After tracking through the eastern Gulf of Mexico and battering much of the west coast of Florida with strong winds and storm surge, Hurricane Helene made landfall in the Big Bend region of Florida’s northwest coast at 11:10 pm EDT about 10 miles west-southwest of Perry, Florida, as a powerful Category 4 hurricane with maximum sustained winds reported at 140 mph by the National Hurricane Center (NHC).  Hurricane Idalia, a strong Category 3 storm at the time, also made landfall in the Big Bend last year.  

Animation of NASA IMERG precipitation rates and totals from Hurricane Helene from Sept. 23 - Sept. 27, 2024. Download video (right-click -> "save as") Credit: NASA

Helene’s formation was somewhat unusual.  In the heart of hurricane season, tropical storm and hurricane formation normally expands eastward across the tropical Atlantic towards the coast of Africa where the vast majority of storms originate from African easterly waves propagating westward off the west coast of Africa.  Helene, however, originated from a broad area of low pressure that developed in the far western Caribbean.  This area of low pressure likely resulted from the combined effects of the Central American gyre and the Madden-Julian Oscillation (MJO).   The Central American gyre is a type of broad monsoon-low that can form seasonally over Central America when winds in the eastern Pacific Ocean become more westerly, which combined with the normal easterly trade winds over the western Caribbean leads to a large-scale counterclockwise or cyclonic flow over Central America.  The MJO acts like a smaller scale ENSO circulation but can propagate eastward from the Indian Ocean and Maritime Continent into the eastern Pacific.  When this happens, it can bring more westerly winds to the eastern Pacific, contributing to the cyclonic flow pattern associated with the Central American gyre.  Persistent thunderstorm activity within this large-scale cyclonic flow over the western Caribbean is what eventually led to the formation of Helene.

The area of thunderstorm activity that later evolved into Helene was first identified as a potential tropical cyclone by NHC on the morning of Monday, Sept. 23.  However, despite the ongoing thunderstorm activity, it wasn’t until 24 hours later that this disturbance became organized enough with a defined circulation center to become a tropical cyclone.  At this point its 45 mph winds were already stronger than a tropical depression, so NHC declared the formation of Tropical Storm Helene at 11:00 a.m. EDT on Sept. 24 with the center located in the far northwestern Caribbean about 170 miles south-southeast of the western tip of Cuba.  Throughout the rest of the day, Helene continued to track to the west-northwest towards Cozumel, Mexico, and slowly strengthen.  However, during the early morning hours of the Sept. 25, Helene began to recurve more towards the north in response to an area of high pressure over Florida and an upper-level trough of low pressure moving into the lower Mississippi Valley.  Helene continued to strengthen throughout the morning of Sept. 25, and at 11:00 a.m. EDT, Helene became a hurricane as it was passing through the Yucatan Channel about 55 miles northeast of Cancun, Mexico, and into the southeastern Gulf of Mexico.

With water temperatures running about 1 to 2o C above normal in the southeast Gulf and low atmospheric wind shear, the environment was quite conducive for strengthening.  However, despite these favorable conditions, Helene’s inner core was still not fully organized, which delayed intensification, and it wasn’t until the following morning on Sept. 26 that Helene’s central pressure really began to fall and induce an increase in wind speed.  But, by 7:00 a.m. EDT, Helene’s maximum sustained winds had reached 100 mph, making it a Category 2 hurricane.  By now, Helene was about 290 miles due west of Key West, Florida and moving north-northeast at 12 mph.  Helene continued to respond to the low wind shear and warm Gulf waters as it tracked to the north-northeast, and by 2:25 p.m. EDT, Air Force Hurricane Hunters found that maximum sustained winds had increased to 120 mph, making Helene a major Category 3 hurricane.  The center was now about 170 miles west-southwest of Tampa, Florida.  Despite Helene’s distance from the Florida coast, Helene’s wind field was very large, especially on the eastern side of the storm.  This resulted in tropical storm-force gusts impacting the west coast of Florida throughout most of the day.  Even along Florida’s east coast, wind gusts exceeded tropical storm-force at times. As Helene continued its steady track towards Florida’s Big Bend, it continued to accelerate ahead of the upper-level trough over the Mississippi Valley and also continued to intensify.  At 6:20 p.m. EDT, with the storm due west of Tampa and moving forward at 23 mph, Helene became a Category 4 hurricane with maximum sustained winds reported at 130 mph by NHC.  However, Helene’s central pressure continued to come down as it approached the northwest coast of Florida, and by 9:00 p.m. EDT, maximum sustained winds reached 140 mph.  It was just after this time that Cedar Key, then about 65 miles east of the center, recorded sustained winds up to 64 mph with a peak gust of 84 mph.  Helene’s central pressure fell slightly more, but the storm was near land and had reached its peak intensity before making landfall.

The animation at the beginning of the article shows surface rainfall estimates associated with Hurricane Helene from NASA’s IMERG precipitation product.  The animation begins at 8:00 p.m. Sept. 22 (00:00 UTC Sept. 23) when Helene was still in the process of formation, progresses through landfall in Florida’s Big Bend, and ends at 8:30 a.m. EDT (12:30 UTC) Sept. 27 when Helene was over northern Georgia.  IMERG shows heavy rain of 6 to 12 inches (shown in purple) over the northwestern Caribbean, including western Cuba and the eastern tip of the Yucatan Peninsula, due to the persistent thunderstorm activity that led to Helene’s formation and the initially slow movement of the storm.  Surprisingly, despite the large wind field IMERG shows that most of central and southern Florida received less than ~2 inches (50 mm) of rain from Helene.  This is due in part to the storm’s distance from the coast but also Helene’s fast forward motion.  Another interesting feature is that the heaviest rainfall, where Helene made landfall in Florida’s Big Bend, is located west of the center.  Normally, the heaviest rainfall would occur east of the center where the storm’s strong counterclockwise circulation should draw in the most moisture from the Gulf. However, a frontal boundary draped over the Florida Panhandle interacted with Helene’s circulation to concentrate the highest rainfall totals, on the order of 6 to 12 inches (shown in purple), west of the center, which was west of Tallahassee over the Florida panhandle.  IMERG also shows heavy rainfall totals of 6 to 12 inches (shown in purple) over northern Georgia and the southern Appalachians where the remnants from Helene will continue to pose a flooding threat over the next few days.  Locally higher amounts on the order of 16 inches or more have been reported over parts of the Florida panhandle and southern Appalachians.

The NASA Disasters Response Coordination System has activated to support agencies responding to the storm, including FEMA and the Florida Division of Emergency Management. Learn more.  

Credits: 

Animation by Jason West (NASA GSFC, PPS, KBR)
Story by Steve Lang (SSAI / NASA GSFC)
Edits by Jacob Reed (Telophase / NASA GSFC)