Potentially Dangerous Vermont Weather Weekend: Severe Storms, Local Floods, Tornado?

Here's something you don't see every day:
NOAA's Storm Prediction Center has
placed a small but real risk of a tornado
or two squarely over Vermont Sunday. 

 The exciting weather continues here in Vermont and the rest of New England as heat has given way to repeated bouts of severe storms. 

That's going to continue through the weekend. Especially Sunday.  An outbreak of severe storms, localized flash floods and even possibly a tornado or two could strike the Green Mountain State tomorrow. 

More on that in a bit. 

First, let's set it up. 

Thursday night's cold front, as expected, stalled in southern Vermont Friday. That suppressed most of the thunderstorms southward. Though there were storms mostly south of Route 4 Friday, the severe ones just missed Vermont, instead hitting the Capital District of New York, and Massachusetts and Connecticut.

Social media also showed what appears to be a tornado in Harwinton, Connecticut. I'm sure the National Weather Service office in Albany, New York will be investigating that. 

TODAY

That front is now beginning to lift north as a warm front. It'll be far enough north to allow for a dangerously hot day on the East Coast as far north as Connecticut.  Here in Vermont, it will touch off showers and a few thunderstorms. 

Storms today shouldn't be severe, but a few will bring some locally heavy rain. There's a chance of isolated flash flooding, but it's not a huge issue. At least today. 

A band of rain was already passing through northern Vermont, well north of the warm front early this morning. More showers and storms could break out at any time, though it won't rain all day. You'll notice the humidity in the air is lingering, but at least it's not as bad as it was this past week.

Yet, anyway.   

More showers and storms will hit overnight as the warm front slowly heads toward the Canadian border. 

SUNDAY

Yesterday, we were waiting for data on where that warm front would stall. If it stopped moving over northern or central Vermont, then the main threat would be flooding from repeated storms moving along it, like those boxcars on the railroad tracks.  

A supercell thunderstorm near Sheldon, Vermont in
May, 2018.  This one was rotating, but did not
produce a tornado. It's possible Vermont could see
a rotating supercell or two Sunday. There's a low
but definitely not zero chance of a tornado or two.
Strong straight line winds are a greater threat.

If it stalled in southern Quebec, the bigger threat would be severe weather. 

As of this morning the consensus is the front will stall just north of the border.  That means we still have the threat of some local flash flooding from torrential downpours, but the main threat is severe storms, at least as it stands now.

The Setup

The conditions actually look right for the risk of rotating supercells. The air will certainly be extremely humid. That means plenty of fuel for those storms. 

The main storm system will be coming in from the west. That means temperatures will begin to cool very high in the atmosphere, further increasing instability.

An important ingredient is that warm front. Often, rotating storms are able to get going just south of this type of stalled front. Winds are from the southwest if you're south of the front. Winds are more easterly north of the front. 

This allows winds to change direction and increase with height.  That puts a "spin" in the atmosphere, helping to create supercells.

That's not to say all storms tomorrow will be rotating supercells. But the risk is there. That's why NOAA' Storm Prediction Center at least for now has us squarely in a zone where a couple brief tornadoes are possible. 

This is a pretty rare set up for Vermont and New England. The last time I  can recall a set up like this was in May, 2018.  No tornadoes ended up touching down in Vermont that day, but we did have a couple rotating storms and lots of wind damage. 

The Result

The chances of a tornado are low, but not zero. Basically, there's a 5 percent chance of a tornado forming Sunday within 25 miles of any given point in Vermont, New Hampshire, eastern New York,  western Massachusetts or extreme southern Quebec. 

I'm fixating on tornadoes, but by far the biggest threat from Sunday's possible severe weather is strong straight line winds. Not everybody will see that, either. But I almost guarantee we'll see at least a couple reports of damaging winds.

Worse case scenario, there could be a quite a number of spots that see gusts strong enough to knock down trees and power lines There could even be a rare report of roof or structural damage from the winds, we'll see. 

Torrential rains will come with many of these storms, so there could be local flash flooding, especially if a particular spot is hit by two or three storms tomorrow, and/or if a particular spot has especially  heavy rain today. 

I also expect lots of cloud to ground lightning with Sunday's storms. I'd postpone that Sunday boating excursion on Lake Champlain to another day. I'd also cancel plans for a hike in the mountains. 

There could be large hail with the strongest storms too, but the risk of that is pretty low.

The Caveats

Forecasters still have a lot of questions as to how widespread any severe weather will become tomorrow. If a lot of clouds, showers and garden variety thunderstorms remain over the area all morning and early afternoon, that would limit the extent of the severe weather. 

If the sun breaks out more than expected ahead of the storms, that would worsen them. If that front stalls out further north or further south than expected, that would change the forecast, too. It's also unclear at this point which part of Vermont would be most at risk for dangerous weather. 

So, there will be updates tomorrow morning, and likely changes to the forecast. But this is how it stands now. 

For now, I'd plan on keeping ear out for any warnings that might come through on Sunday, especially in the afternoon and early evening.