Cut-Off Lows, Atmospheric River And Warming All Helped Drive 5-10" Rainfalls Across Southeastern US This Week

[hatrack]

The Potomac River floods downtown Westernport, Md., after extreme rain on May 13. Credit: Ricky Carioti/The Washington Post via Getty Images

After early signs of drought from Georgia through Virginia this spring, a slow-moving storm brought flooding rain to the Southeast this week. Between 5 and 10 inches of rain fell in parts of Georgia, the Carolinas and Virginia. The Potomac River, from West Virginia to Washington, D.C., flooded, and like many of the rivers between there and Georgia, their levels will remain high through the weekend.

The multi-day rain came from a system that formed in the Deep South, but understanding how it produced so much rain requires looking higher in the sky. The movement of air 3-4 miles overhead, at the jet steam level, governs how storms develop and move. Usually, that air moves west to east across the country with a few smaller waves nudging north and south as it flows east. In most cases, it brings periods of rain for about a day and then exits.

EDIT

In addition to the cut-off low, a deep pipeline of moisture was pulled northward from the eastern Gulf of Mexico into the Southeast. This atmospheric river of moisture is not a new phenomenon, but its name is, having been adopted into the meteorology canon after being first proposed in 1994. Similar to rivers running along the ground, atmospheric rivers are narrow areas of racing water, but in this case, water vapor is being moved rather than liquid water. These firehouses of moisture come in many sizes, but on average, they are 1,200 miles long, 300 miles wide, and 1-2 miles deep. As a result, they move far more water than any river along the ground: The strongest atmospheric rivers discharge water at more than 10 times the rate of the Mississippi River.

EDIT

Precisely how much more rain can be traced to climate change requires a more formal review, but there are similarities between this event and a multi-day heavy rainfall in April that impacted the middle Mississippi Valley, where the Ohio and Tennessee rivers also converge. An analysis of that event from scientists at World Weather Attribution indicated that there was a 9 percent increase in rainfall intensity due to the warming climate. Projecting forward on the current path of planetary warming by 2100, the chance of a similar flooding rain would double, and its intensity would increase by an additional 7 percent.

EDIT

https://insideclimatenews.org/news/16052025/atmospheric-river-southeast-floods/