Two weeks ago, Hurricane Helene slammed into the Deep South and parts of Appalachia, bringing devastating floods to a region often spared from strong hurricanes. A variety of factors combined to make Helene an especially dangerous storm. First, it formed over the Gulf of Mexico, where waters tend to be warmer than the Atlantic. Second, the globe is currently experiencing a transition to La Niña conditions where unusually cold ocean temperatures in the Pacific bring about all sorts of effects on the weather.
Specifically, as it relates to hurricanes, La Niñas decrease what’s known as vertical wind shear over the Atlantic and Gulf of Mexico. Think of vertical wind shear as the difference in wind between the surface and higher altitudes. The reduced vertical wind shear we’re experiencing right now made it easier for Helene to grow into a large, coherent, vertical structure and for the warm, moist air fueling it to rise undisturbed.
The third factor that made Helene especially dangerous for areas far inland was the fact that it followed closely behind another rainstorm. While far less dramatic than Helene, the first storm dumped a lot of rain on the region that oversaturated the ground. After Helene made landfall, instead of being drained of energy, the wet surface continued to fuel the storm as it moved up through Georgia into the Appalachian mountains.
Finally, the Appalachian mountains themselves drove the storm upward to a higher altitude—which cooled the storm quickly. Colder air cannot hold as much water as warmer air, so the result was even more torrential precipitation in the mountainous western Carolina region.
Together, all these factors combined to bring about a storm that dumped six months’ worth of rain on the towns and villages of western North Carolina in a matter of hours. The buildings, roads, and ground itself were unable to handle it. Flash floods
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