91L Near Tropical Depression Status; Entire SE U.S. Coast Could be Impacted

An area of disturbed weather over the warm waters of the Gulf Stream, located about 130 miles east-northeast of Melbourne, Florida on Monday morning (Invest 91L), has grown more organized this morning. Surface pressures are falling, and 91L is close to tropical depression status. Satellite loops on Monday morning showed 91L had a well defined surface circulation, with heavy thunderstorms building and steadily organizing into spiral bands. Long-range radar out of Melbourne, Florida showed these bands were already affecting the coast of Central Florida. Sea surface temperatures in this region were about 1°C above average, 27 - 28°C. The counter-clockwise circulation of an upper level high pressure over Florida was bringing northerly winds over 91L at high altitude, and these winds were creating light wind shear of 5 - 10 knots. However, water vapor satellite loops show that the atmosphere has dried noticeably to the north of 91L since Sunday, and the northerly winds are driving this dry air in the heart of the storm. This dry air is interfering with development and keeping any heavy thunderstorms from developing on the north side of the circulation. The Hurricane Hunters will investigate 91L on Monday afternoon.


Figure 1. Water vapor satellite image of 91L taken at 10:45 am EDT Monday June 30, 2014. A large area of dry air (black colors) was to the north of the system, and was interfering with development. Image credit: NOAA/NESDIS.


Figure 2. Wind forecast for 11 am Thursday, July 3, 2014, as made by the 06Z Monday June 30, 2014 run of the GFS model (left) and 00Z Monday run of the European model (right.) Both models are predicting that 91L will become a tropical storm and threaten the South Carolina coast on Thursday.

Forecast for 91L
Steering currents are weak off of the Southeast U.S. coast, but the models are in good agreement on the track of 91L. The disturbance should continue a slow southward to southwesterly motion on Monday, which would bring the storm very close to the coast of Florida by Tuesday. The system is expected to meander near the coast of Florida on Tuesday and Wednesday, before a trough of low pressure to the north begins pulling the system to the north and northeast on Thursday and Friday. Heavy rains of at least 2 - 4" will likely affect the Northwest Bahamas and eastern coast of Florida Monday through Wednesday. Heavier rains of 4 - 8" are likely, since I expect 91L to develop into a tropical depression on Monday or Tuesday. Heavy rains of 2 - 4" will spread to coastal Georgia, South Carolina, and North Carolina on Thursday and Friday, and tropical storm conditions are possible along the South Carolina and North Carolina coasts on Thursday and Friday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 91L 2-day and 5-day development odds of 60% and 80%, respectively. The 12Z Monday run of the SHIPS model shows the atmosphere surrounding 91L will get even drier this week, with the wind shear staying light to moderate, 5 - 15 knots. This dry air, in combination with potentially moderate wind shear, will slow development of 91L. If 91L makes landfall over Florida on Tuesday or Wednesday, interaction with land will also interfere with development. However, the storm will be over the warm waters of the Gulf Stream when the center is offshore, and we should not dismiss the possibility that 91L could reach Category 1 hurricane strength late this week, as some of the members of the 00Z Monday morning European model ensemble were suggesting. It is more likely, though, that 91L will struggle with land interaction, dry air and wind shear, and be at worst a medium-strength tropical storm named Arthur with 50 - 55 mph winds as it brushes the coasts of South Carolina and North Carolina on Thursday and Friday. There is a lot of uncertainty with this forecast, so stay tuned.

Jeff Masters