PTC 7 Predicted to Cross Gulf, Reach Coast as Tropical Storm

A disturbance just north of Cuba, christened Potential Tropical Cyclone 7 by the National Hurricane Center (NHC), is predicted to become a tropical depression by Monday and strike the upper Gulf Coast as a tropical storm by Tuesday night. A Tropical Storm Watch has been hoisted from Morgan City, Louisiana, east to the Alabama/Florida border, including Lake Pontchartrain and Lake Maurepas.

As of 5 pm EDT Sunday, newly christened PTC 7 was centered about 100 miles north-northeast of Camaguey, Cuba, and about 275 miles east-southeast of Marathon in the Florida Keys. Top sustained winds were 30 mph, mainly on the north side of the disturbance.

The PTC designation, which was introduced by NHC in 2017, is intended to highlight systems that are not yet tropical cyclones but that could intensify to produce tropical-storm impacts within the next 48 hours. In this case, computer models have come into increasing agreement that PTC 7 will intensify as it enters the eastern Gulf, and they consistently show a west-northwest track that would bring the system into the central Gulf Coast around Tuesday night or early Wednesday. The GFS and European model ensembles from 12Z Sunday are tightly clustered around a projected landfall in southeast Louisiana on Tuesday night. The exact landfall location could shift west or east depending on how PTC 7 evolves as a tropical cyclone, but huge swings are unlikely given the well-defined steering flow.

Figure 1. NHC forecast for PTC 7 as of 5 pm EDT Sunday, September 2, 2018. Image credit: NOAA/NWS/NHC.

Not yet TS Gordon, but heading that direction

As of Sunday afternoon, PTC 7 was still a disorganized system. An expanding field of showers and thunderstorms (convection) extended mainly to the north and east of PTC 7’s ill-defined center, with extensive thunderstorms across Cuba. PTC 7's circulation was somewhat more evident aloft than at the surface, where no closed center was evident in ASCAT scatterometer data. We are fortunate PTC 7 is not yet a tropical depression or tropical storm, because it will be passing over the Florida Straits—one of the most notorious breeding grounds for fierce hurricanes, including the 1935 Labor Day hurricane, the strongest ever recorded in the United States. PTC 7’s circulation is expected to become more coherent as it approaches the straits on Sunday night, boosted by the typical nighttime peak in convection for tropical systems. By the time PTC 7 crosses the Florida Keys on Monday afternoon, it will likely be a tropical depression or minimal tropical storm (although no tropical storm watches or warnings were in effect for the Keys as of 5 pm EDT Sunday).  The next name on the Atlantic list is Gordon.

PTC 7 will be passing near an oceanic eddy with high ocean heat content just west of the Florida Keys on Monday; it will encounter a cold eddy Monday night, followed by another warm eddy on Tuesday. Overall, the amount of heat content in the Gulf is at near-record levels, so PTC 7 should have plenty of oceanic fuel to work with.

Figure 2. A comparison of Ocean Heat Content (OHC) levels in the Gulf of Mexico in late August in 2018 (top) compared to 2005. There are two Loop Current eddies in the Gulf this year, compared to one in 2005. There was more OHC in 2018 compared to 2005, though the Loop Current eddy that fueled Hurricane Katrina was slightly larger with more heat energy than the main 2018 Loop Current eddy. Note also the presence of a pronounced cold eddy in 2018; any hurricane passing over this cold eddy will see a sudden reduction in the heat energy available to it. Image credit: University of Miami Rosenstiel School of Marine and Atmospheric Science.

Steering currents will keep PTC 7/Gordon moving at a brisk pace: it’s expected to spend less than 36 hours over water between the Keys and the central Gulf Coast. Sea surface temperatures along its path will be a very warm 30°C (86°F), and wind shear is predicted to remain light to moderate (around 5 – 10 knots) by the 18Z Sunday run of the SHIPS model. Factoring in these favorable conditions as well as the brief time over water, NHC is predicting that PTC 7 will reach southeast Louisiana as a strong tropical storm. If PTC 7 organizes a bit more quickly than expected by Monday, then a landfall as a Category 1 hurricane can’t be ruled out. More likely, PTC 7 will bring tropical-storm-strength winds near and just east of its center.

Rainfall the main threat from PTC 7

Wind is not the main threat from this system—heavy rain is. The coast of Louisiana and southeast Texas is being primed for water problems by a tropical wave now moving west near the coastal TX/LA border. Rains totaled 2” – 4” on Saturday night in the Galveston, TX, area, and high-resolution regional models indicate that another round of noctural convection tonight into early Monday could produce localized amounts in the ballpark of 5” across far southeast TX and southwest LA (the same area slammed by Hurricane Harvey’s rains last year, although these rains will be a small fraction of Harvey’s). The heaviest rains from PTC 7 will likely be across southeast Louisiana and southern Mississippi and Alabama, with local amounts possibly topping 5”. The system’s steady motion will help reduce the odds of widespread extreme totals near the coast, but PTC 7 is expected to slow later this week as it moves inland across Louisiana, and its moisture could bring heavy rain to the Southern and Central Plains by next weekend.

We’ll have a full update on the tropics on Monday morning. See our Sunday morning post for more on Tropical Storm Florence—which will spin harmlessly in the central Atlantic this week but could pose a threat to Bermuda and/or the U.S. East Coast in the 6- to 12-day range—and Typhoon Jebi, which is weakening but still expected to strike central Japan as a Category 1-strength typhoon on Tuesday local time.

There’s also Hurricane Norman, which vaulted unexpectedly to Category 4 strength on Sunday more than 1000 miles east of Hawaii. Norman will be heading west toward the islands through Wednesday, but forecast models are consistent in calling for a northward turn later next week, well before a weakening Norman has a chance to threaten Hawaii.

Dr. Jeff Masters contributed to this post.