Rash of Flash Floods Fueled by Slow-Moving Storms

(June 15) -- Flooding is the most common of all weather disasters in the United States, and flash flooding -- an intense flood that occurs suddenly -- is the nation's leading weather-related killer. All states are susceptible, and an average of 150 Americans die in flash floods each year.

While there have been many cases of flash flooding so far this spring, the three most dramatic -- the Nashville flood in early May, the Arkansas flood of last week and the Oklahoma City flood on Monday -- were all caused by the same overall weather factors, just in different locations on different days.

In each case, a slow-moving storm system combined with abundant moisture associated with a southerly flow of air from the Gulf of Mexico to produce waves of intense thunderstorms over the same area during a relatively short period of time. The result, in each instance, was flooding so intense and so sudden that it couldn't be avoided by residents and outdoor enthusiasts in the same way that a slow-rising river can be avoided.

The Nashville-area flood of May 1-2, which affected parts of Kentucky, Arkansas and northern Alabama in addition to much of Tennessee, was the result of a deluge of thunderstorms during a two-day span, fed by a slow-moving storm system from the west and one of the first tropical surges of moisture from the Gulf of Mexico this spring. Parts of the region received 3 1/2 months' worth of rain in one weekend.

The storm is estimated to have been a once-in-a-millennium event for much of Tennessee and is believed to have caused at least $1.5 billion in damages in Nashville alone.


(Image courtesy of the National Oceanic and Atmospheric Administration)

Last week, a slow-moving storm tapped into very humid air to produce two rounds of thunderstorms in western and central Arkansas on Thursday night, followed by another round of thunderstorms from later Thursday night into the early morning hours of Friday. Twenty-four-hour rainfall totals exceeded 6 inches in northeastern Texas and a large portion of Arkansas, including the Albert Pike Recreational area, where 20 people were killed.

On Wednesday, this storm had produced 10 inches of rainfall in the Hill Country of Texas (to the northwest of San Antonio), resulting in serious flash flooding.

A new storm system repeated the process in Oklahoma on Monday and Monday night. Slow-moving thunderstorms in a very humid air mass moved over the same area, resulting in over 8 inches of rain in the Oklahoma City metro area. Much of the state was pounded with tremendous rain, resulting in numerous flash flood warnings throughout the day and evening. It will most likely be a couple of days before the extent of damage can be determined.