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.

Snow For All.

Often when I write this piece, I have good news for one region of the country and not-so good news for other areas of ski country. Such is the nature of winter weather across a country that stretches 3,000 miles from coast to coast and with so many and varied climate zones. However, just as there are times in the middle of the summer when much of the country is warmer than normal, occasionally we see a pattern in winter where most or all of the skiing regions get lined up for significant snowfall at roughly the same time. We are entering just such a period.

In order for a favorable pattern to set up for such a large area of the country we obviously need an ample supply of cold air. From mid-December until just recently, most of the air flowing across the country was of Pacific origin, not the prime source region for winter weather. The jet stream configuration now in place is tapping cold air from the northwestern corner of the continent. There is still a risk that we could see an intrusion of severely cold arctic air by the end of the month, but at a minimum, enough cold has showed up to make even southern regions cold enough to be on the lookout for fresh snow. It certainly helps that this is the time of year when normal temps in most areas are right at their climatological minimums for the year. To illustrate the extent of the cold air, here is a look at the forecasted 5,000 foot level for Friday, Jan. 29^th

The area shaded in hues of blue indicate where temps will be below freezing. Sub-freezing temps at that level are a proxy for where the column of air overhead is cold enough to support snow, even the southern Appalachians look sufficiently cold.

The other ingredient for widespread snow is a favorable jet stream configuration. Here is a forecast for the jet stream for late next week that I agree with, showing three key components to a snowy pattern from coast to coast.

The trough along the west coast will produce fresh snow from Alaska down to the southern Sierra, and inland to AZ and NM as well. Storms from the southern end of that trough will traverse the country and bring the opportunity for snow to the central and southern Appalachians. The red ridge over northeastern Canada will remain in place and help keep storms from cutting through the Great Lakes, which would produce rain to the east of the track. Northern disturbances will come out of the Gulf of Alaska, move along the U.S./Canadian border, and produce snow in the Great Lakes and Northeast from time to time. Those systems are often called Alberta Clippers because they typically move through that province. If the impulses in the northern and southern branches of the jet combine forces, the trough off the east coast will be pulled westward and potentially lead to a blockbuster coastal storm for the mid-Atlantic and Northeast. The potential is high nationwide. Let’s hope the pattern delivers the goods!!!

Regional Details

Northwest U.S./Western Canada: The trough along the coast produces multiple rounds of meaningful snow over the next couple of weeks. No issues with snow levels due to input of arctic air.

Sierra: Trough extends far enough south to produce a much more active storm pattern to this region.

Rockies: Full length of the mountain range will have frequent snow events into early February.

Midwest: Clipper systems keep coming every two-three days into next month. Each one will produce light snow with backside lake effect, as well.

Mid Atlantic/Southeast:  Cold enough for snowmaking; southern storm track could generate natural snow at times.

Northeast/QB: Clipper systems help northern resorts with light snow at times. Ridge over Canada likely to suppress major storm track too much UNLESS two jet branches combine for coastal storm.