Christmas 2020: Strong Winds, Flood Risk And A Flash Freeze

One model run for early Christmas morning shows
a sharp cold front in western Vermont. Low 50s from the
Greens east, upper 20s far west. Timing of the cold front
and its intensity are still in question.

Details are slowly coming together on an expected nasty Christmas Eve and Christmas Day storm here in Vermont and the rest of the Northeast, but timing questions are huge.  

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Forecasters are pretty sure we're in for a good dousing of rain, some strong, potentially damaging winds and something known as a flash freeze, which I'll get into in just a bit. 

 There will probably be some snow with this, too, but just how much is really up in the air.  

Here's what's going on: A parade of storms has been smacking into the Pacific Northwest, dumping a lot of rain out that way, which is pretty typical for winter.  

These storms, most of them in somewhat weakened and definitely dried out form, are eventually zipping through New England with little fanfare, just some light precipitation. 

One came through yesterday, another goes by a little to our south tonight and tomorrow, no big deal. 

However, one of these Pacific storms has a lot of energy with it.  Plus, a deep dip in the jet stream will form as this storm gets going.  

This will allow deep subtropical and warm air to fly north on strong upper level winds ahead of the storm's cold front.

This will unleash what looks like quite a bit of rain on us Christmas Eve day and night. Storm totals could easily go over an inch. 

That deep snow in southern Vermont will rapidly melt with the warmth and rain.  Flooding is definitely a possibility.  

Already, some river forecasting models depict some flooding along the Otter Creek and Mad River, and it's likely most other rivers will rise. 

Most of the trouble with this thaw and rain will most likely be small stream, street and basement flooding. This would literally put a damper on Christmas celebrations, already seriously cut back due to Covid. 

Remember those strong upper level winds I just mentioned that are bringing that warm, wet air to us?  Some of those winds could mix down to the surface, though it's unclear to what extent that will happen.

The rapidly diminishing snow cover in southern Vermont might, just might help create a temperature inversion that would help keep the strongest winds from reaching the surface. But we're not sure.

At this point, the western slopes of the Green Mountains and the Champlain Valley have the best chance of getting slapped by strong, potentially damaging wind gusts. This would be mostly a tree and power line issue. Again, something you don't want to have at Christmastime. 

We could see temperatures get well into the 50s Christmas Eve night. That would get us close to record highs most days this time of year. Except Christmas Eve is the anniversary of a bonkers 2015 holiday heat wave that brought Vermont temperatures as high as 70 degrees. So no record highs that day. 

I suppose there could be readings close to record highs for Christmas Day in the early morning before temperatures fall.

Incredibly, what seemed like a lock for a white Christmas in southern Vermont, which got two to three feet of snow in many areas last week, now looks questionable.  That will depend on the timing of the storm's cold front. 

The cold front on Christmas Day looks to be a real humdinger. I picture a line of heavy downpours just ahead of it, followed abruptly by crashing temperatures and a changeover to snow.

This is that flash freeze I referred to.  All that water from the rain will instantly freeze on roads and sidewalks, presenting a hazard to anyone driving or walking. 

The rain will change to snow with the passing of the front.  It's unclear how much snow will fall, as forecasts are all over the place. It depends mostly upon the timing and strength of a new storm riding northward along this strong cold front. 

Computer models are all over the place, because they don't have a handle on the situation yet. According to this morning's forecast discussion from the National Weather Service in South Burlington, one model has Burlington receiving somewhere between a half inch and 18 inches of new snow on Christmas Day.

I'm guessing the low end of that range, but that's just a guess right now. 

The timing of the powerful cold front is a question mark, too.  It could enter western Vermont before dawn and sweep right through, or it could wait until during the day.  Which means right now, it's impossible to know what conditions will be like when you wake up Christmas morning.  It could be in the 50s and raining, it could be way below freezing with snow and blowing snow outdoors.

Again, the models are all over the place.  One run of computer models has the temperature in Burlington at around dawn somewhere between the upper single numbers to low 50s. Yeah, no help there. 

Whatever happens, the wind will continue to crank as temperatures fall during Christmas Day.  

We might not yet know the timing or the intensity of this year's Christmas storm. It's 2020, after all, anything can happen.

Just plan on rough weather outdoors.  It will be yet another good excuse to heed the warnings of the Vermont Health Department to just stay home with your immediate family and not venture out to somebody else's house. 

This would prevent the spread of Covid, and also prevent you from getting trapped or in a crash on bad, icy or flooded roads.