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| Flooding in Ellenburg, New York Sunday. Photo by Peter Visconti, via WPTZ. |
Rounds of flash flooding hit the small communities of Ellenburg, Chazy and Bellmont. Fortunately, I'm not aware of any serious injuries, but people were evacuated from campgrounds, numerous roads are washed out and flooded, and sheds, decks and propane tanks were seen being swept away.
This was all highly localized. Plattsburgh, New York, less than 20 miles from parts of the flood zone, received just 0.68 inches of rain Saturday night through Sunday morning.
The flooding came in two parts. Saturday night, it was "training" which might be familiar to readers of this blog. It's when heavy showers and thunderstorms go over the same spot, like boxcars on railroad tracks.
This first round inundated a camp ground in Ellenburg, and washed out several roads in the area.
But we weren't done yet.
New storms fired up late Sunday morning and continued through the afternoon in something called "back building."
I guess the way to explain it is to first come up with a more common scenario than back building, then show the difference.
Let's say you're spending a warm, very humid day in a lovely Vermont town in or near the mountains. Say, Montgomery, or Stowe, Warren, Shrewsbury, Weston or perhaps Wilmington.
At mid-afternoon, a big thunderstorm fires up just to your west and moves in. It's a real gullywasher, dumping a good inch of rain within an hour. Then it moves on. The town road crew might have to patch up the edges of a couple dirt roads, but at least the garden got watered and you can now enjoy a a calm, balmy summer evening.
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| Schematic on how back building thunderstorms work, via Weatherboy.com |
With back building, that first storm fires up to your west. Just as the first piece of it starts to move on, the back edge of the storm grows and expands, and moves in. And it keeps happening, over and over again.
Through some alchemy of stalled weather patterns, a patch of instability, the terrain and plain dumb bad luck, the storm just seems to sit there, re-forming in its original spot, and dumping inches and inches of rain.
The initial storm falls apart as it moves downwind, but a series of new storms form over the same area and keep repeating the process.
That's what happened in a narrow area of northern New York Sunday.
These incidents usually cover a small area, as we saw Sunday in northern New York, The storms weakened as they moved past the flood zone, so places downwind in their path, like the northwestern tip of Vermont up by Alburgh got some rain, but no big deal.
The initiation point of the new storms during the day Sunday gradually moved southwestward. The torrents hit the Ellenburg area first, then moved west toward Bellmont.
Finally, in the late afternoon, the back building storms moved further southwestward into a drier area while weakening. The heavy rains ended.
Back building storms are relatively rare, but do happen occasionally every spring and summer anywhere. They've certainly happened in Vermont.
This type of storm is most likely in July or August, when the overall atmosphere is sluggish and local effects can combine with minor weather disturbances to create back building.
It's another good reason to have a very good app with weather radar, so you can track this sort of thing when it happens.
Today, there is a risk of torrential thunderstorms in one or two spots in Vermont and New York. Any problems should be pretty localized.
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