91L Growing More Organized; Will Bring Heavy Rains to Florida and the Bahamas

An area of disturbed weather over the warm waters of the Gulf Stream, about 230 miles east of Jacksonville, Florida (Invest 91L) has grown more organized since Saturday, and is a threat to develop into a tropical depression early this week. Satellite loops on Sunday morning showed 91L with only a modest amount of heavy thunderstorm activity, but the system had a pronounced spin, and the heavy thunderstorms were organizing into spiral bands. Long-range radar out of Melbourne, Florida showed two of these bands about 75 - 150 miles east of the Central Florida coast. Sea surface temperatures in this region were about 1°C above average, 27 - 28°C--plenty of heat energy for a developing tropical cyclone. Wind shear was light, 5 - 10 knots, but was enough to keep any heavy thunderstorms from developing on the north side of 91L. Water vapor satellite loops showed some modest patches of dry air to the north of 91L, and this dry air was retarding development on Sunday morning. The Hurricane Hunters are on call to investigate 91L on Sunday afternoon.


Figure 1. MODIS satellite image of 91L, taken at approximately 18:30 UTC (2:30 pm EDT) on Sunday, June 29, 2014. Image credit: NASA.

Forecast for 91L
The 12Z Sunday run of the SHIPS model showed shear rising to the moderate range, 10 - 15 knots, beginning on Monday, and remaining moderate until the end of the week. There is a significant area of dry air to the north of 91L over North Carolina that may work its way south and get wrapped into its circulation on Monday and Tuesday, but this dry air should diminish on Wednesday. With the disturbance parked over the warm waters of the Gulf Stream, organization into a tropical depression is a good possibility. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 91L 2-day and 5-day development odds of 40% and 70%, respectively. As of 11am EDT, the appearance of 91L on satellite images had improved to the point where I'd put the 2-day odds of development at 50%. Steering currents are weak off of the Southeast U.S. coast. The models all predict a slow southward drift on Sunday, followed by a southwesterly motion on Monday, which would bring the storm very close to the coast of Florida. The 06Z Sunday run of the GFS model has 91L making landfall over Florida on Tuesday, while the 00Z UKMET and European models stall the storm offshore, then accelerate it to the northeast later in the week, caught by a trough of low pressure to the north. Regardless of the exact track of 91L, the coast of Central Florida and the Northwest Bahamas are likely to receive heavy rains of at least 2 - 4" on Monday and Tuesday from 91L. If 91L develops into a tropical depression or tropical storm, widespread rainfall amounts of 4 - 8" will likely fall in coastal Central Florida and the Northwest Bahamas. Heavy rains are a potential threat for the coasts of Northern Florida, Georgia, South Carolina, and North Carolina late in the week, but there is high uncertainty in this possibility.

Jeff Masters