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Alberto weakens, brings light rains to Georgia and South Carolina

By: Dr. Jeff Masters, 3:31 PM GMT on May 20, 2012

Tropical Storm Alberto continues to move slowly to the west-southwest off the coast of South Carolina, and is bringing light rains to the coasts northern Georgia and southern South Carolina. Recent radar and satellite loops show that Alberto has weakened late this morning, and has lost most of its heavy thunderstorms. Upper level winds out of the west-southwest are creating a moderate 15 - 20 knots of wind shear over Alberto, and these winds are driving dry air into the storm, which has caused it to deteriorate. The dry air impinging on Alberto can be seen in water vapor satellite loops. Yesterday, Alberto was over the warm 81°F (27°C) water of the Gulf Stream, but today, the storm has moved west of the Gulf Stream, and is now centered over cooler waters of 79°F (26°C). This gives the storm a lot less energy to power it, and as Alberto drifts farther to the west-southwest today, ocean temperatures will get even cooler. A hurricane hunter aircraft is scheduled to visit Alberto this afternoon.

Figure 1. Late morning visible satellite image of Alberto.

Forecast for Alberto
Sporadic rain showers from Alberto are likely to affect the Georgia and South Carolina coasts today, and the coasts of North Carolina and South Carolina on Monday. The heaviest rains and tropical storm-force winds should remain offshore, but even if Alberto did make landfall, the storm is too small to cause major flooding problems, particularly since the coast is under moderate to severe drought. Alberto's rains will be generally less than an inch over land areas, which will not be plentiful enough to cause significant drought relief. Wind shear is expected to increase to the high range, 30 - 40 knots, tonight through Monday, which should be able to rapidly disrupt a storm as small as Alberto. Steering currents are weak, and Alberto will wander off the coast of South Carolina through Monday morning, before getting caught up by a trough of low pressure on Monday night which should lift the storm out to the northeast. NHC is giving Alberto a less than 5% chance of reaching hurricane strength. Alberto should cause little or no damage to the coast, except perhaps for some coastal erosion due to high waves.

Figure 2. Late morning total rainfall image of Alberto from the Charleston, SC radar. Alberto's rains have been less than one inch along the coast, and most of the rain has fallen offshore.

Figure 3. Late morning radar image of Alberto from the Charleston, SC 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 Monday morning.

Jeff Masters


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