'Upsloping rainfall' with Helene was a major factor in extreme flooding

Several factors created a "perfect storm" that caused flooding the likes of which the Southern Appalachians hadn't seen in over 100 years.

The incredible rainfall amounts reported ahead of and during Hurricane Helene in the southern Appalachians during late September were enhanced by a process known as "upsloping" and helped lead to extreme flooding that killed dozens of people.

"There were a number of factors that came together to produce the extreme rainfall that began Wednesday night and Thursday, Sept. 25," AccuWeather Chief On-Air Meteorologist Bernie Rayno explained.

One significant factor was what meteorologists call "upsloping."

A cold front had stalled across the Southeast and produced over 6 inches of rain in the mountains before Helene even arrived—something that may have been a "predecessor event," meteorologists say. Predecessor events bring heavy rain inland before tropical storms and hurricanes and are a new type of phenomenon that meteorologists are just beginning to research.

Next, fueled by rich Gulf of Mexico moisture, Hurricane Helene was already producing heavy rain as it began to move over the same area.

As Helene tracked into Tennessee—not a typical track for a tropical storm—tropical air was forced to move up the mountains. When air is forced upward, it gets increaingly moist and causes rainfall to intensify. This "upsloping" setup made the rain heavier than if Helen had tracked over flat land.

The mountainous terrain in the southern Appalachians is known for causing sudden flash flooding. Walls of water that move downstream quickly and are hard to escape from quickly became deadly. Once roads were washed out over a wide area and took utility and communication infrastructure with them, people were cut off from travel, relief organizations and medical assistance.