What's With Vermont's Balmy Winter Nights?

Jackson the Weather Dog out after dark Friday evening
measuring the meager snow cover and checking out
our little-used snow shovel. The lack of snow cover
has helped lead to warm nights, relative to normal in Vermont.

 Friday morning was the chilliest so far this winter in Burlington, Vermont, dipping down to 13 degrees.

  But chilly is an awfully strong word to describe that temperature.  

Considering that temperature is just about normal for a time of year when temperatures have historically gotten into the 20s below zero.   

Simply put, we haven't had any truly cold mornings this winter.  And won't for a little while at least. 

Remarkably, it hasn't even gotten even into the single numbers above zero in Burlington, never mind below.  I don't recall seeing such sustained warmth so deep into any winter.  

WARM WEATHER PATTERN

There's at least a couple reasons for this dearth of cold nights so far. The first is the general weather pattern, which has been really, really persistent all winter so far. Most of the time, air is making a cross country trip from the Pacific to the Atlantic.  

This is preventing frigid air from making its way down from northern Canada.  It's locked away near the North Pole, kept there by a strong polar vortex up there keeping the truly frigid air close to its heart.  

This state of affairs isn't necessarily permanent. It never is.  

As I've previously noted, some, but not all long range forecasts have the polar vortex weakening or stretching out into an elongated mess.  If that were to happen, it would be easier for bitter air to head south toward the United States. 

The latest long range forecasts (keep that grain of salt handy) have the polar vortex doing that weakening thing, shifting and allowing frigid air to plunge into the western and central parts of Canada and the United States. This would start this coming Wednesday or Thursday. 

This should lead to a volatile weather pattern featuring intense cold in the northwestern United States and lots of storminess across most of the Lower 48.

We should expect a fair amount of storminess in Vermont out of this, but it remains to be seen whether any of that bitter air makes it as far east as New England. We shall see!

NO SNOW COVER 

Another big reason for the warm nights is a lack of snow cover. On any clear night, snow cover helps reflect any lingering heat back up to space, so temperatures crash. If there's a fresh, deep snow cover on a clear, calm winter night, temperatures would really hit rock bottom.

A 2017 article from meteorologists at WCCO in Minnesota helped explain this further. 

"Snow gives out what's called long-wave radiation. Essentially, it's letting go of whatever heat it nay have and cools the air even more. This is especially true with fresh snow."

Bare ground exposed the air to the warmth of any moisture in the soil. Which means if there's no snow on the ground, it's harder to get colder at night. 

If the forecasts are right, most of Vermont should finally see a decent snow cover by Sunday afternoon.  In the short term, though, that won't lead to any particularly cold nights. Southern Vermont might get a little chilly - single digits above zero, if skies clear Sunday or Monday nights. 

Otherwise the weather pattern has featured cloudy nights, which tend to be warmer. That'll be the case heading into midweek. Plus, the parade of storms is, for now, pumping mild air our way, keeping nighttime temperatures warm. Despite the snow cover, which might end up melting Wednesday depending on how warm it gets during the day,

CLIMATE CHANGE TRENDS 

 Though there are a few years thrown in with a higher number of frigid nights, the trend in subzero temperatures in Vermont is downward, due largely to climate change.   . 

Take last winter for example.   The temperature never got below zero until February 1 in Burlington. It was just one of  small handful of winters that lasted that long without a first zero of the season. 

 I don't know how long we'll last this winter without a zero or below, but this isn't your grandparents' winter. They would have froze their bloomers off on many mornings by the first week in January. 

In the past, most years had their first zero in Burlington sometimes in December, or perhaps early January in the warmest years. 

Only one winter in Burlington, 2001-02 stayed above zero all winter. I doubt we'll repeat that winter, as subzero nights have historically happened as late as March 29 in Burlington. It's still awfully hard to get through a winter without at least one spell of frigid Arctic air. 

But there's still no sign of any major cold for at least a week around here in New England.