Timing: The snow could begin in the western suburbs as early as late morning, but should get going in most spots during the afternoon. The heaviest precipitation should fall between mid-afternoon and early evening. Snow should end between sunset and mid-evening (8-10 p.m.), from west to east.

Accumulation potential: Just a little non-accumulating snow to as much as an inch in the immediate Washington/Baltimore metro regions and to the southeast. From western Montgomery county through central Loudoun county and to the northwest, a heavy coating (0.5”) to 2” or so is possible.

Impacts: In the immediate metro area, any snow accumulation should occur primarily on grassy areas. However, as darkness approaches, it’s not out of the question temperatures get closer to freezing - especially if the snow is falling at steady clip. So in addition to reduced visibilities, some slick travel cannot be ruled out during the PM rush hour Wednesday. The potential for slick travel is greatest north and west of the District, especially as you head northwest along I-270, and west on I-66 and 267 outside the beltway.

National Weather Service snow accumulation forecast

Temperatures: Temps may rise into the upper 30s to near 40 midday before the snow begins. As snow becomes steadier in the afternoon, temperatures should fall back into the mid-30s around town, with low-to-mid 30s in the colder N & W suburbs. As the sun starts to set, temperatures will get closer to freezing closer to town - and when the best chance of accumulation will occur.

What could go wrong?

Weaker low scenario, tracking north

This scenario would reduce precipitation amounts, and raise temperatures, resulting in little more than conversational snowflakes, or even some rain in the immediate metro area and south and east. The colder north and west suburbs would get a coating to 1” in this scenario.

Stronger low scenario, tracking south

This scenario would bring a thump of snow in the immediate metro region, cooling temperatures to near freezing, and producing as much as 2” or so. Significant PM commuting problems could arise.

Model discussion from Wes Junker, CWG’s winter weather expert:

The models continue to be quite different in their handling of the precipitation and surface temperatures with tomorrow’s storm making any definitive call a low confidence one.

The NAM has waffled from run to run on how much precipitation/snow might fall. Last night’s 1 a.m. run had similar amounts to the European and GFS models which were much lighter than the amounts forecast by this morning’s NAM. The latter NAM run more than doubled the amounts and also trended colder for Reagan National (DCA_ and is now forecasting temperatures to fall from 37 or so at the storm’s onset at noon to around 33 or 34 during the period of heaviest precipitation.

This morning’s RGEM (Canadian model) looks very similar to the latest NAM run. Both would suggest a couple of inches of snow could fall across portions the area especially north and west of the city. None of this morning’s Short Range Ensemble System’s members support as much precipitation as the NAM. More than half predict less than 0.10” which is about what this morning GFS predicted for Washington.

The models with the lighter precipitation amounts are also forecasting warmer temperatures and would argue for little or no accumulations. That’s the puzzle, whether to believe the NAM and RGEM, which often overforecast precipitation or to believe the other models.

Right now I’m leaning towards the lighter solutions. That does not preclude higher elevations to the north and west from possibly seeing an inch or so of snow but suggests that DC will probably see little if any accumulation except possibly on grass. However, this is a low confidence forecast as there is some potential for the NAM to be right.