TD 3 Forms in SE Gulf; Tropical Storm Warnings on Florida Gulf Coast

A large swath of coastline from the Florida panhandle to the state’s west coast was placed under a tropical storm warning on Sunday morning by the National Hurricane Center (NHC) with the commencement of advisories for newly declared Tropical Depression 3, expected to become Tropical Storm Colin by Sunday night. The tropical storm warning extends from Indian Pass (southeast of Panama City) to Englewood (between Tampa and Fort Myers). As of 11 AM EDT Sunday, the center of circulation for TD 3 was located just off the north coast of Mexico’s Yucatan Peninsula, or about 550 miles southwest of Tampa, FL. Virtually all of the shower and thunderstorm activity (convection) associated with TD 3 was positioned more than 100 miles east of this center, over the very warm waters of the Yucatan Channel, the extreme southeast Gulf of Mexico, and the northwest Caribbean (see Figure 1 below). The convection was broadening and intensifying on Sunday, suggesting that TD 3 is approaching tropical storm strength.


Figure 1. Visible satellite image of TD 3.


Figure 2. WU tracking map for Tropical Depression 3 based on the forecast issued by NOAA/NHC at 11 AM EDT Sunday, June 5, 2016.

The wide extent of TD 3’s convection could have set the stage for a large and powerful hurricane if the system had a long time to organize under favorable conditions. However, larger tropical depressions tend to take longer than smaller ones to consolidate their energy, and TD 3 will not have a great deal of time before it moves inland. Models have been quite consistent in making TD 3 a weak or moderate tropical storm prior to landfall, including HWRF, the top-performing intensity model of 2015.

A broad southwest flow at upper levels will channel TD 3 toward the upper west coast of Florida, with landfall possible as soon as Monday evening, perhaps in the sparsely populated Big Bend section of Florida’s coast. There is high confidence on this general track, with the large-scale features driving it already in place and model guidance holding firm. The large southward extent of the tropical storm warning reflects the storm’s marked asymmetry, which will keep the heaviest thunderstorms and the highest winds well to the east of the center. There is a chance that the low-level center of circulation of TD 3 could reorganize beneath the heaviest convection, which might result in a landfall closer to the southern part of the warning area. However, any such shift would result in a shorter track over water, which would give TD 3 even less time to organize over the Gulf. Hurricane hunters from the U.S. Air Force were en route to TD 3 early Sunday afternoon; the data they collect should give us a better sense of TD 3’s structure and its potential for strengthening.


Figure 3. Infrared NOAA GOES image of TD 3 as of 1600Z (noon EDT) Sunday, June 5, 2016. Image credit: NASA/MSFC Earth Science Office.

Potential impacts in Florida
High tides on Florida’s west coast are at their highest values of the month because of this weekend’s new moon, so even a weak Tropical Storm Colin could produce noticeable storm surge flooding. NHC is predicting that water heights at high tide along the immediate coast could reach 1 to 3 feet above ground level from Indian Pass south to Tampa Bay and 1 to 2 feet above ground level from Tampa Bay south to Florida Bay.

Regardless of its exact track and strength, TD 3 will bring very heavy rains across much of Florida, especially on Sunday night and Monday. There is some risk of nocturnal tornadoes over South Florida on Sunday night, as southeasterly low-level flow is overtopped by stronger southwest flow at upper levels. Here and throughout the state, convection will be fueled by a swath of extremely rich tropical moisture moving north from the Caribbean. On Monday, the bulk of the Florida peninsula will be swaddled by an air mass with precipitable water between 2.00” and 3.00”. These PW amounts could match or exceed record values for late spring and early summer at several locations. (Precipitable water, or PW, is the amount of moisture in the atmosphere in the form of water vapor above a given point.) Rainfall amounts are likely to be in the 4” - 6” range on the right-hand side of TD 3’s track across central and/or northern Florida, with isolated 6” - 10” amounts possible.


Figure 4. Projected rainfall for the period 8 AM EDT Sunday, June 5, 2016, through 8 AM Wednesday, June 8. Image credit: NOAA/NWS.

After the Florida landfall, what next?
TD 3 may reach its maximum strength after it traverses Florida rather than beforehand. Nearly all models are predicting that TD 3 will restrengthen after it emerges off the east coast of Florida, so we could be dealing with a Tropical Storm Colin roughly paralleling the southeast U.S. coast on Tuesday. Here again, the lopsided structure of TD 3 will tend to keep the heaviest thunderstorms and strongest winds on the offshore (southeast) side of the center, which would minimize any potential impact from Georgia to North Carolina even if the center were to hug the coastline. Steering flow around a large eastern U.S. trough means there is very high confidence in the overall northeastward direction of motion. By Wednesday, the system is expected to be shooting northeastward between Bermuda and Nova Scotia as it rapidly evolves into a post-tropical storm.

I’ll be back with a brief update on Sunday night should TD 3 be upgraded to Tropical Storm Colin, with our next full update midday Monday. One other note: residents of the mid-Atlantic should be on alert for potentially severe thunderstorms on Sunday afternoon and evening. The NOAA Storm Prediction Center has an enhanced risk of severe weather (see map below) extending from central North Carolina to central New Jersey, including the Washington, D.C., and Philadelphia metro areas. Damaging winds and a few tornadoes are possible with the strongest storms.

Bob Henson