Arlene strengthens as it heads toward the US...

TS Arlene passed over the extreme western tip of Cuba early this morning and has now entered the Gulf of Mexico, still bearing down on the US coast.  A Tropical Storm Warning has been issued for eastern LA over to St. Marks FL, and a Hurricane Watch has been issued for MS, AL, and the western FL panhandle.

The anticipated ridge to the north of the storm is indeed forming, and will steer Arlene slightly west of due north, and decrease the vertical wind shear as it does so.  This is the reason for the Hurricane Watch... it is possible that Arlene could intensify more rapidly as the shear lets up.  Currently, convection is still displaced to the east and north of the low-level center.

At 15Z today, it was located at 24.0N 84.9W and tracking N at 11kts.  Intensity is 50kts and 1000mb.  The forecast is for gradual strengthening, acceleration, and nudging to the NNW.  Landfall is expected Saturday afternoon as a strong tropical storm or even minimal hurricane, and the most likely area to experience the worst conditions still appears to be the MS/AL coasts.

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TD1 becomes TS Arlene...

This morning, a ship near TD1 reported winds high enough to require upgrading TD1 to a Tropical Storm, the first of the season.  Arlene is still over very warm SSTs, but the shear has not relaxed as much as expected and the low-level center is exposed to the southwest.  This is still forecast to lessen in time as a ridge builds to the north of the storm.  The ridge would put Arlene in a more friendly shear environment, and will also be responsible for steering it toward the AL/MS coast.

At 21Z today, TS Arlene was located at 20.2N 84.2W and tracking N at 7kts.  Intensity estimated by satellite (a plane is en route) is 35kts and 1001mb.  It should accelerate and gradually intensify, making landfall on Saturday evening as a strong TS.

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First Tropical Depression forms in western Caribbean...

An Air Force aircraft flew a recon mission into the system this afternoon, reaching it at about 4pm EDT and found a closed circulation, making it TD1.  The low-level center is located at 17.2N 84.0W.  Convection is not very centralized, but outflow aloft is improving in all quadrants.  Initial intensity measured by the aircraft is 25kts and 1004mb.

The vertical wind shear has relaxed to 15kts and is still decreasing as the subtropical jet lifts out of the area.  The SSTs are about 29C there, and the 26C water is roughly 100m deep (26C is the rule-of-thumb threshold for tropical cyclogenesis), so there is ample energy available to it even if it stalls for a bit.  The warm water is much shallower in the Gulf, so once there it must move more quickly to avoid cool upwelling.

The forecast is still for gradual strengthening and drifting north toward the US coastline.  Timing and location are the key questions, but the canned answer I'll give is "it's too early to know for sure".  Most likely is on Saturday between New Orleans and Appalachicola as tropical storm.

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