Flash Flood In A Drought? Happens Often, Actually. Even Here In Vermont

Flash flooding in Burlington, Vermont back in 2012. Very localized
flash flooding can occur even during very dry conditions,
if the downpour is torrential enough. 

I've been whining since at least May about how dry it is here in Vermont and surrounding areas.

Despite a bit of recent rain, the southern half of Vermont is in a moderate drought, while the northern half is abnormally dry, not quite a drought, according to the most recent U.S. Drought Monitor. 

So why does the subject of flash flooding keeping popping up during this weather regime?

The reason: Tis the season for local downpours. It's the summer doldrums, when the humidity is highest and scattered thunderstorms often move very slowly, or sometimes not at all.

The best way to end a drought or a dry spell is to get an area wide, steady, soaking rain.  A torrential downpour will mostly just run off.

We've seen that a few times already this summer in Vermont.  The more intense thunderstorm rains can overwhelm the drainage in a particular area pretty quickly.  Plus, if the thunderstorm parks itself over a particular spot for an hour or two instead of moving on as usual, the rain can get pretty extreme.

We saw some flash flooding around Starksboro in late June from a stalled thunderstorm while surrounding towns remained dry.  I spotted some minor flash flooding in downtown St. Albans during last week's thunderstorms.  A flash flood warning went up very early Sunday morning for a small area of the Adirondacks due to a pocket of heavy rains.

You'll get a situation where it  rains so hard and so fast in one town that there's washouts and high water on small streams.  Meanwhile, two towns away, people are watering their dusty, wilting gardens.

The reason I bring all this up today is that, interestingly, the National Weather Service's daily Excessive Rain Outlook puts almost all of Vermont and New Hampshire, northern New York, western Maine and central New England in a slight risk zone for flash flooding over the next 24 hours.

A slight risk zone means there's a 10 to 20 percent chance of rainfall heavy enough to trigger flash flooding within 25 miles of any point within the slight risk zone.

These risk levels are akin to severe thunderstorm risks. Both are a five point scale going from marginal, to slight, to elevated, to moderate to high risk. So Vermont is at level two of a five point scale later today and tonight.

As the slight risk implies, there might well be no flash flooding. Based on current forecasts, there might be a few areas in Vermont that actually get  no rain at all between now and tomorrow.

Most of us will see some showers and storms as an upper level disturbance moves slowly into the region.  Moisture will feed into this from the southeast, forecasters are telling us. Also, an upper level disturbance consists of a pool of colder air aloft, so the temperature contrast will increase between ground level and several thousand feet above of us. That encourages showers to form.

If any locally torrential rains develop at all, I bet any instances of that would be isolated at most. The forward motion of the showers and storms will be slow, which is why there's that slight risk guidance on flash floods.

While the the National Weather Service's  U.S. level guidance has that slight risk of a pocket or two of flash flooding around here, the local office of the National Weather Service in South Burlington is less bullish with the idea.

The local office definitely has showers and a few isolated thunderstorms in the forecast today thruogh Tuesday, there's no mention of torrential rains. The South Burlington office of the NWS in this morning's forecast discussion correctly describes the expected showers as beneficial, as we definitely need the rain.

The bottom line: Most of us should see some rains in the next 24 hours that won't amount to anything too spectacular.  There's a low risk somebody could get unlucky with too much water.