Snow still a legitimate possibility Sunday night into St. Patrick’s Day

There’s a reasonably solid computer model consensus that a storm late Sunday into Monday will bring wintry weather back to the D.C. area. That doesn’t mean crippling snow, or even accumulating snow, but the odds that we’ll see at least one more period of the white stuff look pretty good.

The critical ingredient for snow, showing up in all the models, is strong arctic high pressure to the northwest. As we approach spring, such a feed of cold air is necessary to counter the ever-strengthening sun.

GFS model shows snow and mixed precipitation over the region at 2 a.m. Sunday morning (WeatherBell.com)

GFS model shows snow and mixed precipitation over the region at 2 a.m. Sunday morning. Notice the strong arctic high over Canada (WeatherBell.com)

Models are also unified that low pressure will develop along a cold front draped across the South, and will move northeast to a position off the coast of the Carolinas. There are subtle differences in the timing, track and intensity of the low which are important for pinning down specifics with respect to when, where, and how much precipitation will fall.

What we can say right now is the following:

  • It gets colder Sunday, with highs only in the 40s, and a good chance of rain developing in the afternoon.
  • Sunday night, as even colder air filters in, rain likely changes over to snow, with a possible period of sleet mixing in during the transition. The changeover happens first in areas to the northwest (Loudoun and Frederick counties) and last in the southeast (southern Maryland).
  • Precipitation – probably in the form of snow – ends Monday morning; the exact end time is uncertain – some models suggest not long after midnight, others keep it going through midday.
  • There is some potential for heavy precipitation and amounts
  • The best chance for accumulating snow is in our colder north and west suburbs; around the city accumulation on grassy areas and more “conversational” snow is favored as temperatures may be slow to fall to freezing or below – but we cannot rule out accumulating snow if precipitation is heavy enough
European model shows the freezing line well south of the D.C. area at 8 a.m. Monday morning, with light precipitation having accumulated in the previous several hours, mostly as snow. (StormVistaWxModels.com)

European model shows the freezing line well south of the D.C. area at 8 a.m. Monday morning, with light precipitation having accumulated in the previous several hours (2-8 a.m.), mostly as snow. (StormVistaWxModels.com)

Says Wes Junker, CWG’s winter weather expert: “The upper level center has a favorable track for snow, the high pressure is in a good position to supply cold air, and the surface low is tracking to our south.  That’s a favorable combination for getting snow in Washington.”

Why is there a chance significant snow *could* fall with this storm?

Take a look at this GFS ensemble (GEFS) plume diagram for the evolution of precipitation at Dulles Airport. Each of the lines on the diagram below represents one out of a grouping of simulations from the GFS model. Blue lines represent snow, green lines rain, and purple lines ice; when they are rising, it indicates precipitation is falling, when they are flat, it is dry.

GEFS plume diagram shows different model simulations (colored lines) depicting accumulating precipitation in multiple forms (blue snow, green rain, pink ice) Sunday (3/17) night into Monday (3/18). (Penn State/NOAA)

GEFS plume diagram shows different model simulations (colored lines) depicting accumulating precipitation in multiple forms (blue snow, green rain, pink ice) Sunday (3/17) night into Monday (3/18). (Penn State/NOAA)

Notice the steep ascent of numerous blue lines between March 16 and 17; this shows that numerous simulations favor snow falling in the region. The average simulation puts out 0.78″  of liquid equivalent precipitation in the form of snow at Dulles, which would convert to several inches, at least.

Also, the Canadian model suggests fairly hefty precipitation falling as snow in the region, but the  snow/mix line settles over the heart of our region when precipitation is heaviest – which means forecasting amounts will be complicated.

Canadian model shows mixed precipitation over the region at 2 a.m. Monday morning. (Environment Canada)

Canadian model shows mixed precipitation over the region at 2 a.m. Monday morning. (Environment Canada)

Stay tuned for additional updates. Tomorrow we will assign odds at different snowfall amount possibilities. Right now, I’d say the chance of at least an inch in the city is around 20-30 percent and right around 50 percent in our colder suburbs…

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