Hint: Cold Air Coming Your Way.

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I assume that this week’s headline has many of you scratching your head wondering “What in the world is a stratwarm?” I will try and explain but the bottom line is that this phenomenon is in large part responsible for the colder than normal pattern that covers much of the country, much to the delight of snow lovers. The past ten days or so have brought fresh snow to just about every resort in the country, and the coming two-three weeks look very promising, as well.

Okay, what is stratwarm? The technical term is Sudden Stratospheric Warming (SSW), and it is an almost exclusively northern hemispheric winter event that typically occurs in about six out of every 10 years. Unusually warm air develops in the top 10 per cent or so of the atmosphere (the stratosphere) over the high latitudes. The warming works its way down in the atmosphere and then reverses the winds circling the North Pole from westerly to easterly. Those winds weaken as well, which disturbs the polar vortex. If you imagine the strong westerlies around the pole acting as a dam of sorts, keeping the coldest air trapped at the top of the world. When the wind shifts direction and weakens, the dam breaks and cold air heads southward to the mid latitudes. Here is a map of the current temperature anomalies at the 10 millibar level of the atmosphere, right near the top

Notice the orange/brown swatch draped over the North Pole. The warm air is in place and has been for a couple of weeks. Stratwarm episodes take weeks to develop and mature, and this one started in late December. You may have heard that both Europe and China experienced severe cold outbreaks last month as pieces of the vortex broke loose and reached the mid-latitudes. The next thrust of cold is now aimed at Canada and the U.S. It will first move into the northern Plains next week before spreading east-southeastward. Here is a forecast of surface temperature anomalies for Monday, the 8^th

The greens/blues/purples indicate where temps will be below normal, with the core of the coldest air moving into the Dakotas. A week later, the cold has pushed southward to envelope most of the lower 48. Have a look.

Now, cold air doesn’t always lead to snow, but with an upper trough likely to cover much of the same area as the cold mid-month, the likelihood of significant snow events from the Rockies to the east coast will be enhanced. Resorts west of the continental divide and along the west coast will be more influenced by upper ridging over the eastern Pacific, so snow events will be harder to come by, except in the Pacific NW, where a typically snowy La Nina winter will carry on. Stratwarms can cause some uncomfortably cold days on the slopes, but this one will also lead to a cold and snowy February for a majority of the slopes we love.

REGIONAL HIGHLIGHTS:

Pac NW/B.C.:

Snow this weekend, only B.C./Washington resorts pick up snow next week as west coast ridge builds.

Central and southern Sierra:

Very snowy late January morphs into dry first half of February as new ridge on west coast keeps the flakes away.

Rockies:

Occasional rounds of light snow across the south, somewhat heavier spells of snow central and north from small disturbances embedded in cold northwesterly flow.

Midwest:

Cold shot from the arctic arrives this weekend and dominates for more than a week. Several rounds of light snow during that time.

Northeast:

Light/moderate snow event early next week. Arctic air gradually spreads into region thereafter. Potential for sizable storm around the 15th.

Mid-Atlantic/Southeast:

Higher elevation snow early next week, colder weather follows and dominates through mid-north. Next shot at significant snow around the 15th.