Tropical Storm Helena formed in the western Caribbean early Tuesday and was expected to intensify into an unusually large major hurricane that would bring historic storm surges to a large swath of Florida's Gulf Coast on Thursday. Flooding of 5-8 feet (1.5-2.6 m) in Tampa Bay is expected to reach levels not seen since 1947.
Helena's formation date was September 24, more than a month later than the average formation date for the season's eighth named storm (August 21).
As of 11 a.m. ET Tuesday, Helena was located 180 miles (295 kilometers) east-southeast of Cozumel, Mexico, moving northwest at 12 mph (19 kph) with maximum sustained winds of 45 mph (75 km/h). Satellite and radar imagery of the Cayman Islands showed widespread severe storms in Helena, which became more organized and brought heavy rain to the Cayman Islands as well as parts of Cuba and Mexico's Yucatan Peninsula. Upper-level winds from the west produced moderate shear of 10-15 knots on the storm, limiting nearly all severe thunderstorm activity to the east side of the circulation.
As of 8 a.m. ET Tuesday, 5.57 inches (141.5 millimeters) of rain had fallen at Grand Cayman's Irvine Robert International Airport in the 24 hours. A personal weather station on the northwest tip of Grand Cayman reported 7.05 inches (179 millimeters) of rain in the 24 hours at 12:30 noon ET on Tuesday.
Helena's trajectory prediction
Once Helena establishes a clearer center of circulation Tuesday afternoon, there won't be much mystery about its whereabouts. Initially, the storm will push northwest through a high pressure system to the north toward the Yucatan Channel and southern Gulf of Mexico. As this high pressure system moves eastward, the clockwise flow around the high pressure, combined with the counterclockwise flow exerted by the trough northwest of Helena, will force the storm's track further north and then north-northeast. Computer models are tight on forecasts for an impact in the Big Bend, Florida, area Thursday afternoon or evening. Only one isolated member passed south of the densely populated Tampa Bay area on Tuesday in the latest run of 6Z's GFS and European ensemble models (Figure 1). However, these gathering track forecasts could still change modestly if the center of Helena moves further east or west as the storm continues to organize Tuesday afternoon.
Helena intensity forecast
Because Helena formed from a large circulation known as the Mesoamerican Gyre, its large initial size will cause it to slowly strengthen Tuesday night. The latest run of some professional hurricane models on 12Z on Tuesday showed Helena could hit the northeastern tip of the Yucatan Peninsula on Wednesday morning, significantly delaying hurricane intensity. Once Helena appears in the Gulf of Mexico, conditions will be very favorable for its intensification. Ocean temperatures in the northwest Caribbean and eastern Gulf of Mexico are at or near record highs – about 30-31 degrees Celsius (86-88°F), with large amounts of warm water extending to great depths (high ocean heat content). Wind shear is expected to weaken Tuesday night into Thursday afternoon, with wind speeds remaining in the low range of 5-10 knots. The atmosphere over the Gulf of Mexico will be very moist, and land interactions with Cuba and Mexico's Yucatan Peninsula will be reduced as Helena emerges over the open southeastern Gulf on Wednesday. These conditions will allow Helena to become Hurricane Helena on Wednesday night and a major hurricane on Thursday afternoon.
In the hours before Helena was named, unusually favorable intensifying conditions led the National Hurricane Center to make its most aggressive intensity forecast to date for the unnamed tropical disturbance (see tweet above). The official forecast from the National Hurricane Center (NHC) at 11 a.m. EDT on Tuesday said Helena would reach peak Category 3 hurricane intensity Thursday morning into Thursday evening, with winds of 115 mph (130 kph) ), but the more advanced intensity predictions of the forecast model predict a wide range of possible maximum values. The DTOPS model gives a 97 percent chance that Helena's winds will strengthen to at least 65 mph (105 km/h) in the 48 hours ending at 8 a.m. ET Thursday, making it 110 mph. hour (180 km/h) as a high-level Category 2 hurricane.
Dangerous storm surge in Florida
Given its large initial size and low pressure (central pressure was 1,000 mb at 11 a.m. EDT Tuesday, unusually low for a developing tropical cyclone), Helena will be a large storm capable of producing severe storm surge . With such a widespread storm, it is especially important not to focus only on the “thin line” of the central track, as the greatest impacts may extend east of Helena's track, including the west coast of Helena, Florida.
The NHC ranks Helena's size at the 90th percentile of its current latitude and predicts tropical storm force winds extending up to 255 miles (410 kilometers) from the center of the storm Thursday night. This massive wind field, combined with large areas of shallow water off Florida's west coast, will produce massive and damaging storm surges along much of Florida's Gulf Coast, hundreds of miles from downtown Helena.
Helena's storm surge and shock waves will cause severe damage and erosion to the beaches and dunes of Big Bend, Florida, which were severely impacted by Category 1 Hurricane Debbie on August 5 of this year and on August 30 of this year. Idalia, a Category 3 hurricane, brought a storm surge of 8 to 12 feet (2.4-3.7 m) to the Big Bend and 3 to 5 feet (0.9-1.5 m) to Tampa Bay. Storm surge.
Helena will likely produce a larger storm surge than Idalia, which is a fairly small storm. On August 29, 2023, when Idalia passed 125 miles (201 km) west of Tampa as an intensifying Category 2 hurricane, tropical storm winds extended 160 miles (257 km) toward Tampa. Helena is expected to pass about 150 miles (240 kilometers) west of Tampa Thursday afternoon, 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 8 feet (1.5-2.6 m) to Tampa Bay and up to 15 feet (4.6 m) to Cedar Island. The NHC gave a 10 percent chance that much of the Tampa Bay area could see flooding more than 6 feet above normal drought levels, with flooding in some low-lying areas possibly exceeding 9 feet.
The timing of Helena's impact will be an important factor in determining the extent of coastal flooding. Cedar Key in Florida's Big Bend area is expected to experience high tides at 8:12 a.m. ET on Thursday and 10 a.m. ET on Friday; low tide will be at 4:36 p.m. ET on Thursday. The difference in water level between high and low tide is almost three feet (0.9 m). The largest surge (higher than normal water levels) will occur at impact, with the NHC currently forecasting Thursday around 8 p.m. ET. However, maximum water levels (swell heights above normal dry ground) may occur within a few hours of impact, when the tide is approaching.
High tide in St. Petersburg, Florida, is 8:42 a.m. ET on Thursday and 1:58 a.m. ET on Friday; low tide is 5:39 p.m. Thursday. The difference in water level between high and low tide is about two feet (0.6 m). Peak storm surge in Tampa Bay during Hurricane Idalia in 2023 began as the hurricane's center moved westward, then peaked when Idalia made landfall about 150 miles northwest. We expect similar conditions to occur in Helena, although water levels may be higher. If the timing of the current NHC forecast is correct, a storm surge of more than 3 feet will begin impacting Tampa Bay around 2 p.m. ET on Thursday, when the storm is just west of the city. Storm surge will peak around 8 p.m. ET Thursday and is expected to impact the Big Bend area. The six-hour peak swell period coincided with low tide, so that was lucky. However, for Helena, the uncertainty in the timing of the storm (along-track uncertainty) is greater than the uncertainty in the location of the impact (path uncertainty), so the timing of impact needs to be monitored closely.
Florida, Georgia face heavy rain threat
As Helena approaches the Florida coast on Thursday, the storm will flow strongly northwest from a trough of low pressure. This will accelerate Helena from a forward speed of approximately 10 mph (16 km/h) to a rapid speed of 20 mph (32 km/h). This rapid movement will limit the time Helena can bring heavy rain, and freshwater flooding may not be the storm's main threat; storm surge and high winds will cause damage. With record sea surface temperatures in the eastern Gulf of Mexico, Helena will still be able to bring significant rainfall totals across Florida and the southeastern United States: 4 to 8 inches (102-203 mm) of rainfall are expected along the Helena path , extending 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 take an unusual left curve around an isolated upper low and into the central Mississippi Valley.
destructive inland winds
Due to Helena's expected size, Category 3 intensity, and rapid forward motion upon impact, damaging winds will extend farther inland than typical hurricanes. If Helena follows a track east of the Florida Panhandle, Tallahassee could be at particular risk. Based on data dating back to 1945, the strongest sustained winds and strongest gusts recorded in Tallahassee were 58 mph (August 8, 1962) and 83 mph (September 11, 1990), respectively. . Helena may exceed both values. Given Tallahassee's large population and extensive tree cover, widespread power outages and damage from downed trees are a real possibility. The impact could extend into interior southern Georgia.
