Sunday, June 18, 2017

Sunday Update On Severe Storm, Flood Threat In Northeast

Aftermath of severe flash flooding in Barre, Vermont in
May, 2011. There remains a flash flood risk in Vermont
and most of the rest of New England Monday, but let's
hope it doesn't get as bad as in this photo. 
Flash flood watches are flying for a big area of the Northeast today through Monday as a slow moving batch of severe storms with torrential rains still seem like a great bet.

The flood watch zone extends through much of New York, and from New Jersey up through all but southeastern New England.

This potential flood alert zone includes northwestern Connecticut, western Massachusetts, all of Vermont, all but eastern New Hampshire and the all of Maine except a zone within roughly 40 miles of the coast

Not every town in this zone will get a flash flood today or tomorrow, of course, but some storms will repeatedly bulleye some towns and regions, resulting in flash floods.

Yesterday, I noted there would be some adjustments to the forecast position and timing of the strongest storms and sure enough there are.

Broadly, the forecast is on track. NOAA's Storm Prediction Center has at least a slight risk of severe thunderstorms today along the entire western half of the Appalachian chain.

We're still expecting strong and severe thunderstorms to develop in New York State this afternoon, but they'll move a little faster toward the east than I thought yesterday.

This puts Vermont in play for severe thunderstorms later this afternoon and evening, especially the western half of the state. (Had the storms come through later, after daytime heating had ended, the storms would have been weaker.

The Storm Prediction Center has nudged the risk zone of severe thunderstorms into Vermont.

The upshot is across northern New York and Vermont, there could be some strong, damaging winds and torrential rains with the storms this afternoon and evening. The best shot of severe weather in Vermont is probably toward evening.

The only thing that could limit severe weather is that temperatures don't cool all that much with height. (That's called the lapse rate and would increase instability in the atmosphere.)  That could get in the way of a lot of storms forming, which means this evening wouldn't be that big a deal.

But don't count on such a reprieve.

As the National Weather Service in South Burlington, Vermont notes,  there are plenty of examples when conditions are like today's: A so-so lapse rate, but relatively strong upper level winds, hot and humid conditions and an incoming weather disturbance that have touched off a lot of severe storms. So beware.

And we ain't done after this evening.

I broader chance of severe thunderstorms exists in New England Monday.  The flash flood threat rises Monday afternoon almost regionwide, too.

If enough sun breaks through for a time Monday morning, the storms in Vermont and eastern New York could quickly become severe by afternoon. At this point I think the biggest risk of severe thunderstorms, mostly with damaging winds, would be across the eastern half of Vermont, western New Hampshire and western Massachusetts.

I actually wouldn't be totally surprised if there were a brief tornado spin up in western or central Massachusetts.

The broader threat remains localized flash floods on Monday. The air in the Northeast has far more moisture in it than normal, and the deep moisture extends high up into the atmosphere.

That means any storms that do form could contain torrential rains that come down at a rate of one to two inches per hour.

Here in Vermont, the threat of severe storms might diminish during the course of the afternoon, but there will probably still be thunderstorms around with those torrential downpours.

Since the parent cold front is moving so slowly and the air flow is parallel with this weather disturbance, we'll get into that dreaded "training" that I've referred to in past spells of flash flood threats.

As a reminder of what training is, thunderstorms often form into a line. These lines usually move generally west to east perpendicular to the orientation of the line. That means any one spot would only get a thunderstorm for an hour or less. No biggie.

When training happens, thunderstorms within the line move parallel to that line, which often stalls out. The effect of this is one thunderstorm after another moves over the same spot, like a long line of box cars on railroad tracks.

This can last for a few hours, and can result in six inches or more of rain falling in that time, leading to a local flash flood.

Some of Vermont's worst recent flash floods, including a May, 2011 disaster in and around Barre and the epic flash floods during the summer of 1998, were brought on by training thunderstorms.

There's no guarantee there will be any training box car type thunderstorms in Vermont or the rest of New England Monday, but it's something to watch.

Things start to calm down Tuesday, though the threat of heavy rain and flash flooding might linger over eastern New England.

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