Tag Archives: Georgia

Subtropical Storm Ana Forms Off Southeast U.S. Coast

Thunderstorms developed closer to a broad area of low pressure off the southeast U.S. coast and the National Hurricane Center (NHC) classified it as Subtropical Storm Ana.  At 11:00 p.m. EDT on Thursday the center of Subtropical Storm Ana was located at latitude 31.5°N and longitude 77.6°E which put it about 250 miles east of Savannah, Georgia, about 180 miles southeast of Charleston, South Carolina and about 200 miles south of Wilmington, North Carolina.  Ana was moving toward the north at 2 m.p.h.  The maximum sustained wind speed was 45 m.p.h. and there were gusts to 60 m.p.h.  The minimum surface pressure was 1004 mb.  A Tropical Storm Watch has been issued for the portion of the coast from Edisto Beach, South Carolina to Cape Lookout, North Carolina.

More thunderstorms are being generated near the center of circulation, but the strongest winds are about 90 miles from the center.  So, the structure does not match a classical tropical cyclone where the strongest winds are closer to the center and NHC classified it as a subtropical storm.  The center of circulation is over Sea Surface Temperatures (SSTs) near 26°C which are warm enough to support the development of a tropical cyclone.  So, Ana could make a transition to a tropical storm during the next 24 to 48 hours.  The upper level winds are lighter and vertical wind shear has decreased.  However, because of the proximity to the coast some drier air is being pulled around the western side of the circulation.  Only modest intensification is expected due to marginal SSTs, some vertical wind shear and drier air.

A high pressure system north of Ana is likely to limit the motion of Ana for the next day or two.  Eventually, the high will shift eastward and an upper level trough approaching from the west will accelerate Ana toward the northeast.  Persistent northeasterly winds along the coast could create problems with beach erosion.  The slow movement also means some locations could receive locally heavy rainfall.

 

Possible Development of Low Pressure East of Florida on Wednesday

A number of numerical models have been predicting the development of some type of low pressure system east of Florida this week.  The surface pressure has decreased by about 4 mb during the past 24 hours at several locations in the northern Bahamas, which could be an early sign that a surface low is in the formative stages.  When the low forms, it will likely be classified as either an extratropical cyclone (a typical mid-latitude low with a cold core) or a subtropical cyclone (a type of hybrid low with some tropical characteristics).  An upper level trough over the eastern Gulf of Mexico is generating strong southwesterly winds across the region.  At the same time a surface high pressure system north of the area is producing northeasterly winds in the lower levels.  The combination of southwesterly winds in the upper levels and northeasterly winds in the lower levels means the vertical wind shear over the likely development region is very high.  Since tropical cyclones form in regions of little vertical wind shear, the low is not likely to be a tropical cyclone when it develops.

However, the surface high north of the system is likely to inhibit the northward motion of the low.  If the low moves slowly while it is over warm Sea Surface Temperatures, the thunderstorms in the circulation could release enough latent energy to warm the upper levels of the circulation.  If, at the same time, an upper level ridge over the Central U.S. moves eastward and causes the upper level wind speed to diminish, then the wind shear over the low could be reduced.  Less shear and warmer temperatures in the upper levels could cause the structure of the low to taken on a more tropical form.  It is possible that it could make a transition to a tropical cyclone if the environment is just right.

The forecast track of the low is uncertain because it is unknown precisely where the low will form initially.  It does appear that after the low forms it will move northward slowly for several days.  The expected track could bring the low near the Mid-Atlantic Coast by the end of the week.  Given the Sea Surface Temperatures, the wind shear, and the time of year, the low is likely to be be of tropical storm strength by the end of the week.  However, both the track and intensity forecasts will be highly uncertain until the low pressure system organizes.  Even if the center of the low stays off the coast, northeasterly winds at the surface could generate some beach erosion.