A tropical wave, previously known as Invest 95L, has been slowly moving northwest north of the Greater Antilles over the past several days, and today has been deemed with a likely chance of becoming a tropical depression or storm over the next few days. The National Hurricane Center has since dubbed the system as Potential Tropical Cyclone 09L. The disturbance currently has maximum winds of 30mph and is moving northwest across the Bahamas, which will eventually include Great Abaco and Grand Bahama, both of which were devastated by Hurricane Dorian about 10 days ago. The forecast currently indicates that it’ll move into Florida and then proceed back offshore, but the forecast confidence is not very high at this point. Chances for development are near 80% through the next 5 days. Another system, a tropical wave southwest of Cabo Verde, currently has a medium shot of development and could be a long-range threat for portions of the northern Caribbean by next week.
09L is currently a broad tropical wave centered over the central Bahamas. While there is decent rotation at the surface, no observations currently suggest that it has attained a closed low-level center. It appears as if the pressure center and area of maximum rotation is located near the island of Rum Cay and moving relatively slowly. The convective pattern is slowly improving with time, although there is quite a bit of shear coming from a strong upper-level low over the northeastern Gulf of Mexico. This is causing most of the convection to be forced off towards the northern end of the wave axis. However, this is where vorticity is naturally maximized, so this is part of the reason that the system has been able to organize.
As time goes on, the system will track towards the northwest and gradually organize. The upper-level low currently inducing the southwesterly shear over the disturbance will continue to trek westward across the northern Gulf and gradually weaken as a stronger trough propagates eastward across the continent. This will both reduce wind shear and increase upper-level divergence as air advects east around a weak ridge to the southeast and another upper-level low south of Bermuda. Sea surface temperatures in the area have rebounded up to 29C since Hurricane Dorian stalled near Grand Bahama just over a week ago, which is plenty warm enough to fuel a tropical cyclone at the surface level. The current forecast indicates that 09L becomes a tropical depression during the day tomorrow near Nassau and a tropical storm after that near Grand Bahama by tomorrow night.
Similarly to Dorian, the forecast track confidence is relatively low. Because the system has not yet developed, the models are having a difficult time resolving the track because they all generate surface centers in different locations. The trough currently weakening the upper low over the Gulf will be kicking out from the Dakotas to southeastern Canada. This will allow the ridge to the southeast to expand and steer 09L northwest, possibly into the Florida East Coast by Saturday. However, the storm is then expected to move inland and slow down over or near North Florida by Sunday. This is because a strong, deep-layer ridge will build over most of the CONUS after the trough exits, and another ridge will build over the southwestern Atlantic. These ridges will create competing flows atop the storm that essentially cancel out. The forecast then indicates a curve offshore near Georgia or South Carolina by Monday as both ridges break down from another trough moving through Maritime Canada. This weakness should pull the system offshore, but it may be severely weakened from land interaction. In short, a stronger system is more likely to remain offshore because the tropospheric extent will be more elevated and will feel more of the influence of the current trough near Quebec. Most of these features are well removed from each other, which is another reason why the forecast is so tricky. After it moves offshore, long-range models suggest intensification and a northward turn, but the certainty on this is even lower given the high initial uncertainty. And all of this is assuming the system develops at all. The GFS, which is much weaker with the system, has it tracking across South Florida, into the Gulf of Mexico, and impacting the Gulf Coast as a disturbance or tropical depression. This solution, however, is less likely given recent trends.
Impacts in the Bahamas will be mostly confined to heavy rains and squally weather in general. Tropical Storm Warnings have been issued for Eleuthera, New Providence Island, and unfortunately, Great Abaco and Grand Bahama, islands that have only just begun recovering from the catastrophe that was Hurricane Dorian. While 09L will certainly be nowhere near as bad as Dorian, any type of tropical cyclone impact at this time is certainly unwelcome. As for the southeastern United States, impacts are still uncertain, but are highly dependent on track. Heavy rainfall is likely over portions of Eastern Florida this weekend, though exact totals can vary widely depending on track and intensity. Right now, the WPC calls for 4-6″ of rain across the Bahamas and North Florida. Winds up to tropical storm force are likely across the Northwestern Bahamas and Eastern Florida with stronger gusts. This system is only expected to peak as a moderate tropical storm for now, but this could change if it remains further offshore or is delayed in arrival.
Another tropical wave, currently located in the eastern tropical Atlantic, has also caught the attention of global models. It’s relatively broad and diffuse for now, but models show it gaining intensity as it approaches the Caribbean Sea early next week as conditions become more conducive for development. Because of the long-range time frame for the forecast, confidence is also low. The NHC currently gives it a 40% chance to become a tropical cyclone in the next 5 days. Regardless, interests in the Eastern and Northern Caribbean should keep an eye on this system in case it attempts to develop.
If 09L does gain a name, which appears likely at this point, it’ll be “Humberto”. Interests in both the Bahamas and the southeastern United States should pay attention to this system and re-exercise cautions used during Dorian, even though this will not be quite as strong. Listen to the advice given by your local officials and emergency management agencies for the latest information on this system.