Quick hitting snow tonight, coating to a few inches possible

* Winter Weather Advisory issued from 10 p.m. to 6 a.m. for metro region *


Radar & lightning: Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

9:10 p.m. Update: The models we rely on for short-range forecasts (i.e., 0-24 hours out) have trended drier for tonight’s system. That doesn’t mean we won’t achieve the snow accumulation ranges shown in our map below. But it does make it less likely we’ll hit the high end of the ranges or get into “boom” territory, and some places may end up at the low end or a bit lower, especially the western suburbs where the drying trend has been most pronounced. The timing of 11 p.m.-6 a.m. from west to east still seems on target.

From 2:56 p.m. … A fast-moving weather disturbance swings by to the north overnight, bringing a period of snow to much of the D.C. region.

Areas under winter weather advisory in blue

Areas under winter weather advisory in blue

Key points:

  • Snow moves in between around 11 p.m. and 2 a.m., west to east
  • Snow exits between around 4 a.m. and 6 a.m., west to east, possibly ending as a wintry mix (sleet and freezing drizzle) near and east of I-95
  • More mixed precipitation (snow, sleet, freezing rain and plain rain) is likely in southern Maryland and east of the Chesapeake Bay. A few slick spots are possible in these areas.
  • Generally, most areas in the immediate D.C. area should get between a coating and a couple inches of snow; however, 3″ or so can’t be ruled out where heavy bands set up.  We assign slightly higher amounts in the colder north and west suburbs because it will be colder there, but – to be honest – confidence in where the heaviest amounts occur is low and 3″ amounts could focus closer to the I-95 corridor or even east.
  • The ground should be cold enough for most snow that falls to stick.
  • Untreated roads are likely to have slicks spots for the AM commute, but main roads should be OK.  Precipitation will have ended, which will help conditions gradually improve.
  • Precipitation is gone by sunrise across the region, with decreasing clouds in the morning and a notable warm-up  Tuesday afternoon, as temperatures rise to 45-50.
Capital Weather Gang snow accumulation forecast for Monday night issued 3:00 p.m. Monday

Capital Weather Gang snow accumulation forecast for Monday night issued 3:00 p.m. Monday

SchoolCast

D.C. Schools, Alexandria, Arlington, Stafford, Calvert, St. Mary’s counties

schoolcast-1.5-apples

1.5 apples: 50/50 chance of delay. 1 in 4 chance schools close.

Elsewhere

schoolcast-2-apples

2 apples: Good chance (75 percent) of delay, 50/50 chance for schools. Do your homework

FedCast

fedcast-2-domes

2 domes: Good chance of unscheduled leave option, 50/50 chance for delayed opening.

Technical discussion, by Wes Junker

A low is forecast to track into the Great Lakes region which is not a favorable track for significant snowstorm in the D.C. area.  However, the associated warm advection (push of warm air) in advance of the incoming front should bring a period of snow.

Surface temperatures look cold enough to allow 1 to 3 inches of accumulations around the area.  Like most potential snowstorms for this area, there are a few complications.  One is the models are trying to form a secondary low just to our southeast which could enhance the precipitation a little on the east side of the city.  Also, the NAM sounding forecast for tomorrow morning showed an unstable layer suggesting there could be a brief period of moderate to heavy snow.

A third possible fly in the ointment is whether the precipitation stays all snow or changes to freezing rain or drizzle towards the end.  The bottom line is this certainly is not a classic D.C. snowstorm but still could lead to a messy early commute.

The NAM model forecast sounding (below) for a location near Frederick Maryland at 4 a.m. tomorrow morning shows an unstable layer.  Note how the red line is slanted more to the left than the curved dashed line just to its right.  Such a sounding always offers the potential for getting a brief period of convective-type precipitation rates.  That’s a big complication,  basically, the pattern is one favoring 1-3 inch amounts but if a band with moderate to heavy precipitation rates develops someone could end up with 3 or 4.

wes-sounding1While most of the precipitation is expected to fall as snow, a number of Short Range Forecast ensemble members favor snow transitioning to freezing rain or drizzle before ending. The NAM forecast sounding (below) helps explain why the precipitation might end as drizzle. The sounding below indicates that at 4 a.m. temperatures will be below freezing throughout the atmosphere. The sounding is a snow sounding. However, a dry layer is starting to move in aloft. That dry layer may end up causing the precipitation to eventually switch to light freezing rain or drizzle.

wes-sounding2Ice crystals have a hard time forming if the cloud top temperature is warmer than -10 degrees C. The cloud tops, the region just below where the red and dashed line separate on the sounding below is around -12 degrees C. That is still cold enough to support snow. However,  the dry layer aloft is slowly eroding the cold cloud layer with time. If the dry layer continues to grow, the cloud tops will lower and warm. If the cloud tops end up warming above -10 degrees C, nucleation (crystal formation) might stop leading to freezing drizzle towards the end of the event.

The bottom line is early tomorrow morning, driving conditions are likely to be slick. You might want to give yourself extra time for your commute.

Also on Capital Weather Gang

D.C. area forecast: Some snow tonight, but then a work week warm-up