The Season Of The Flash Flood

Flash flooding  covers a road near
Jericho, Vermont on July 4, 2013.  

In much of the nation, we've entered the season of the flash flood.  

Yes, flash floods can and do occur any time of year in almost any part of the country. But summertime, especially from now on into September, is when you are at most risk of such floods.

Right now, there are flash floods, or the threat of such, going on in parts of the country. The area in and around Memphis, Tennessee is having big problems this Sunday morning as torrential thunderstorms fill underpasses and creeks with water.

In the Upper Midwest and northern Plains, soaked by repeated rounds of bad storms since May, more thunderstorms threaten renewed flash flooding over the next few days. 

True, it's the hottest, sunniest part of the year in many parts of the country, too. So things dry out fast. So how do you get floods?

Easy. Weather systems move more sluggishly than they do in the colder months. A collection of thunderstorms might move slowly over a particular area, or not move at all. The result is inches and inches of rain, and a flash flood.

We had that here in Vermont last summer, several times during June and the first part of July. It was a very wet early summer, and slow moving storms causing several destructive flash floods in different sections of the state.

The hot, often humid air of summer can hold a LOT of moisture. A thunderstorm or complex of storms can collect much of this wetness and dump it onto the land below, and there you go, another flash flood.  

Another motorists takes a chance with
a flash flood in Richmond, Vermont in June, 2013.  

In the southwestern United States, the heat causes the air pressure to fall. Gradually, that draws in moisture from the Pacific and Gulf of Mexico to places like Arizona and southern Nevada, and surrounding areas.

This moisture sets off thunderstorms. The desert landscape can't hold onto the water that pours from these storms, and yes, you get a flash flood.

Finally, later in summer, the tropics get active. A tropical storm or hurricane moving inland can drop huge amounts of rain and cause a flash flood. In fact, inland floods are arguably the biggest threat from tropical storms and hurricanes.

Where I am in Vermont, we're too familiar with this phenomenon, too. In August 2011, Hurricane Irene moved into New England. By the time it got into Vermont there wasn't much wind, but boy was there rain!  It turned into a chaotic flood, easily among the Top 5 natural disasters in the Green Mountain State's history.

These tropical storms, or more accurate, their remnants, can set off floods a long, long time after they've seemingly dissipated.

In 1995, Tropical Storm Dean hit Texas with little fanfare. An area of disturbed weather, the remnents of Dean, slowly crossed the center of the country, where if finally collided with a cold front over northern Vermont.

The result was a huge, destructive flash flood, especially in the Lamoille River basin.

Flash floods, as is obvious, come on fast. If you see water going over a road, or any kind of flood threat when you're out and about, remember that hoary but true National Weather Service admonition: Turn around, don't drown.