More snow tonight and Saturday, but may not stick near the city

UPDATE, 2:40 p.m: The National Weather Service has issued a winter weather advisory for northern Maryland (Frederick,Carroll and Baltimore counties) and extreme northwest Virginia (map), for 2-4 inches of snow between 10 p.m. tonight and 10 a.m. Saturday.

From 12:27 p.m.: A wound-up upper level disturbance will zip by to the south late tonight and early Saturday, in good position to produce another period of snow for the region.

GFS model shows strong low pressure moving off the coast midday Saturday, and starting to intensify (WeatherBell.com)

GFS model shows strong low pressure moving off the coast midday Saturday, and starting to intensify (WeatherBell.com)

However, especially for locations inside the Beltway and to the south and east, temperatures may be a little mild for accumulation and snow could mix with or change to rain at times.

Key points:

  • Snow or mixed rain and snow starts between 11 p.m. and 3 a.m. tonight (perhaps somewhat later east of town) and most likely tapers off early afternoon Saturday.
  • For the immediate metro region and south and east, generally less than 1″ is expected due to temperatures near or slightly above freezing, and the chance of rain mixing in; overall a nuisance snow, low impact
  • D.C.’s north and west suburbs may receive 1-2″, and far north and west suburbs 2-5″ (highest amounts in hills/mountains); overall a low to moderate impact
  • With these kinds of systems, a narrow band of heavier snow can develop (see Wes Junker’s technical discussion below), leading to boom potential; on the other hand, areas outside the narrow zone of steady precipitation may see only very light precipitation, leading to equal chances of a bust.
  • As the storm moves off the Mid-Atlantic coast and races northeast, it may rapidly intensify, and produce blizzard conditions in eastern New England.  Very cold and windy conditions will follow this storm. Winds may gust to 30 mph Saturday afternoon with temperatures Saturday night dipping into the teens to near 20.
Capital Weather Gang forecast for snow amounts Friday night into Saturday morning.  Issued 12:20 p.m. Friday.

Capital Weather Gang forecast for snow amounts Friday night into Saturday morning. Issued 12:20 p.m. Friday.

Accumulation maps from other sources

NAM model high resolution

High resolution NAM model snow forecast (WeatherBell.com)

High resolution NAM model snow forecast (WeatherBell.com)

Canadian model

Canadian model snow forecast (WeatherBell.com)

Canadian model snow forecast (WeatherBell.com)

GFS model

GFS model snow forecast (WeatherBell.com)

GFS model snow forecast (WeatherBell.com)

National Weather Service

National Weather Service snowfall forecast for Friday night and Saturday

National Weather Service snowfall forecast for Friday night and Saturday

WUSA9

 

Technical discussion, from Wes Junker

The models dive an energetic upper level system to our south on a track favorable to give us snow as its surface low passes across North Carolina.

Clippers are tricky little systems since they usually start off moisture starved.  They generally produce a stripe of light snow across the area usually on the order of a dusting to 3 inches.   However, occasionally they can get amped enough to give us stripe of moderate snow 3-5 inches but the track has to be perfect.  The heaviest precipitation with clippers is usually found in a relatively narrow stripe making forecasting snowfall amounts with them complicated.

For this event, marginal temperatures for snow are an added worry with various models and model runs at odds whether surface temperatures will be below freezing during the event.

This morning’s Short Range Ensemble Forecast (SREF) system simulations illustrate how much slight changes in the strength and track of the clipper can have on precipitation.  Below, each line represents one model run with either slightly different physics or slight differences in their initial conditions.  The scale on the left indicates the amounts of precipitation that member is predicting during its run.

SREF model simulation of precipitation.  Where lines are rising steeply, precipitation is mounting most heavily.  Where lines are flat, precipitation is not falling.

SREF model simulation of precipitation. Where lines are rising steeply, precipitation is mounting most heavily. Where lines are flat, precipitation is not falling.

Three of the 21 members predicted the rare boom scenario.  The 06Z NAM was in that heavy precipitation grouping but this morning’s dropped the precipitation to the below the thick black line (the mean of all the model runs).

We’ve already had two higher order (more intense) clipper systems this season.  The SREF guidance suggests there is some potential for such an event but the more likely result is for the clipper to produce between .07 to .25” inches of liquid equivalent.  That would translate to a dusting to 2 or 3 inches if the surface temperatures stay below freezing.  These marginal events often produce more snow outside the beltway then within as the slightly warmer urban temperatures keep down snowfall amounts.   That could again be the story with this storm even if we end up in that narrow band of heavier precipitation.

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