Foliage Stripping Vermont/New England Storm To Cause Bit Of Flood Risk

This rainfall forecast map from the National Weather 
Service office in South Burlington predicts more than 1.5
inches of rain from the upcoming storm, with local
amounts to 2 inches. 

You'll notice the wind picking up in Vermont today. It was already gusting to 20 mph right near Lake Champlain as of 9 a.m.  

That's the first sign of a pretty powerful storm that will give us a blast of wind, some heavy downpours and take down lots of autumn leaves.

As clouds increase today, it's your last chance to enjoy the Vermont foliage unless you're in the Champlain Valley and warmer southern valleys. 

That fact that so many leaves will come down brings a unique problem. The fallen leaves could contribute to local flood problems. 

We're expecting anywhere from 1.5 to 2.5 inches of rain out of this storm in Vermont between tomorrow and Friday.  Since it hasn't really rained much over the past couple of weeks, we wouldn't normally expect a flood threat out of this storm.

In fact, the main rivers in Vermont are pretty low, so this storm won't cause any problems on waterways like the Winooski, Lamoille, Missisquoi and Otter Creek. They water levels in those rivers will go up, but won't spill over their banks.

The problem, in fact is the leaves. Before the rain even gets here Thursday afternoon and evening, wind gusting to 20 to 30 mph for most of us, and 45 mph in some areas, will take down a lot of leaves.

When the heavier rain starts Thursday evening, all those leaves can easily clog storm drains, causing urban flooding. Since many storm drains normally work well when not clogged by autumn leaves, some urban flooding might occur in neighborhoods that seldom if ever have trouble with street flooding. 

A lovely autumn view Tuesday from atop a hill in
Richmond, Vermont, looking west toward the Adirondacks.
A windy storm with heavy rain Thursday night will
take down many of these leaves, and create the trheat
of local flooding. 

Along small brooks and streams, sticks and branches off lie across them.   A multitude of leaves that fell will float downstream from the storm runoff and get stuck against those branches.  The leaves compressed against these obstacles can create some surprising strong little dams.  

Water collects behind the mini-dams. The pressure gets too much and breaks through, causing a surge of water that breaks through other leaf dams down stream.  The result is you can get a localized, brief flash flood as the water from those broken leaf dams surges downstream.

This won't be a widespread problem, but I can envision it happening in a few spots. NOAA's Weather Prediction Center has us in a slight risk zone for excessive rain and a flash flood chances because of this. 

The flood risk, low as it is, will reach its crescendo later Thursday night. Along and just ahead of the front, they will probably be a narrow band of especially heavy rain, with rainfall rates going up as high as one inch per hour. Luckily, the band of torrential rain won't last more than an hour in any given location, but still, it can cause issues with all those fallen leaves. 

On the bright side, we needed some big autumn rains this year and so far, we're getting them.  Most of Vermont was in drought or abnormally dry through much of the summer and early autumn.  

September turned out to be quite rainy in the Green Mountain State, which eased the dry conditions quite a bit. Still, as of last week, the U.S. Drought Monitor was still detecting a continued need for additional rain in southern and central Vermont especially.

This upcoming storm will put another dent in that problem.

Additional rain is expected early next week, but it won't be as heavy as Thursday night's storm.