December 2009 snowfall picture

Pedestrians walk on a snow-covered Grandin Road on Dec. 19, 2009, when 16 inches of snow accumulated by that morning. That snowfall occurred when the weather set-up was similar to the forecast for this weekend.

It’s just what we should expect in an El Nino winter.

A big, wet low-pressure system enters U.S. airspace over southern California, then lumbers over the southern tier of states, lifting moisture northward ahead of it, first tapping the Pacific and later the Gulf of Mexico. Where Arctic air is pressed southward by high pressure over the central and eastern United States, that moisture falls largely as snow.

That is what we face this weekend, at its simplest level, with the potential for significant snowfall in or very near our region.

It will probably snow this weekend in Southwest Virginia, with Saturday night into Sunday appearing to be the most likely time period. It might even linger into early Monday.

Beyond that, there is not much yet that can be reasonably projected about the storm system’s exact effects on our region, with track and evolution uncertainties lingering. Hopefully some of that can be determined reasonably well in the next couple of days. The upper-end potential for this winter storm, however, could be well over 6 inches of snow, maybe over a foot, somewhere — if not in your front yard then somewhere within a couple hours drive (on a dry day).

You may see some snowflakes flying through the air on Wednesday morning, or even wake up to some whiteness on the ground in some spots. This is completely unrelated to the weekend snow threat, except in one way: It represents the front edge of Arctic air building into our region.

This cold air will be firmly in place through the weekend, with temperatures bouncing around in the 20s and 30s, only as high as the 40s when the sun is out.

The cold air is not in question this time. It will be firmly placed and thick enough for snowfall in Southwest Virginia this weekend. What is in question is the track and evolution of the storm system, which will make the difference in us getting a big snow, having some snow become mixed with sleet or freezing rain, or getting just a fringe of somebody else’s snowstorm.

Though an El Nino winter has not officially been declared yet, with the warming of equatorial Pacific waters not having achieved a baseline level for the minimum length of time necessary, we have quite likely started one that will become official in a couple of months.

And whether it is official yet or not, the atmospheric setup this weekend is reminiscent of the first week of December 2002 or a little later in the month in December 2009, other recent El Nino winters.

The differences in those two events highlight some of the challenges facing forecasters this weekend. The 2002 storm dumped 4 to 8 inches of snow, followed by a crust of ice, while the 2009 storm dropped 1 to 2 feet of almost entirely snow.

The 2002 low moved northeastward into Tennessee, then transferred its energy to a coastal low that headed for the Northeast. The 2009 storm was a continuous track from the gulf up the East Coast, the classic “Miller A” track that produces some of our biggest snow events.

Neither of these evolutions look likely in full, with atmospheric blocking associated with the Arctic air mass preventing anything from going too far north of us, but either evolution is possible to a partial extent.

Let’s throw in a couple more “overrunning” type snowstorms with a low tracking to the south throwing moisture over us, in early January 1988 and late January 2010, that also occurred in El Nino winters.

The 1988 storm went well south of us, giving our region amounts generally 2 to 6 inches while dumping some foot-plus amounts on parts of the Carolinas. The 2010 storm was similar, but shifted northward — our region got 8 to 12 inches with lesser amounts to the south where it mixed with sleet. Tracks similar to either of these are possible at this point.

Computer forecast models have done what models love to do with details, bounce all over the place with storm track and precipitation amounts. You could at once earlier this week see a forecast model map with 26 inches pasted on Roanoke and another with zilch.

But the overall atmospheric pattern is solid for a likely, though not certain, snow event in our region this weekend.

The federal government’s Weather Prediction Center, the National Weather Service’s central office monitoring precipitation events, has been assigning chances above 50 percent for getting at least a quarter-inch of liquid equivalent for snow and sleet — or 2.5 inches if all snow — in Southwest Virginia since Sunday.

Nearly a week in advance, that is remarkably high confidence.

The outside possibility of no effect at all, of course, still remains as well, if the system veers well south of us. But these big wet El Nino lows rolling across the southern U.S. usually prove hard for us to miss.

There are still miles to go before we sleep on this one, whether our woods fill up with snow or not.

Weather Journal appears on Wednesdays.

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Since 2003, Kevin Myatt has penned the weekly Weather Journal column, and since 2006, the Weather Journal blog, which becomes particularly busy with snow. Kevin has edited a book on hurricanes and has helped lead Virginia Tech students on storm chases.

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