Late weekend storm could bring another winter weather mess

Jet stream is taking a big dip over the eastern third of the U.S., setting up a cold, stormy pattern. (WeatherBell.com)
Jet stream is taking a big dip over the eastern third of the U.S., setting up a cold, stormy pattern. (WeatherBell.com)

The calendar says spring, but we’re locked into an extreme weather pattern which keeps cranking out opportunities for wintry precipitation. After a chance for a few conversational snow showers tonight, a possible bout of winter storminess arrives Sunday and Monday.

For meaningful snow accumulation above and beyond what we’ve experienced this March, exceptional – rather than just extreme – circumstances are required. Average high temperatures are closing in on 60 degrees, and for snow to accumulate – especially near the city – both an extraordinary cold air mass and heavy precipitation rates are needed (it also helps if snow falls at night rather than during the day, when it has better prospects of sticking). Snow can accumulate a little more easily in locations north and west, especially those with elevation (above 700-1,000 feet).  We’ve already seen that play out twice this month.


The Arctic Oscillation: observed levels in recent months (in black) and projection (in red) 10 days into the future. (NWS)

It turns out we have an extreme-to-exceptional weather pattern in place.  The Arctic Oscillation, which serves as an indicator of how contorted the jet stream is, is its second most negative on record for March at -5.2 writes Wunderground’s Jeff Masters.  Due to a massive area of blocking high pressure over Greenland, the jet stream is taking a huge excursion to the south over eastern North America, driving much colder than average weather to our latitude.  (CWG’s Rick Grow will have more on this unusual weather pattern in a feature tomorrow).

But again, for a snowstorm at this time of year, abnormal cold by itself is not enough; we must also have a big storm that follows a track to deliver a sustained period of heavy precipitation over our region coinciding with any arctic blast.

As we analyze the  weather models for Sunday and Monday, it doesn’t appear the cold air will be quite cold enough or the precipitation quite heavy enough for a headline grabbing late March snow event.  On the other hand, another winter weather mess like we’ve experienced twice already this month is quite possible. That would mean some accumulating wet snow west of the city and into the mountains, and perhaps a conversational coating of snow, sometimes mixing/alternating with rain, closer to town.  Of course, in the spirit of communicating uncertainty, the potential Sunday-Monday storm is still 5 days away and, as the devil is in the details, the forecast is subject to change.


GFS Model shows snow falling Sunday morning with the freezing line running through NW Washington, D.C. (StormVistaWxModels.com)

Both the GFS and European models show  a similar set up.  Low pressure will cut across the south and then head northeast into the Tennessee and Ohio Valley.  Enough cold air is in place for a period of snow west of town and snow or rain elsewhere as the low approaches.  Surface temperatures are flirting with the freezing mark in model simulations, with places like Leesburg and Frederick most consistently below freezing when it’s precipitating.


The European Model ensemble mean shows snow or a rain/snow mix (near the I-95 corridor) Monday morning, with the freezing line in Loudoun county (WeatherBell.com)

As the low pressure center reaches the Ohio Valley, it dies out and a new area of low pressure begins forming off the Mid-Atlantic coast.  During the transfer of energy, our region gets dry-slotted and temperatures warm, so any snow or mixed precipitation transitions to drizzle or just cloudy skies.

Sometimes when a new area of low pressure forms off the coast, it explosively develops and roars up the coast, with an area of heavy precipitation expanding inland.  But, because of the strong blocking pattern, the new low pressure center heads east-northeast out to sea, throwing back little or no precipitation.  So the odds of a prolonged period of heavy precipitation to give us an anomalous snow event seem low.

I should note that the GFS and Euro models differ somewhat on the storm timing.  The GFS simulates a period of snow or mixed precipitation early Sunday morning into midday, wheras the Euro model is slower with the storm system, not bringing much of precipitation to the region until Sunday afternoon and night into early Monday.  We’ll get a better idea on the exact timing in the next couple of days. (Snow lovers should hope most precipitation falls at night to max out accumulation potential).

The bottom line is that this is a complicated set up, but there are elements which make it interesting and worth keeping a watchful eye on. This is especially true if you live west of the city and towards the mountains where some accumulating snow is a realistic possibility, while more of a long shot along the I-95 corridor.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · March 20, 2013