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Tropical Storm Alberto forms off the South Carolina coast

By: Dr. Jeff Masters, 9:41 PM GMT on May 19, 2012

The first named storm of the 2012 Atlantic hurricane season is here. Tropical Storm Alberto formed this afternoon off the coast of South Carolina--a little going-away present for outgoing NHC director Bill Read, who retires on June 1! Alberto has the potential to hit North Carolina as early as Monday, but since the storm is so small, it would only affect a small area of the coast with high winds and heavy rain. Upper level winds out of the southwest are creating a moderate 15 - 20 knots of wind shear over Alberto, 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 to sustain a tropical storm. The system is tangled up with an upper level trough of low pressure, which is pumping cold, dry air into the storm, slowing development. The dry air impinging on Alberto from the southwest can be seen in water vapor satellite loops. Heavy rain showers from Alberto are located about 50 miles offshore of the coast of South Carolina, as seen on Wilmington radar. At times today, 93L has had a cloud-free center resembling an eye on radar, but this was not a true eye.

Figure 1. True color satellite photo of Alberto taken at 1:50 pm EDT Saturday May 19, 2012. Image credit: NASA.

Forecast for Alberto
Rain showers from Alberto are likely to move onshore between Charleston and Wilmington Saturday night and Sunday, bringing 1 - 3 inches of rain to portions of the coast. The storm is too small to cause major flooding problems, particularly since the coast is under moderate to severe drought. Alberto's rains will not be plentiful enough to cause significant drought relief, except perhaps over a small region near the coast, where (and if) the storm makes landfall. Wind shear is expected to remain in the moderate range, 15 - 20 knots, through Monday, which is low enough to allow some slow development. Since the storm is very small, it is highly vulnerable to even a modest increase in wind shear or dry air, which could rapidly disrupt it. Steering currents are weak, and Alberto 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. The moderate wind shear and dry air are likely to keep Alberto below hurricane strength. NHC is giving Alberto a 5 - 10% chance of reaching hurricane strength before dissipating on Thursday as it scoots northeast out to sea.

Figure 2. Late afternoon radar image of Alberto from the Wilmington radar.

Alberto in historical context
Alberto is earliest-forming tropical storm in the Atlantic Basin since Ana in 2003, which formed on April 21. Alberto is one of only three Atlantic tropical storms to form in May in the past 31 years. The others were Tropical Storm Arthur of 2008, and Tropical Storm Arlene of 1981. There was also a subtropical storm, Andrea, that formed in May of 2007. Formation of an early season tropical storm from an old frontal boundary, like occurred with Alberto, is not a harbinger of an active hurricane season--it's more of a random occurrence. Early season storms that form in the Caribbean, though, often signal that a busy hurricane season may occur.

I'll have an update Sunday morning.

Jeff Masters


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