Tropical Storm Helene forms in the Western Caribbean » Yale Climate Connections

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Tropical Storm Helene formed in the Western Caribbean on Tuesday morning, and is expected to intensify into an unusually large major hurricane that will bring a historic storm surge to a large swath of Florida’s Gulf Coast on Thursday. The 5-8 feet (1.5-2.6 m) of inundation predicted for Tampa Bay would be their highest water level since record-keeping began in 1947.

Helene’s formation date of Sep. 24 comes over a month later than the average formation date of Aug. 21 for the season’s eighth named storm.

Helene steadily growing more organized

At 11 a.m. EDT Tuesday, Helene was located 180 miles (295 km) east-southeast of Cozumel, Mexico, moving northwest at 12 mph (19 kph), with top sustained winds of 45 mph (75 kph) and a central pressure of 1000 mb. Satellite imagery and Cayman Islands radar showed that Helene had a large area of heavy thunderstorms, which were steadily growing more organized and bringing heavy rains to the Cayman Islands and portions of Cuba and Mexico’s Yucatan Peninsula. Upper-level winds out of the west were creating moderate wind shear of 10-15 knots over the storm and keeping almost all of the heavy thunderstorm activity confined to the east side of the circulation.

Owen Robert Airport on Grand Cayman Island received 5.57 inches (141.5 mm) of rain in the 24 hours ending at 8 a.m. EDT Tuesday. A personal weather station on the northwest end of Grand Cayman Island received 7.05 inches (179 mm) of rain in the 24 hours ending at 12:30 p.m. EDT Tuesday.

Figure 1. Track forecasts out to five days for Helene, from the 6Z Tuesday, Sep. 24, 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 Helene than the European model. (Image credit: weathernerds.org)

Track forecast for Helene

Once Helene establishes a more sharply defined center of circulation Tuesday afternoon, there will not be much mystery about where it is going. The storm will initially be steered northwestward into the Yucatan Channel and the southern Gulf of Mexico by a high-pressure system to the north. As this high slides to the east, the clockwise flow of air around the high, combined with the counter-clockwise flow imparted by a trough of low pressure to Helene’s northwest, will force the storm on a more northerly and then north-northeasterly track. The computer models are tightly clustered around a projected landfall Thursday afternoon or evening in the Florida Big Bend region. The latest 6Z Tuesday runs of the European and GFS ensemble models (Fig. 1) have only one stray member passing to the south of the heavily populated Tampa Bay area. However, a modest shift in these clustered track forecasts could still occur if Helene’s center reforms more to the east or west on Tuesday afternoon, as the storm continues organizing.

A chart showing the intensity predictions for Helene. Most models predict the storm will peak at a Category 2 or higher storm.
Figure 2. Model intensity forecasts for Helene available as of 8 a.m. EDT Tuesday, Sep. 24, 2024. (Image credit: tropicaltidbits.com)

Intensity forecast for Helene

Since Helene is forming from a large circulation known as a Central American Gyre, its initial large size will make it slow to intensify through Tuesday night. The latest 12Z Tuesday runs of some of the specialized hurricane models showed Helene might hit the northeastern tip of the Yucatan Peninsula on Wednesday morning, significantly delaying intensification. Once Helene emerges into the Gulf of Mexico, conditions will be very favorable for intensification. Ocean temperatures in the northwest Caribbean and the eastern Gulf of Mexico are record to near-record warm – about 30-31 degrees Celsius (86-88°F), with a substantial amount of warm water extending to great depth (a high ocean heat content). Wind shear is predicted to abate and be in the low range, 5-10 knots, Tuesday night through Thursday afternoon. The atmosphere over the Gulf of Mexico will be very moist, and land interaction with Cuba and Mexico’s Yucatan Peninsula will decrease as Helene emerges into the open southeastern Gulf on Wednesday. These conditions should allow Helene to become Hurricane Helene by Wednesday night, and a major hurricane by Thursday afternoon.

A few hours before Helene was named, the unusually favorable conditions for intensification led the NHC to make their most aggressive intensity forecasts ever for an unnamed tropical disturbance (see Tweet above). The 11 a.m. EDT Tuesday official forecast from NHC called for Helene to peak Thursday morning through Thursday evening as a Cat 3 with 115 mph (130 kph) winds, but the top intensity models give a wide range of potential top intensities, from Cat 2 to Cat 4 (Fig. 2). The DTOPS model gave a 97% chance that Helene would intensify by at least 65 mph (105 kph) in the 48 hours ending at 8 a.m. EDT Thursday, making it a high-end Cat 2 with 110 mph (180 kph) winds.

A map showing that climate change has made ocean temperatures much higher in the Gulf of Mexico
Figure 3. The Climate Shift Index: Ocean for sea surface temperatures on Sep. 22, 2024, along Helene’s future path in the Western Caribbean and southern Gulf of Mexico show that sea surface temperatures were about 1-2 degrees Celsius (1.8-3.6°F) above average, and climate change made these ocean temperatures at least 200-700 times more likely. (Image credit: Climate Central)

A dangerous storm surge for Florida

Given its initial large size and low pressure (its central pressure of 1000 mb at 11 a.m. EDT Tuesday was unusually low for a developing tropical cyclone), Helene will be a large storm capable of generating a significant storm surge. With such a sprawling storm, it’s especially crucial not to focus just on the “skinny line” of the center’s path, as major impacts could extend far to the east of Helene’s track, including along Florida’s west coast.

NHC mentioned that Helene was in the 90th percentile for size for its current latitude, and predicted that tropical-storm-force winds would extend out up to 255 miles (410 km) from the center of the storm on Thursday evening. This huge 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, several hundred miles from where the center of Helene passes.

Helene’s surge and battering waves will cause significant damage and erosion to beaches and dunes in Florida’s Big Bend that were heavily impacted on August 5 this year by Category 1 Hurricane Debby, and on August 30 last year by 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.

Helene is very 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 Helene is predicted to pass about 150 miles (240 km) west of Tampa on Thursday afternoon, 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 5-8 feet (1.5-2.6 m) to Tampa Bay up to 15 feet (4.6 m) to Cedar Key. NHC gave a 10% chance that significant portions of the Tampa Bay area could see flooding of over six feet above normally dry ground, with some low-lying areas getting more than nine feet of inundation (Fig. 4).

Figure 4. Hurricane Helene has a 10% chance of bringing a storm surge over 6′ to significant portions of the Tampa Bay region, according to the latest NHC potential inundation graphic for Helene.

The timing of Helene’s landfall will be a significant factor in determining how much coastal flooding occurs. High tide at Cedar Key in Florida’s Big Bend region, where the highest storm surge is expected, 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. EDT Thursday. The difference in water level between high and low tide is almost three feet (0.9 m). The peak surge (the height of the water above what would normally occur) will occur at the time of landfall, which NHC is currently predicting will be around 8 p.m. EDT Thursday. However, the highest water levels (height of the surge above normally dry ground) may occur a few hours after landfall, since the tide will be coming in at that time.

High tide at St. Petersburg, Fla., is Thursday morning at 8:42 a.m. EDT and again at 1:58 a.m. EDT Friday; low tide is at 5:39 p.m. Thursday. The difference in water level between high and low tide is about two feet (0.6 m). During Hurricane Idalia in 2023, the highest storm surge in Tampa Bay began when the center of the hurricane passed due west, then peaked when Idalia made landfall about 150 miles to the north-northwest. We can expect a similar pattern for Helene, though potentially with higher water levels. If the timing of the current NHC forecast is correct, a storm surge in excess of three feet will begin affecting Tampa Bay around 2 p.m. EDT Thursday, when the storm will be due west of the city. The surge will increase to its maximum level around 8 p.m. EDT Thursday when landfall is expected in the Big Bend region. This six-hour period of the highest storm surge straddles low tide, so that would be fortuitous. However, with Helene, there is more uncertainty over storm timing (along-track uncertainty) than landfall location (across-track uncertainty), so the landfall timing will need to be monitored closely.

A heavy rain threat for Florida and Georgia

As Helene 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 Helene to accelerate from a forward speed of about 10 mph (16 kph) to a brisk 20 mph (32 kph). This rapid forward motion will limit the amount of time Helene 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, Helene will still be able to generate some respectable rainfall totals over Florida and the Southeast U.S.: 4-8 inches of rain (102-203 mm) is possible along Helene’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).

Damaging winds far inland

Because of Helene’s expected large size, category 3 intensity, and fast forward motion at landfall, damaging winds will extend much farther inland than is usual for a hurricane. Tallahassee may be at particular risk if Helene takes a track into the eastern Florida Panhandle. In data going back to 1945, the strongest sustained wind and wind gust on record for Tallahassee are 58 mph (August 8, 1962) and 83 mph (September 11, 1990), respectively. Helene could exceed both of these values. Given Tallahassee’s substantial population and extensive tree canopy, widespread power outages and tree-fall damage are quite possible. Such impacts could extend well inland into southern Georgia.

John produces Mexico’s southernmost major hurricane landfall on record; deadly floods, mudslides still possible

After a burst of very rapid intensification that brought to mind Hurricanes Patricia (2015) and Otis (2023), Hurricane John nudged ashore just southwest of Marquelia, Mexico, around 11:15 p.m. EDT Monday. Less than 12 hours after it was a tropical storm, John made landfall as a Category 3 hurricane with top sustained winds estimated at 120 mph (195 kph). John is the only major hurricane in modern records to make landfall on Mexico’s Pacific coast southeast of Acapulco.

As of 11 a.m. EDT Tuesday, John had weakened to minimal tropical-storm strength, with sustained winds of 40 mph (65 kph). Roughly paralleling the Mexican coast, John was centered just inland about 40 miles (65 km) northwest of Acapulco and was moving northwest at 10 mph (16 kph). Amid weak steering currents, John is expected to move erratically near the coast over the next several days as a weak tropical storm or perhaps a tropical depression. The main threat will continue to be extreme rainfall of up to 30 inches (483 mm) in pockets near and east of John’s center, raising a risk of life-threatening localized floods and mudslides across the coast-facing mountains of southeast Guerrro and southwest Oaxacan states. Widespread power outages and at least two mudslide-related deaths has been reported as of midday Tuesday, according to the Associated Press.

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