Tropical storm warning flags are flying over western Cuba and Mexico's Yucatan Peninsula. Severe weather in large areas of the western Caribbean is moving towards the Gulf of Mexico and is expected to make landfall in western Florida on Thursday under the name “Helen”.
The storm, initially labeled Invest 97L, was renamed Potential Tropical Cyclone 9 (PTC 9) by the National Hurricane Center (NHC) at 11 a.m. ET on Monday. The PTC designation is used for a system that can bring a tropical storm or hurricane into a land area within 48 hours but has not yet formed a tropical cyclone.
PTC 9 Keep Organizing
As of 11 a.m. ET Monday, PTC 9 was located 130 miles south-southwest of Grand Cayman, moving north at 6 mph (9.7 kph) with maximum sustained winds of 30 mph (48 km/h) with strong winds. Satellite and radar imagery of the Cayman Islands shows PTC 9 with a large area of intense, highly rotating storms that are stabilizingly organizing to bring heavy rain to parts of the Cayman Islands and Cuba. Conditions were favorable for development, with near-record ocean temperatures near 30.5 degrees Celsius (87°F), wind shear of 10-20 knots, and a very humid atmosphere (mid-level relative humidity of 75%).
Trajectory prediction of PTC 9
Initially, PTC 9 will be guided by the high pressure system to the north to the Yucatan Strait to the northwest, and then as the high pressure system moves northeast, after entering the Gulf of Mexico, most of its direction will move northward and the low pressure trough deepens. northwest of. For systems in the development stage, there is often considerable uncertainty in the trajectory (Figure 1). Possible landfalls in the United States are almost entirely in Florida, from the West End to Tampa. The latest Monday 12Z forecast from the GFS model shows PTC 9 moving uncomfortably close to the densely populated Tampa Bay area.
PTC 9 Strength Forecast
Because PTC 9 formed from a large circulation known as the Mesoamerican Gyre, its larger initial size will cause the intensification to be slower. Assuming PTC 9 successfully crosses 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 be at or near record highs – 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 and a very moist atmosphere are expected, with reduced interactions with land in Cuba and Mexico's Yucatan Peninsula as PTC 9 emerges over the southeastern Gulf on Tuesday. These conditions will allow PTC 9 to become Hurricane Helene on Tuesday night. One possible factor inhibiting intensification is the upper-altitude outflow from Tropical Storm John in the eastern Pacific, which is expected to make landfall in southeastern Mexico on Tuesday (see our post on John). John's outflow will likely produce more wind shear than PTC 9 forecast through Tuesday night.
At 11 a.m. ET on Monday, the official forecast from the National Hurricane Center (NHC) said that the PTC Category 9 hurricane will peak on Thursday morning, with an intensity slightly lower than that of a major hurricane, and will be a Category 2 hurricane with wind speeds of 110 mph. hours (177 km/h). However, this forecast may be conservative, with multiple intensity models predicting that PTC 9 will become a Category 3 or 4 hurricane (Figure 2). The DTOPS model gives a 95% chance that PTC 9 winds will increase by at least 75 mph (121 km/h) in the 72 hours ending at 8 a.m. ET Thursday, bringing it to at least Category 2 speeds , with wind speeds of 105 mph (169 km/h). Note, however, that these models were developed assuming the existence of tropical cyclones with well-defined surface circulation, and PTC 9 does not yet meet this definition. Assuming surface circulation has developed by then (as expected), we should have a better idea of how things will develop by Tuesday.
Dangerous storm surge in Florida
Given its large initial size and low pressure (the central pressure at 11 a.m. ET on Monday was unusually low for a developing tropical cyclone), PTC 9 is likely to be a large storm capable of producing severe storm surge. The NHC forecast tropical storm-force winds extending 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, damaging storm surges along much of Florida's Gulf Coast. Severe storm surge is possible along portions of Florida's west coast east of the storm's center.
According to the NHC's current track forecast, storm surge and waves are likely to cause significant damage and erosion to the beaches and dunes that were severely impacted by Category 1 Hurricane Debbie on August 5 this year and last year as a Category 3 hurricane made landfall. Hurricane Idalia brought 8 to 12 feet (2.4-3.7 m) of storm surge to the Big Bend area and 3 to 5 feet (0.9-1.5 m) of storm surge from Tampa to the Bay. 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. Such a large wind field would be capable of bringing 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 push button below). arts). 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. EST; low tide is at 4:36 p.m. Thursday. The difference in water levels between high and low tide is nearly 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. 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 be trapped by strong northwestward airflow from a trough of low pressure. This will accelerate the PTC 9 from its current 9 mph to a blistering speed of 15-20 mph (24-32 km/h). This rapid forward movement will limit the duration of PTC 9's heavy downpours, and freshwater flooding may not be the storm's main threat: storm surge and wind damage are. With record sea surface temperatures in the eastern Gulf of Mexico, PTC 9 will still be able to bring significant rainfall totals across Florida and the southeastern United States: between 4 and 8 inches (102-203 mm) of rainfall are expected along the stretch. . There is considerable uncertainty in the location of rainfall inland beyond Florida and Georgia, as the system may make an unusual left turn around an isolated upper low and into the central Mississippi Valley (see Figure 1).
In the Tropical Climate Outlook released at 8 a.m. Eastern Time on Monday, the National Hurricane Center gave the chance of PTC 9 development as 80% within two days and 90% within seven days. Hurricane hunters are scheduled to fly to PTC 9 Center for the first time Monday afternoon, and NOAA aircraft will be tasked with sampling the environment around the storm starting Tuesday morning to improve forecasts from storm models.
