Fast-emerging system likely to slam Florida Gulf Coast as a dangerous Hurricane Helene on Thursday » Yale Climate Connections

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Tropical Storm Warning flags are flying for western Cuba and Mexico’s Yucatan Peninsula as a large area of disturbed weather in the western Caribbean heads toward the Gulf of Mexico and an expected landfall in western Florida on Thursday under the name Helene. Originally labeled Invest 97L, the disturbance was re-christened Potential Tropical Cyclone 9, or PTC 9, by the National Hurricane Center, or NHC, at 11 a.m. EDT Monday. The PTC designation is used for systems that could bring tropical storm or hurricane conditions to land areas within 48 hours but that are not yet tropical cyclones.

PTC 9 steadily growing more organized

At 11 a.m. EDT Monday, PTC 9 was located 130 miles south-southwest of Grand Cayman Island, moving north at 6 mph (9.7 kph), with top sustained winds of 30 mph (48 kph) and a central pressure of 1004 mb. Satellite imagery and Cayman Islands radar showed that PTC 9 had a large area of heavy thunderstorms with plenty of rotation, which were steadily growing more organized and bringing heavy rains to the Cayman Islands and portions of Cuba. Conditions were favorable for development, with near-record ocean temperatures near 30.5 degrees Celsius (87°F), moderate wind shear of 10-20 knots, and a very moist atmosphere (a midlevel relative humidity of 75%).

Figure 1. Track forecasts out to five days for PTC 9, from the 6Z Monday, Sep. 23, 2024, run of the European ensemble model (left) and GFS ensemble model (right). Individual forecasts of the ensemble members are the lines color-coded by the wind speed in knots they predict; red colors correspond to a Category 1 hurricane. The time in hours from the model initialization time is in gray text. The GFS model forecasts were much more aggressive in intensifying PTC 9 than the European model. (Image credit: weathernerds.org)

Track forecast for PTC 9

PTC 9 will initially be steered northwestward into the Yucatan Channel by a high pressure system to its north, then mostly to the north once it enters the Gulf of Mexico as the high pressure system moves to its northeast, and a trough of low pressure digs in to its northwest. As usual for a system that is in the developing stages, there is considerable track uncertainty (Fig. 1). The locations of a potential U.S. landfall are almost entirely in Florida, though, from the western panhandle to Tampa. The latest 12Z Monday forecast from the GFS model (see Tweet below) shows PTC 9 coming uncomfortably close to the heavily populated Tampa Bay area.

Intensity forecast for PTC 9

Since PTC 9 is forming from a large circulation known as a Central American Gyre, its initial large size will make it slow to intensify. Assuming PTC 9 manages to scoot through the Yucatán Channel between Cuba and Mexico without spending much time over land, conditions are very favorable for intensification. Ocean temperatures in the Gulf of Mexico will be record to near-record warm – about 29.5-30.5 degrees Celsius (85-87°F), with a substantial amount of warm water extending to great depth (a high ocean heat content). Wind shear is predicted to be a moderate 10-20 knots, the atmosphere will be very moist, and land interaction with Cuba and Mexico’s Yucatan Peninsula will decrease as PTC 9 emerges into the open southeastern Gulf on Tuesday. These conditions should allow PTC 9 to become Hurricane Helene by Tuesday night. One potential brake on intensification: upper-level outflow from the Eastern Pacific’s Tropical Storm John, which is expected to make landfall in southeastern Mexico on Tuesday (see our post on John). The outflow from John may create more wind shear than predicted for PTC 9 through Tuesday evening.

The 11 a.m. EDT Monday official forecast from NHC called for PTC 9 to peak on Thursday morning just below major hurricane strength, as a Cat 2 with 110 mph (177 kph) winds. However, this forecast may be conservative, and a number of intensity models predict that PTC 9 will become a Cat 3 or Cat 4 hurricane (Fig 2). The DTOPS model gave a 95% chance that PTC 9 would intensify by at least 75 mph (121 kph) in the 72 hours ending at 8 a.m. EDT Thursday, making it a least a Cat 2 with 105 mph (169 kph) winds. Keep in mind, though, that these models were developed assuming that a tropical cyclone with a well-defined surface circulation was present, and PTC 9 does not yet meet that definition. We should have a better idea by Tuesday of how things will evolve, assuming a surface circulation has taken shape by then (as expected).

Intensity forecasts for PTC 9
Figure 2. Model intensity forecasts for PTC 9 available as of 8 a.m. EDT Monday, Sep. 23, 2024. (Image credit: tropicaltidbits.com)

A dangerous storm surge for Florida

Given its initial large size and low pressure (its central pressure of 1004 mb at 11 a.m. EDT Monday was unusually low for a developing tropical cyclone), PTC 9 is likely to be a large storm capable of generating a significant storm surge. NHC was predicting that tropical-storm-force winds would extend out up to 255 miles (410 km) from the center of the storm on Thursday morning. This large wind field, combined with the large area of shallow waters off the west coast of Florida, will produce a large and destructive storm surge for a substantial portion of the Florida Gulf Coast. Significant surge could extend along parts of Florida’s west coast well east of the storm center.

Given the current NHC track forecast, it appears likely that the surge and battering waves of the storm will cause significant damage and erosion to beaches and dunes that were heavily impacted on August 5 this year by Category 1 Hurricane Debby and last year by the landfall of Category 3 Hurricane Idalia, which brought a storm surge of eight to 12 feet (2.4-3.7 m) to the Big Bend and three to five feet (0.9-1.5 m) to Tampa Bay.

PTC 9 is likely to generate a larger storm surge than Idalia, which was a fairly small storm. When Idalia passed 125 miles (201 km) west of Tampa on August 29, 2023, as an intensifying Cat 2, tropical storm-force winds extended out 160 miles (257 km) towards Tampa. When PTC 9 is predicted to pass about 150 miles (240 km) west of Tampa on Thursday morning, tropical storm-force winds are predicted to extend out 255 miles (410 km) toward Tampa. A wind field this large will be capable of bringing a storm surge of five to 10 feet (1.5-3 m) to Tampa Bay and over 15 feet (4.6 m) to Cedar Key (see Fig. 3 and Tweet below). NHC gave a 10% chance that significant portions of the Tampa Bay area could see a storm surge over nine feet (Fig. 3).

Storm surge forecast
Figure 3. Hurricane Helene has a 10% chance of bringing a storm surge over 9′ to significant portions of the Tampa Bay region, according to the latest NHC potential inundation graphic for PTC9.

High tide at Cedar Key in Florida’s Big Bend region is Thursday morning at 8:12 a.m. EDT and again at 10 a.m. EDT Friday; low tide is at 4:36 p.m. Thursday. The difference in water level between high and low tide is almost three feet (0.9 m), so the timing of PTC 9’s landfall will be a significant factor in determining how much coastal flooding occurs. It’s too early to talk about the likely timing of PTC 9’s landfall, though.

A heavy rain threat for Florida and Georgia

As PTC 9 approaches the Florida coast on Thursday, the storm will get caught in a strong steering flow from the trough of low pressure to its northwest. This will cause PTC 9 to accelerate from its current forward speed of 9 mph to a brisk 15-20 mph (24-32 kph). This rapid forward motion will limit the amount of time PTC 9 has to dump very heavy rains, and freshwater flooding is not likely to be the storm’s main threat — storm surge and wind damage will be. Since sea surface temperatures in the eastern Gulf of Mexico are record warm, though, PTC 9 will still be able to generate some respectable rainfall totals over Florida and the Southeast U.S.: four to eight inches of rain (102-203 mm) is possible along PTC 9’s track, extending several hundred miles inland into Georgia, and possibly into Tennessee, South Carolina, and North Carolina. There is large uncertainty over inland rain locations beyond Florida and Georgia, as the system could take an unusual leftward bend around the cut-off upper low and move toward the mid-Mississippi Valley (see Fig. 1).

In its Tropical Weather Outlook issued at 8 a.m. EDT Monday, the National Hurricane Center gave PTC 9 two-day and seven-day odds of development of 80% and 90%, respectively. The first hurricane-hunter flight into the center of PTC 9 is set for Monday afternoon, and a NOAA aircraft is tasked with sampling the storm’s surrounding environment beginning on Tuesday morning in order to improve model predictions.

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