By: Lead Forecaster Daniel Crawley
We continue a multi-part series looking back at Hurricane Helene, its evolution and how it turned into the largest natural disaster in the history of Western North Carolina replacing major weather events such as the floods of 1916 and 1940.
Between now and Saturday (Sept 27), the official 1-year anniversary, we will look back at the storm from a forecaster’s perspective here at Foothills Weather Network. We will go over the factors that created a three-day rain event that brought historic flooding to many river basins in WNC along with the impact of strong winds as Helene itself moved in. We will also re-live the post-storm impacts that Helene brought to the region, including several of the counties that we cover daily at Foothills Weather Network.
Part 2 of the series starts with a look back at September 23rd, 2024; four days before Helene made landfall in the Florida Big Bend Region. Helene had yet to be officially designated as a tropical cyclone by the National Hurricane Center, however model and forecaster confidence was strong enough to where the NHC did initiate advisories for a “Potential Tropical Cyclone.” In this case, PTC Nine came into existence and the initial forecast of PTC Nine lined up with guidance showing a northward moving storm that would come inland and traverse through the Southeast US.
With there being no change in the synoptic players during 9/24, confidence continued to increase that heavy rainfall would move into the Southeast US…but the question was exactly where that would set up?
That all leads to today’s topic on the Precessor Rain Event (PRE) that occurred ahead of the core remnant of Helene. The term PRE is used when synoptic conditions allow abundant precipitation to occur far ahead of any low pressure. The setup with Helene was straight forward; a cutoff upper low (providing its own atmospheric lift) to the west along the Mississippi River in coordination with Western Atlantic Ridge will funnel any moisture from the Gulf or Western Caribbean due north.
As of late Monday, the forecasted position of the upper low/Atlantic Ridge funneled heavy rainfall toward North Georgia, East Tennessee and the Smokies of Western North Carolina beginning as early as Wednesday afternoon, with the North Carolina Foothills falling just east of the PRE event. Even though the region at that exact point was not highlighted for a lot of rain out ahead of the developing tropical feature, the trends were concerning to forecasters as a whole.
As local forecasters, we had the feeling that a significant flash flood event was becoming more likely. Foothills Weather Network issued a briefing that evening highlighting the increased confidence in a significant flood event somewhere along the Blue Ridge.
Coming Up: Part 3 of the series, we will talk about the further development of Helene as it finally becomes a tropical cyclone. Meanwhile continued trends and confidence begins to put into play the idea that Helene’s impact could have historical significance. Read more about that when we bring you the next segment in this series on Wednesday.