Tropical Storm Leslie continues to move extremely slowly in the west-central Atlantic north of the lesser Antilles. She remains sheared from the west by a deep-layer ridge in the central Caribbean and an upper trough to her northeast, and the low-level center is exposed to the west of the convection. This has been the story with Leslie for days now, but conditions will be improving in the near-future. As the upper trough to her northeast splits and exits, sending a cut-off low southward to the east of Leslie, it should alleviate some of the shear and provide a good ventilation pattern between that upper low to her east and another one to her west over the northern Bahamas. The farther north Leslie moves, the farther away she will be from the Caribbean ridge, which will also relax the shear over her. Cold water upwelling due to Leslie’s slow movement may also be a hindering factor right now, and the storm will continue to move slowly under weak steering currents during the next 3 days, but after that will begin to speed up, and with favorable environmental conditions, should easily become a hurricane eventually. The global models all agree on aggressive deepening. The ECMWF even makes Leslie a Cat 4 near and north of Bermuda, which may be overdone, but illustrates the conducive pattern for strengthening.
Leslie’s track has been a slow one as she has missed one trough and is now stuck south of a ridge passing by to her north, which is causing the weak steering currents. As this moves on, a new trough will dig into the eastern U.S. and allow Leslie to ride northward up the western flank of the subtropical high. This trough is forecasted to cut off into a deep upper low that will not be moving much, and should provide a clear, static steering pattern that should take Leslie around the subtropical high near Bermuda, and eventually into the Canadian Maritimes. Cut-off upper lows over the eastern U.S. can tend to bring hurricanes farther to the west towards New England than usual, but the steering pattern here looks to have the subtropical high too far south to allow a direct hit on New England. However, high surf and fringe effects from the western side of the storm may impact New England during Leslie’s passage. Leslie’s track will likely take her very close to Bermuda, and with nearly 5 whole days of slow movement ahead before she reaches the island, Bermuda should be prepared for a direct hit from a likely strengthening hurricane. The Canadian Maritimes will likely get a direct hit from Leslie afterwards, though she will be moving much quicker by then. This would be the first direct tropical impact on these two places this season.
Elsewhere….Tropical Storm Michael has formed east of Leslie from a baroclinic low turned tropical, and is not a threat to land.
The remnants of Hurricane Isaac are drifting back southward over Alabama, and will be moving back out over the Gulf of Mexico during the next 3 days. The GFS and ECMWF hint at regeneration of this low over the northeastern gulf, which would certainly be a possibility to watch for. With Leslie recurving to the east, the pattern favors this low getting pulled back out of the gulf to the northeast fairly quickly, but it would not be out of the question to see a tropical depression or weak tropical storm develop.
The models continue to show the threat for additional tropical developments from African waves coming westward during the next two weeks as we pass through the climatological peak of the hurricane season.
We shall see what happens!