFS ensembles have trended a little south. The GFS does skim northern Hispaniola with Erika and has for a few runs. The 12z Euro appears to be slightly north of the recent GFS runs but not by much.

I believe that the GFS tracks Erika farther south because it opens it up into a wave quicker and allows it to get caught in the easterly trades and track farther south (like what happened with Danny). Given I think Erika can potentially try to at least maintain itself as a marginal tropical cyclone for the next few days, I feel it has a much better chance of missing Hispaniola and Cuba than Danny as if it maintains itself, the modelled steering currents suggest Erika staying on a similar heading, which would take it just north of Hispaniola’s and Cuba’s mountains.

If Erika opens up into a tropical wave and tracks farther south, a track over so many mountains while dealing with a marginal at best environment could well kill the system for good (see Danny); if something forces the ridging ahead of Erika to be a bit stronger, it could still hit the mountains even if it maintains itself as a tropical cyclone and then degenerate into an open wave. So, my 60% probability as laid out above is a conservative estimate based on these several sources of uncertainty, and if anything is more conservative than the NHC forecast due to the GFS and 12z Euro runs showing more substantial interaction with the higher terrain of Hispaniola and Cuba and degenerating Erika into an open wave. Although I don’t have access to the individual members, mean maps of the 12z Euro ensembles suggest that many members have an evolution very close to the 12z Euro with Erika degenerating into an open wave before getting to Hispaniola, skirting Hispaniola and barely missing Cuba before the north. It should be noted that the GFS and Euro and many ensemble members do still bring what’s left of Erika into the eastern Gulf of Mexico by Monday, however if Erika tracked over that much land it may be too disheveled to spin up into anything strong in the Gulf.

Erika TCHP

As for the potential intensification of Erika down the road if it can survive and find a lower shear environment, the waters off the Florida coast, over the Bahamas and into the eastern Gulf of Mexico are extremely warm and warm to a deep depth. Any tropical cyclone moving through this area would have no shortage of heat to work with from the ocean.

The GFS and Euro, as shown above, both show Erika emerging into a lower shear environment in the 72-96 hour timeframe (Saturday-ish) as it finally passes the TUTT axis and heads towards either the Florida Straits or Bahamas. Both models maintain the low shear look as they move Erika into the eastern Gulf by Monday:

Erika GFS 120

Erika Euro 144

The GFS is very sheepish in terms of re-intensifying Erika early next week near Florida or over the Gulf, but the 12z Euro did try to spin up a weak tropical cyclone over the eastern Gulf early next week. The HWRF and GFDL both maintain Erika as a marginal tropical cyclone through Saturday and then go bananas over the Bahamas Sunday into Monday and attempt to develop Erika into a major hurricane off the Florida coast. The SHIPS and LGEM intensity models are more middle of the road and show an intensifying tropical storm in five days.

At this point, I see a few scenarios in terms of the intensity forecast, in no particular order:

  • Erika maintains as a tropical cyclone north of the Greater Antilles, but the environment doesn’t improve over the Bahamas as much as the GFS and Euro currently imply and we see a tropical storm possibly threatening Florida Sunday-Monday.
  • Erika maintains as a tropical cyclone and the GFS and Euro are correct in showing a low shear, good outflow environment over the Bahamas late this weekend along with very high heat content water, and Erika intensifies into a hurricane Sunday into early next week.
  • Erika degenerates into an open wave either due to the environment or due to ridging being stronger than anticipated at pushing it into Hispaniola/Cuba, and then either doesn’t redevelop over the Gulf or perhaps spins up into a weak tropical storm in the Gulf early to mid next week.

So, the intensity forecast is tied into the track forecast some, as I believe any chance for a stronger system comes off the table if Erika gets caught up in the Greater Antilles and completely loses its surface circulation.

I’m fairly sure that Erika won’t recurve completely east of the US at this point due to the pattern that will develop late this weekend into early next week:

Erika ECM ENS 120

The Euro ensembles (shown) and GFS ensembles from Monday afternoon are in reasonable agreement on a strongly –PNA and modestly negative NAO pattern late this weekend into early next week. The strong –PNA forces a ridge over the central US and essentially renders the polar jet useless east of the Rockies, meaning there won’t be a strong trough over the eastern US. The modest –NAO attempts to carve out a trough over the NE Atlantic, but the –PNA really dominates things and doesn’t allow the trough to dip far enough south to pick up a tropical cyclone over the Bahamas. This suggests that Erika will likely end up over Florida or the Gulf early next week, however at what intensity is in question.

Earlier models tried intensifying Erika quicker this weekend and recurving it east of the US before the –PNA really starting building ridging over the central and eastern US, however due to the hostile conditions Erika will face over the next few days I see that possibility as a remote outlier.

So, to summarize:

-Erika will struggle through Friday. If Erika misses the Greater Antilles and survives until Saturday or Sunday, the environment may favor intensification.

-The pattern supports a track towards Florida or into the Gulf, but given the first bullet and all the uncertainty discussed above, at what intensity this occurs at is in question. We could see anything from an open wave that doesn’t re-develop in the Gulf to potentially a hurricane that threatens Florida or the Gulf.

Interests in the Southeast US and along the Gulf Coast should monitor the evolution of Erika regardless!

Erika scenarios

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