Morning Flash Flooding

An area of training thunderstorms this morning put down nearly 4.5″ of rain in Salem over the course of 2 hours. An unstable atmosphere coupled with a weak area of convergence just north of the Sound lead to an impressive burst of Monday morning rain.

Here’s the doppler radar estimate for precipitation this morning. The yellow is >2.5″ of rain and the red is >3.5″ of rain. Our spotter in the southern part of Salem reported 4.5″ of rain.

While that was a lot of rain in a short period of time we didn’t receive any reports of serious flooding. The biggest reason was that the area of heaviest (4″+) rain was relatively small geographically.

Here’s the river stage hydrograph on the Eightmile River on the East Haddam/Lyme border.

While the spike is impressive it’s well short of the record stage of 11.12 feet from June 1982 or even other high crests in the last decade. This map of the watershed from eightmileriver.org shows why the flooding on the Eightmile wasn’t more substantial.

The river gauge is located on the main stem just upstream from the confluence with the east branch. The water flowed through the gauge had run off from all of the eastern half of East Haddam and the southern part of Colchester. While over 4″ of rain likely fell at the gauge site itself most of the watershed upstream of the gauge saw substantially less rain – on the order of 1.5″-3.5″. Had 4.5″ fell across the entire basin upstream from the gauge at North Plain the flooding would have been more significant.

When predicting flooding in a given location on a river it’s all about how much rain has fallen in an entire basin – or at least the portion of the watershed upstream from you! For example, in the March 1936 Connecticut River flood there really wasn’t any notable amount of heavy rain or snow melt in Connecticut. All the rapid run off was upstream in New Hampshire and Vermont and that was enough to push the Connecticut River well out of its banks and through the city of Hartford.

I poke fun at our weather watcher Steve in Moosup for his hydro fetish. He’s affectionately known as the Sultan of Sandbags – so I hope he enjoyed this post 🙂

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