Nation's Flash Flood Season Is In Full Swing

A flash flood near St. Albans, Vermont in 2014.  A stalled thunderstorm
caused this one. 

Now that we are full on into summer, the peak of severe thunderstorm season has passed, but we're into what I regard as peak flash flood season.

We can and do often get severe storms and tornadoes in the United States through the summer. But big outbreaks of severe weather are now less common.  That's because there are no - or at least few - big storm systems with immense clashes of cold and hot/humid air to generate tornado barrages.

Instead, smaller weather fronts and weak low pressure areas meander sluggishly, or sometimes not move at all through much of the rest of summer.

There's usually plenty of very humid air to work with around these weak, slow moving systems. The result is torrential downpours.

If these downpours don't last super long, this is a good thing, as it keeps crops and gardens watered down, and tamps down the risk of wildfires. Often, though, you can get several inches of rain in just a couple hours. And these downpours sometimes tend to linger in one place, compounding the trouble.

Many of the culprits behind these flash floods, especially in the middle of the nation, are caused by something called Mesoscale Convective Systems, or MCS's. 

These are huge complexes of thunderstorms that regularly develop late in the day somewhere in the Plains or Midwest and continue on all night.

These things are very common in the summer and all things considered, have a lot of benefits. They keep the corn belt well watered during the critical growing season.

Some of the storms are really intense in these MCS's though. They can dump tremendous amounts of rain in a short period of time and cause almost daily pockets of flash flooding in the summer.

Yesterday, an MCS caused flash flooding east of St. Paul, Minnesota.  This morning, one is causing flash floods in Illinois. 

Other areas of flash flooding could develop today and tomorrow in parts of North Dakota, Kentucky,  and mountainous areas of Idaho and western Montana.  Flash flood risks are usually scattered in random fashion like this during the summer in the United States.

Slow moving cold or warm fronts, or stationary fronts, can also unleash flash floods. In these cases, thunderstorms move along the same path, one after another, over the same area like boxcars passing by on train tracks.

These are called training storms literally because they resemble a train moving down the tracks.

Weak low pressure systems can also cause cause flash flooding by generating showers and thunderstorms rotating over the same area. Early season tropical storms can cause similar problems.

Even areas that are drier than normal - and in drought - are prone to flash flooding this time of year. If it rains hard enough, creeks, streams and river can become overwhelmed with runoff, as can city streets where the storm drains can't handle the deluge.

Most of Vermont is now in drought or at least abnormally dry.  But an area of thunderstorms prompted flood advisories in some sections of southeastern Vermont yesterday.  And there's a marginal risk - very slight - of flash flooding today in much of Vermont today and tomorrow due to the risk of slow moving storms.

As is typical with most flash floods, any risk in Vermont would be limited to very localized areas, while surrounding areas just a few miles away stay relatively dry.

Climate change is probably worsening flash floods and making them more frequent.  A warmer atmosphere can hold more water vapor.  That means more water is available for downpours when something triggers a storm, such as a stalled front or MCS.

The resulting downpours are heavier than they otherwise would be, so the flooding is worse. And more frequent.

Vermont, like many mountainous areas, is especially prone to flash floods because a stalled thunderstorm over the Green Montains can send torrents of water down the slopes, ripping away roads, bridges and sometimes houses.

Also, our small urban areas can become flash flood zones during heavy thunderstorms.  Some nasty ones have happened in Burlington in July, 2012 and in Rutland in 2009.

At least some localized flash flooding happens in Vermont almost every year. Even dry ones.

So while we pray for continued rains this dry summer, let's just hope it won't be too much of a good thing.