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Disturbance 93L off South Carolina coast could become a tropical storm

By: Dr. Jeff Masters, 5:11 PM GMT on May 19, 2012

A hybrid low pressure system with both tropical and extratropical characteristics has formed in the waters off the coast of South Carolina, about 120 miles southeast of Myrtle Beach--a little going-away present for outgoing NHC director Bill Read, who retires on June 1! Residents along the South Carolina and North Carolina coast should pay attention to 93L, as it has the potential to strengthen into a tropical storm and hit the coast on Sunday or Monday, with North Carolina at highest risk. NHC designated this system Invest 93L Saturday morning. Wind shear is a moderate 20 knots over 93L, and the storm is over the warm waters of Gulf Stream, which are 81°F (27°C), just above the 26°C threshold usually needed for a tropical storm to form. The system is tangled up with an upper level trough of low pressure, which is pumping cold, dry air into the storm, inhibiting development. Wind shear is expected to remain in the moderate range, 15 - 20 knots, through Monday, which is low enough that 93L has a decent chance of developing into the Atlantic's first tropical depression. Since 93L is very small, it is highly vulnerable to even a modest increase in wind shear or dry air, which could rapidly disrupt it. Conversely, the storm's small size and favorable positioning over the warm waters of the Gulf Stream mean it is also capable of suddenly organizing, and it would not be surprise to see 93L become Tropical Storm Alberto by Sunday. NHC is giving 92L a 50% chance of developing into a tropical depression or tropical storm by Monday. Since 93L is so small, the computer models are having trouble resolving it, and we don't have very good forecasts of the storm right now. Steering currents are weak, and I expect 93L will wander off the coast of South Carolina through Sunday, before getting caught up by a trough of low pressure on Monday which should lift it out to the northeast. Heavy rain showers from 93L are located about 50 miles offshore of the coast of South Carolina, as seen on Wilmington radar. These showers will probably move onshore between Charleston and Wilmington Saturday night and Sunday, bringing 1 - 3 inches of rain to portions of the coast. At times today, 93L has had a cloud-free center resembling an eye on radar, but this was not a true eye, and winds are probably near 35 - 40 mph in the heaviest rain squalls near the center.

Figure 1. Afternoon radar image of 93L from the Wilmington radar.

Jeff Masters


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