Fast-emerging system could hit Florida Gulf Coast with dangerous Hurricane Helene Thursday » Yale Climate Connection

12z GFS continues to shift more and more east affecting much of the west coast of Florida including the Tampa Bay area. This would bring significant to destructive impacts to the area including significant storm surge if this track plays out. #flwx #PTC9 pic.twitter.com/v9C9Tx7TnK

— StormHQ ☈ (@StormHQwx) September 23, 2024

PTC 9 Strength Forecast

Because PTC 9 formed from a large circulation known as the Mesoamerican Gyre, its initial large size will cause it to intensify slowly. Assuming PTC 9 can quickly cross the Yucatan Strait between Cuba and Mexico without spending too much time on land, conditions are very favorable for intensification. Ocean temperatures in the Gulf of Mexico will reach near-record warm levels – about 29.5-30.5 degrees Celsius (85-87°F), with large amounts of warm water extending to great depths (high ocean heat content). Wind shear speeds of 10-20 knots, a very moist atmosphere and reduced land interactions with Cuba and Mexico's Yucatan Peninsula are expected as PTC 9 moves into the open southeastern Gulf on Tuesday. These conditions will allow PTC 9 to become Hurricane Helene on Tuesday night. One potential factor inhibiting intensification: upper-air flow from Tropical Storm John in the eastern Pacific, which is expected to make landfall in southeastern Mexico on Tuesday (see our post on John ). From PTC 9 into Tuesday night, John's outflow may produce more wind shear than forecast.

At 11 a.m. ET on Monday, the NHC's official forecast said PTC 9 would peak Thursday morning at just below major hurricane strength, with Category 2 winds of 110 mph (177 km/h). However, this prediction may be conservative, as many intensity models predict that PTC 9 will become a Category 3 or 4 hurricane (Figure 2). The DTOPS model gives a 95 percent chance that PTC 9 will strengthen to at least Category 2 wind speeds in the 72 hours ending at 8 a.m. ET Thursday, by at least 75 mph (121 kph). Wind speed was 105 mph (169 km/h). Keep in mind, however, that these models were developed assuming the existence of tropical cyclones with well-defined surface circulation, and PTC 9 does not yet meet that definition. Assuming surface circulation has developed by then (as expected), we should have a better idea of ​​how things will evolve by Tuesday.

a dangerous storm surge Florida

Given its large initial size and low pressure (its central pressure was 1004 mb at 11 a.m. EST Monday, unusually low for a developing tropical cyclone), PTC 9 is likely to be a storm surge capable of producing severe Large storm. The NHC expects tropical storm-force winds to extend as far as 255 miles (410 kilometers) from the center of the storm Thursday morning. This massive wind field, combined with large areas of shallow water off Florida's west coast, will produce large and damaging storm surges along much of Florida's Gulf Coast. Severe surf is possible along portions of Florida's west coast east of the storm's center.

Based on current NHC track forecasts, the storm's rough swells and violent waves could cause severe damage and erosion to the beaches and dunes that were severely impacted by Category 1 Hurricane Debbie on August 5 this year and by Hurricane Debbie last year. The serious impact of Debbie's landing. ) storm surge.

PTC 9 could produce greater storm surge than Idalia, which was a fairly small storm. On August 29, 2023, when Idalia passed 125 miles (201 km) west of Tampa as Category 2 intensity, tropical storm winds extended 160 miles (257 km) toward Tampa. PTC 9 is expected to pass about 150 miles (240 kilometers) west of Tampa Thursday morning, with tropical storm winds expected to extend 255 miles (410 kilometers) toward Tampa. Wind fields this large can bring storm surges of 5 to 10 feet (1.5-3 m) to Tampa Bay and over 15 feet (4.6 m) to Cedar Island (see Figure 3 and the tweet below ). The NHC gave a 10% chance that much of the Tampa Bay area could experience storm surges in excess of 9 feet (Figure 3).

High tide in Cedar Key, Florida's Big Bend area, is Thursday at 8:12 a.m. ET and Friday at 10 a.m. ET; low tide is at 4:36 p.m. Thursday. The difference in water levels between high and low tide is almost three feet (0.9 m), so the timing of PTC 9's landfall will be an important factor in determining the extent of coastal flooding. However, it is too early to talk about the possible timing of PTC 9 landing.

Florida, Georgia face heavy rain threat

As PTC 9 approaches the Florida coast on Thursday, the storm will become entangled in a strong northwestward turn from the trough of low pressure. This will accelerate PTC 9 from its current forward speed of 9 mph to 15-20 mph (24-32 km/h). This rapid forward movement will limit the duration of PTC 9's heavy downpours, making freshwater flooding unlikely to be the main threat from the storm, but storm surge and wind damage. However, with record sea surface temperatures in the eastern Gulf of Mexico, PTC 9 will still be able to produce some decent rainfall totals over Florida and the southeastern United States: 4 to 8 inches (102-203 mm) of rain are possible along the way. PTC 9's tracks extend hundreds of miles inland into Georgia and possibly into Tennessee, South Carolina and North Carolina. There is considerable uncertainty about the location of inland rainfall outside of Florida and Georgia, as the system may make an unusual left bend around the upper low pressure cutoff and move toward the central Mississippi Valley (see Figure 1).

In the Tropical Weather Outlook released at 8 a.m. ET on Monday, the National Hurricane Center gave PTC 9 a two-day and seven-day development probability of 80% and 90% respectively. The first hurricane search flight to the center of PTC 9 is scheduled for Monday afternoon, with NOAA aircraft tasked with sampling the environment around the storm starting Tuesday morning to improve model forecasts.

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