Tropical Storm Dorian Forms while Invest 98L Struggles Off the SE Coast

5 day cone of uncertainty issued by the NHC for Tropical Storm Dorian. The NHC is forecasting a category one hurricane to form.
5 day cone of uncertainty issued by the NHC for Tropical Storm Dorian. The NHC is forecasting a category one hurricane to form.

At 5pm Saturday afternoon, Dorian was classified as a minimal 35kt (40mph) tropical storm. This classification came only six hours after a tropical depression was declared at 11am. As of 11pm Saturday, 98L was an area of interest (AOI) off the eastern coast of Florida. The NHC has given the system a 70% chance of development by Monday night, and a 90% chance of development by Thursday night. Invest 90L will not develop into a tropical system as the circulation has moved onshore in Eastern Texas.

8-25 TI

Tropical Storm Dorian

It should be said right away that Dorian's future is extremely complicated. Dorian is a small tropical storm that is only about 100 miles across. Small storms like this present a forecasting nightmare due to their variability and rapid weakening/strengthening. If Dorian's circulation becomes unexpectedly entangled with dry air, it could very well weaken significantly (in a short time). On the other hand, if convection is able to persist while wind shear is low, the storm could rapidly intensify. It is a fragile process that is near impossible to predict outside the short term of a few hours.

TRACK

Dorian is currently in the Atlantic Main Development Region (MDR), at 11N 51W. It is forecast to move WNW at 10-15mph until Monday morning where it will likely take a more northwest trajectory. By Tuesday Dorian will increase its forward speed slightly as it passes the Lesser Antilles. After this point model guidance splits due to differences in intensity. Those models indicating a stronger storm show it surviving the poor conditions in the NE Caribbean and tracking the storm further north into Puerto Rico or Eastern Hispaniola. Models showing a weaker storm bring it further south and west into Hispaniola and Jamaica as a weak tropical storm or post tropical trough. Observations on Saturday night show that Dorian is moving slightly faster than expected, and it was further west than expected. This may favor a more southern track close to Barbados. Past day five, uncertainty is extremely high. Assuming Dorian survives the trek across the Greater Antilles and NE Caribbean, it will emerge into a portion of the Atlantic with favorable conditions for intensification. A TUTT (Tropical Upper Tropospheric Trough) will be extended across the SW Atlantic as Dorian approaches. This TUTT acts as a wall of shear that must be overcomed by Dorian in order to remain at its intensity. If Dorian is stronger, it will have an ample anticyclone situated above it to help shove the TUTT into two parts. The AC can act as a protective bubble to save the storm from any damaging wind shear. In this scenario, an upper level low pressure system forms over the North-Central Caribbean and steers Dorian NW. At the same time, a ridge is rebuilding over the Atlantic (the ridge is presumably just been gaped by 98L and a trough a few days before. This ridge will steer Dorian due west, and most likely prevent this storm from landfalling in the US north of Florida. Positioning of the storm when the ridge "bridges over" is essential to where it will track. If it moves further west and is just north of Hispaniola at the time, it will be shoved into Cuba and pushed into the Gulf of Mexico. If the storm is well north of the islands, it may be able to track into the Eastern Seaboard of Florida. Speculation of track after day 7/8 is unnecessary.

At the moment the most likely scenario is a track into Hispaniola.

INTENSITY

Currently the NHC expects gradual intensification to 75mph sustained winds by Tuesday night. After that they expect it peaks slightly stronger before weakening due to land interaction and shear. As I mentioned above, Dorian is a very small and fragile storm, and intensity is difficult to predict. In the short term, Dorian will slowly gain strength as it builds a convective envelope. Models are all over the place with some bringing Dorian up to a Category 4 hurricane in the NE Caribbean, while others disintegrate it before the weekend is over. Conditions on Sunday and Monday will be conductive for growth as there will be close to no shear, slower forward speed, plenty warm waters, increasing OHC, and an abatement of dry air. SHIPS shows up to a 20-30% chance of Rapid Intensification in the coming days.

 

*Subsequent updates will be issued as conditions warrant. Hurricane watches will probably be issued on Sunday*

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