* Winter weather advisories for D.C.’s south and east suburbs this evening through 6 a.m. Wednesday *

UPDATE, 3:24 p.m.: Winter weather advisories have been expanded west and now include Anne Arundel, Charles, Prince George’s and Stafford counties, for the potential for 1-2 inches overnight. The latest NAM model came in slightly snowier than its previous simulation suggesting our snowfall map below *might* be a bit conservative. However, we’d like to see the GFS model coming in in about an hour before making any adjustments. Our PM Update (around 4:45 p.m.) will contain our latest snowfall forecast and SchoolCast.

From 11:58 a.m.: Models have shifted a little north with the major winter storm affecting the South – far enough north that some snow flakes should fly over much of the metro region tonight.  Expect more snow the farther southeast of the District you go, with significant accumulations possible for the southern Delmarva down to around Williamsburg and Virginia Beach, where winter storm warnings are in effect.

Snow amounts

Around the District and immediate metro area, some conversational flakes to around 1 inch is most likely, with the best chance of 1 inch east of the District. Inside the Beltway,  a dusting or so seems like the best bet  but 1 inch or a bit more isn’t totally out of the question.  No, this is not a repeat of the January 25, 2000 surprise snowstorm.

Capital Weather Gang snowfall forecast Tuesday night, issued 11:58 a.m. Tuesday

In Southern Maryland, including St. Mary’s and Calvert counties,  several inches are possible and a winter weather advisory is in effect for this area. However, the accumulation forecast – even in southern Maryland –  is tricky. Slight shifts in the location of heavier snow bands could make the difference between 2-5 inches and 1-3 inches in that area.

Generally speaking, this is the kind of storm where we could easily see the amounts on the map, and we could just as easily see a sharp cut off set up that leaves the entire area except for lower Southern Maryland with a dusting or less.


Temperatures are and will remain well below freezing so even light snow will likely coat surfaces.

Start time of snow

* Developing in St. Mary’s County and southern Calvert County between late afternoon and early evening

* Developing in southern Fauquier, Stafford, Spotsylvania, Prince William, Prince George’s and Anne Arundel counties between around 7 and 10 p.m.

* Developing in rest of metro region, from southeast to northwest between 10 p.m. and 1 a.m.

End time of snow

* Tapering off between 1 a.m. and 4 a.m. from northwest to southeast in most of the metro region, but perhaps as late as 4 a.m. to 7 a.m. in eastern areas adjacent to the Chesapeake Bay

Programming note: We’ll post a SchoolCast and FedCast in our PM update around 5 p.m. this evening.

Technical discussion

The NAM model overall is snowier than the GFS model, forecasting around 1 inch of snow in D.C. and 2-5 inches in Southern Maryland.  The GFS would suggest a dusting in D.C. and perhaps 1-3″ in Southern Maryland.

Unfortunately, the differences between the NAM and GFS forecasts are fairly subtle – so it’s hard to know which one is right.

Note how similar the two upper level pattern setups (500 mb height fields) are in the model simulations shown below.  Each model portrays that a weak upper level impulse (bottom left and right panels below) will be approaching our area later tonight and that feature will induce the development of a weak surface low well off the coast (in the top panels).   However, the NAM model (top left) keeps the low slightly farther west which allows a little more of a westward creep to the precipitation shield than the GFS (top right).  I’m always leery of being near the edge of a precipitation field as minor changes can either leave you high and dry or in this case might offer light accumulations across the city if the NAM is correct.

Why are we leaning slightly towards the GFS around D.C.?

Most of the models are predicting lighter precipitation amounts than offered by the morning’s NAM which predicted 0.18” of liquid equivalent at DCA, a bullish forecast that would probably net the city a couple of inches if it were correct.  This morning’s GFS would only offer a dusting to an inch at most and last night’s European model predicted a dusting for the area.  Both models on average have a better track record on forecasting precipitation amounts than the NAM.

Also, most of the Short Range Ensemble Forecast (SREF) members from the early morning run were generally not as heavy as the NAM but most predicted some snow for the area.    They suggest the probability of DCA receiving over 0.10” liquid equivalent is less than 30%. Remember ensemble members are just identical models having a lower resolution than the operational models but each member has its initial conditions or physics tweaked slightly to try to get a feel for how chaos might impact upon the forecast.  They also provide some idea of the probability of precipitation.  The figure below shows the probability of precipitation accumulating 0.10” during the 12 hour period ending at 4 a.m. Wednesday.  Note that only 20 to 30 percent of the members supported the NAM forecast.  That seems like a reasonable assessment.

Percent of SREF model simulations predicting over 0.1″ liquid equivalent (around 1″ of snow) across Southeast and Mid-Atlantic

The air over D.C. is very dry and it will require quite a push of moisture into the region to get accumulating snow.  We may see several hours of virga – that is, snow showing up on radar, but that evaporates before reaching the ground.

So what’s our bottom line?

Light snow is expected during the night starting in Calvert and St Mary’s counties around or a little by early this evening, spreading northward in the D.C. area around 10 p.m.  There should be enough snow to whiten the ground but the snow across most regions is likely to be more conversational.  This  is likely sweepable as opposed to shovelable snow.  The exception is towards the southeast where Calvert and St Mary’s are likely to see 1-4 inches, with the heavier amounts concentrated the farther south and east you get.