Abnormal weather investigation
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The NHC predicted that within just 60 hours, PTC9 would strengthen at a record pace, increasing from less than 35 knots (about 65 kilometers per hour) to hurricane-force winds of at least 100 knots (185 kilometers per hour) or more.
This was the fastest expected spin-up from a disturbance to a major hurricane in NHC history.
And those predictions were correct. Fueled by the deep, super-hot ocean waters of the Gulf of Mexico, Helen went into overdrive, unencumbered by any shear winds that could impede a storm’s growth.
Here are three things to keep in mind as Helen continues her plunge into the southeastern United States.
Rapid intensification of hurricanes is becoming the new normal.
The NHC defines rapid strength when a storm’s maximum sustained wind speed jumps by at least 56 km/h (35 mph) in less than a day (SN: 23/9/13).
With continued record-breaking tropical water temperatures, numerous storms have met and even exceeded this definition over the past few years.SN: June 15, 2023). For example, in 2023, Atlantic hurricanes Idalia and Lee strengthened at approximately 58 kilometers per hour within 24 hours.
Helen is not just a textbook example of such rapid intensification, she is a star student.
Scientists are gritting their teeth in anticipation of just such an event, given the ultra-hot water conditions in 2024. What spurred Helen’s anger was Record high temperatures in the Gulf of Mexico. Sea surface temperatures in the Gulf are high, in some places up to 2 degrees Celsius above the September average of about 29 degrees Celsius. But more importantly, the Gulf’s excess ocean heat isn’t just skin deep, keeping the ocean extremely warm. As it reaches deeper into the water column, it increases the heat content of the entire ocean, providing even more fuel for swirling storms.SN: 7/2/24).
Another favorable factor for hurricane formation in the Gulf is the lack of wind shear, changes in wind direction, or higher velocity changes into the atmosphere. Faster winds in the upper atmosphere can eat away at tightly swirling low pressure systems by blowing much-needed heat and moisture away from the center.
Smaller cyclones may tend to develop even more rapidly.
While Helen took center stage, forecasters were reeling from the sudden increase in strength of another tropical cyclone, Hurricane John, which made landfall on Mexico’s South Pacific coast on September 23.
This was two days earlier than the researchers expected.
The storm was also fueled by warm ocean waters and developed into a Category 3 hurricane just hours after being classified as a tropical storm. Its dramatic, sudden surge in power and speed caught both scientists and authorities off guard, and they scrambled to issue a warning ahead of landfall.
As Helen and John demonstrate, storms can be large or small and can intensify quickly. But recent research suggests there may be a reason why forecasters were even more caught off guard by John’s sudden increase in voice. And it may have something to do with the size of the storm.
A 2014 analysis of the size and intensity of tropical cyclones from 1990 to 2010 suggested that small, compact storms like John were only a fraction of Helen’s width. Sudden intensification of symptoms may be particularly likely. This is because it can confuse predictions.
In particular, it may be related to how large the inner core, the eye of the hurricane, is initially. That may be because storms with larger inner cores are more resistant to structural changes caused by external forces. Such forces may include heat transfer from seawater.
New projections of inland impacts show that intensification is not just a coastal problem.
In August, the NHC announced an experimental hurricane forecast cone. This cone includes not only the expected path of the storm toward land, but also areas where strong winds could be felt inland. The purpose of this new type of forecast, which the center announced in February, is to Increasing public awareness of the dangers of hurricanes They can exist far away from the eye of the storm or even long after landfall (SN: 2/29/24).
This was especially important for Hurricane Helen, which was predicted to bring a devastating storm surge of up to 20 feet when it made landfall in Florida’s Big Bend region, making it one of the largest storm surge predictions the center has ever made. They became one. This is the equivalent of a two-story wall of water coming ashore.
Helen is also home to a huge wind farm, and tropical storm-force wind gusts extended about 500 kilometers from the center of the storm, covering almost the entire state. The storm is estimated to be the fifth largest storm on record in the Gulf of Mexico.
Empirical predictions suggest that Helen’s danger extends throughout the southeastern United States. On September 27, several hours after landfall, Helen was downgraded to a tropical storm as it continued northward, causing high winds, power outages, torrential rain, and flash flooding across Georgia, South Carolina, and North Carolina.
