What’s The Matter With The Polar Vortex?
The past week brought light to moderate snows to much of the West, including some much needed snow in the Sierra, a region that has missed out on storms for much of this winter. A three-day storm last weekend was very productive from the snow belt areas downwind of the Great Lakes through the higher peaks of northern New York and northern New England. Stowe, Smuggler’s Notch, and Jay Peak in Vermont were the lottery winners, with over three feet of snow piling up. A good friend and Stowe skier told me that every turn in the woods last Saturday produced a “face shot” of powder! My favorite resort in the world, Mont Sutton in Quebec, just over the border from Jay, had a three foot dump, as well, and their phenomenal “su bois” (glade) skiing was the best of the season.
While most of the West has enjoyed frequent snows through the heart of the winter, the eastern half of the country has had trouble lining up storms on favorable tracks, and snowmaking temps have been spotty, especially south of I-90. There have been several major snows across the far north, and at elevation, but even in those areas, mixed precip events have also passed through. The main problem has been pretty simple: a lack of deep cold air masses AND a lack of staying power of the cold air that managed to push south. In this piece, I thought I would touch on the reasons why cold air has been hard to come by this winter.
For most of the season, a low pressure center at the upper levels of the atmosphere has been sitting over the western portions of the Arctic. The Polar Vortex typically meanders around the high latitudes, and occasionally weakens and breaks down, and that is when chunks of very cold air migrate southward into the mid-latitudes, often supporting snowstorms in the process. Occasionally, the vortex moves bodily southward, and that is when we see the coldest air reach the U.S. This winter, the PV has been remarkably persistent in its size and position, and that has greatly limited the amount of available cold air. Here is a map of the jet stream showing the current PV.
So, cold air has been scarce and when it has managed to find the eastern half of the country, it has seldom lasted more than a couple of days before milder air returned. The lack of staying power is related to the lack of a blocking pattern over the North Atlantic. A high pressure ridge over Greenland does a nice job of buckling the jet stream over the U.S. in winter, allowing cold air masses to hang around and help produce multiple snow events. That ridge has been missing much of the winter, resulting in a flat jet stream flow across the country, which has tapped milder Pacific air and pumped it into the pattern. Here is a diagram to illustrate.
Notice there’s a low pressure center near Greenland, not the usual high pressure ridge. The counter-clockwise circulation around that feature helps sweep any cold off the east coast and out to sea. There are signs that the flow may be buckling more from mid-month on, which would help generate late snows and sustain the eastern season into April. We shall see.
Here Are The Regional Details.
Northwest U.S./western Canada: Trough offshore brings moderate to heavy snow into next week. After a couple of dry days, a stronger trough comes ashore with heavy snow later next week. The epic season continues.
Sierra: Trough digs down the coast from the NW over the weekend, producing light to moderate snows. Dry early next week, with southern branch trough moving inland late next week, bringing significant snow to the southern Sierra.
Northern Rockies: Weakening troughs produces light snow over the weekend into early next week. Next shot at major snow late next week from northern branch system.
Central and southern Rockies: Ridge aloft early with mild temps. Light snow later this weekend. Next significant storm late next week.
Midwest: Mild weekend with surfaces softening. Lower Lakes looking at potential snow event later next week. No lake effect events in next week.
Northeast/Quebec: Nice weekend on the slopes. Next potential storm mid to late next week, likely another elevation event.
