*September ends with plenty of tropical activity in the Atlantic Basin...a look back on the tropical season so far...a recap of Helene, and a look ahead*

Hurricane Helene not only impacted the state of Florida in a significant way, but also the southern Appalachians where tremendous amounts of rainfall fell and flooding was extreme in places like western North Carolina, eastern Tennessee, southwestern Virginia, upstate Georgia, and upstate South Carolina. Map courtesy Dr. Ryan Maue, X; weathermodels.com

Overview

Hurricane Helene made landfall on Thursday night as a category 4 “major” hurricane near Florida’s Gulf coast town of Perry and then pushed north through Georgia before grinding to a halt over the Tennessee Valley. Tremendous rainfall fell over the southern Appalachians as the result of strong and persistent upsloping winds associated with Hurricane Helene with as much as two feet in some spots and flooding was extreme. The month of September comes to an end with plenty of tropical activity to monitor in the Atlantic Basin and another system is destined for the Gulf of Mexico by this weekend or early part of next week. The tropical season in the Atlantic Basin has now resulted in 11 named tropical storms with four hurricanes having made landfall in the US, and there certainly may be other opportunities to add onto those numbers.

Lots of tropical activity in the Atlantic Basin as we end the month of September. Map courtesy NOAA/NHC

Discussion

Helene strengthened into “major” hurricane status during the day on Thursday and then made landfall on Thursday night as a category 4 storm system near the Florida Gulf coast town of Perry. By reaching the coast of Florida as a “major” hurricane, Helene became the 9th such storm to do so since the year 2000 with the most recent being Idalia during late August of last year. The “Big Bend” region of Florida’s Gulf coast was impacted by the powerful winds of Helene, heavy rainfall amounts, and significant storm surge of as much as 15 feet. Hurricane Helene then pushed rather quickly northward into the state of Georgia and then became heavily influenced by blocking high pressure to the north and forcing the remnants to park over the Tennessee Valley by the early part of the weekend. Given the northward track of Helene through the state of Georgia, upsloping winds became strong and persistent across the southern Appalachians helping to “squeeze” out of the atmosphere tremendous amounts of rainfall in places such as western North Carolina, eastern Tennessee, southwestern Virginia, and upstate regions of Georgia and South Carolina. There were reports of as much as two feet of rain in some of these areas and flooding was extreme in many locations.

Accumulated cyclone energy (ACE) is one metric that is used to assess overall tropical activity in a given region and it is currently running at below-normal levels in the Atlantic and Pacific Basins and across the entire Northern Hemisphere. Data courtesy Colorado State University, NOAA

As we end the month of September, multiple tropical systems exist across the Atlantic Basin including Tropical Storm Isaac, Tropical Depression Joyce, and the newest named Tropical Storm Kirk which makes “11” as to the number of named systems in 2024. Of these named tropical systems, six have reached hurricane status and 4 made landfall in the US including Beryl as a category 1 in Texas on July 8th, Debby as a category 1 in Florida on August 5th, Francine as a category 2 in Louisiana on September 11th and most recent, Helene, as a category 4 in Florida late last week.

Hurricane Helene made landfall in Florida on Thursday night as a category 4 storm and it shows up as 16th on this list of most intense US landfalling hurricanes since the 1800’s. Data courtesy NOAA

Perhaps a better metric to use to assess overall tropical activity in a given basin is the “Accumulated Cyclone Energy (ACE)”. The ACE metric - which was originally created by Dr. William Gray and associates at Colorado State University and later tweaked by NOAA - not only factors in the intensity of a tropical cyclone, it also takes into account its longevity. A “major” hurricane, for example, that lasts for a long time will have a much bigger impact on the accumulated cyclone energy than a short-lived and weaker tropical system. As it stands now, the “accumulated cyclone energy” is actually below-normal for this time of year in the Atlantic Basin despite the recent uptick in tropical activity. In addition, ACE is below-normal across the Pacific Ocean which has a much bigger impact on overall global statistics given its immense size compared to all other oceans and this leads the way to a below-normal measurement of ACE for the entire Northern Hemisphere as we get ready to begin the month of October.

Looking ahead, as has been the case for much of this tropical season, the Gulf of Mexico/Caribbean Sea will likely be the focus of attention as we head into the upcoming weekend and early part of next week. There are some signs that point to the development of another tropical system in central or western part of the Gulf of Mexico which could very well end up in the same eastern part of the Gulf of Mexico as with Hurricane Helene; however, this potential system would likely not be nearly as strong.

Stay tuned for more on this potential weekend/early next week threat and others that may come as we head through the first half of October.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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