St. Patrick’s Day snow threat uncertain, but gains some more support

GFS model shows snow over the D.C. area Sunday evening (WeatherBell.com)

GFS model shows snow over the D.C. area Sunday evening (WeatherBell.com)

Yesterday, the European and Canadian models were predicting snow in the D.C. area late Sunday into Monday morning (St. Patrick’s Day).  The GFS (American) model was a hold-out, but today it has jumped on board with the others and suggests some snow is possible.

Just because the three primary operational models are all forecasting snow doesn’t mean it’s a guarantee.  For one, the storm is still 5 days away, meaning the models could shift. Some members of the various model ensembles (simulations with their data feed tweaked) suggest the storm could remain squashed to our south. Secondly, a snowstorm at this time of year is fighting climatology – in other words, it gets increasingly difficult to achieve accumulating snow in our region in mid-to-late March.

European model simulates a band of snow over the region between 2 a.m. and 8 a.m. Monday morning. (StormVistaWxModels.com)

European model simulates a band of snow over the region between 2 a.m. and 8 a.m. Monday morning. (StormVistaWxModels.com)

“Even with the growing model consensus, there still is considerable uncertainty especially in how much precipitation might fall and on the timing of onset of precipitation,” says Wes Junker, Capital Weather Gang’s winter weather expert. “This time of year, you need either intense snow or snow at night to get accumulation….usually you need both to get it to stick on the roads. These uncertainties make any attempt to forecast amounts this early nearly impossible.”

The models generally predict low pressure will develop in Mississippi and Alabama and then move northeast off the coast of the Carolinas, spreading precipitation into the Mid-Atlantic.  At the same time, arctic high pressure to our northwest will supply cold air.  This is a good snow set up for the D.C. area and, in many respects, mirrors the March 3 storm which dropped 4-6 inches of snow in the region.

But this storm is happening two weeks later than the last one and the cold air supply won’t be as intense.  So it may well be challenging to attain significant accumulation, especially in the city.  The colder north and west suburbs, which have been hammered this winter, once again stand the best chance of getting enough snow shovel.

Canadian model shows heavy snow and mixed precipitation over the region Sunday night (Environment Canada)

Canadian model shows heavy snow and mixed precipitation over the region Sunday night into Monday morning (Environment Canada)

Also, it’s unlikely all of the precipitation will fall as snow.  High temperatures on Sunday will warm to around 50 ahead of the storm, so precipitation beginning during the day Sunday and/or evening (depending on the exact timing) is likely in the form of rain and/or sleet before enough cold air arrives for a changeover to snow some time overnight (earliest north and west, latest south and east).

What do the models show right now?

  • The European model forecasts rain developing Sunday evening, changing to sleet and then snow from northwest to southeast pre-dawn Monday morning.  It would suggest a possible period of accumulating snow, especially in our colder western areas through mid-to-late morning Monday before tapering off.
  • The GFS model suggests a cold rain during the day Sunday, transitioning to light snow Sunday night into early Monday morning before tapering.
  • The Canadian model suggests rain developing late Sunday afternoon or early evening, changing over to heavy, wet snow and/or sleet overnight and a second wave of snow during the day Monday, which might mix with rain.

Stay tuned for daily updates for more specifics on this snow possibility.

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Gusty showers likely this evening, followed by intense wind, cold and possible power outages