* Winter storm watch Sunday night-Monday *

Yes, another winter storm is coming. And most signs now point to it being a moderate to high impact event.

The setup for this storm is the clashing of arctic air from the north and mild air from the south, combined with remnant energy from the system that has been slamming California since yesterday. The end result here in the D.C. area looks to be a period of rain Sunday afternoon into evening, changing to a wintry mix Sunday night, and culminating in a period of potentially moderate to heavy snow Monday morning into afternoon, accumulating several inches as temperatures crash down into the 20s. And oh by the way, followed by a deep freeze Monday night.

How much snow?

Below is our best guess. Amounts could be lower if the storm is not as intense, or if the cold air takes longer to arrive, than we currently expect. Amounts could be higher if storm is stronger, or if the cold air arrives earlier, than we currently expect. This is a first-call map, and we will continue to reassess this evening and tomorrow

CWG’s first-call snow map, issued 2:55 p.m. Saturday

Storm Timeline

Here is the storm timeline the way see it now, of course subject to change as we get closer…

2-6 p.m. Sun: Rain develops from northwest to southeast; temps well above freezing

6-9 p.m. Sun: Rain likely; temps falling toward or into the 30s

9 p.m. Sun-1 a.m. Mon: Rain changes to wintry mix from north to south; temps falling toward or near freezing

1-4 a.m. Mon: Wintry mix; temps near freezing

4-8 a.m. Mon: Wintry mix changes to snow, from north-northwest to south-southeast across the area; temps falling below freezing

8 a.m.-3 p.m. Mon: Snow, moderate to heavy at times; temps plummet into the 20s

3-6 p.m. Mon: Snow ends from west to east; temps in the 20s to near 20

Mon night: Hard freeze with lows in the teens

What about the Monday morning commute?

This is probably the toughest part of the forecast. As noted in our timeline above, we are forecasting precipitation to change from a wintry mix to snow, and for temperatures to fall below freezing, from the north-northwest (e.g. Frederick County) to the south-southeast (e.g., southern Prince George’s and Charles counties) between approximately 4-8 a.m.

If this changeover occurs a couple hours earlier than forecast, then the commute could be an icy mess for pretty much the entire area. If it occurs a couple hours later than forecast, then much of the area except far north and west may make it through the commute with mainly wet roads rather than icy. As it stands now, we feel the most likely outcome is for rush hour roads to start out worse north and northwest of the District than from D.C. toward the south and southeast, but travel could be difficult everywhere, especially the middle to later part of the commute.

Either way, most everyone turns to all snow by mid-morning and temperatures crash into the 20s, likely leading to significant accumulation even on treated surfaces. Note that after 7-8 a.m. or so, the snow may get heavy fast, with rapidly deteriorating travel conditions.

What will schools do?

It’s usually as hard to predict what schools will do as it is to predict the weather. But if the current forecast holds, we expect many schools will cancel Monday, and disruptions are possible into Tuesday as well. Keep in mind, though, we’re still more than 30 hours away from the wintry portion of this storm, so things can still change.

Wild temperature ride

We have a wild temperature ride in store according to one model, with highs near or south of D.C. possibly reaching 60 on Sunday afternoon…

Forecast temperatures from the NAM model at 3 p.m. Sunday. (WeatherBELL)

…and then dropping to near 20 by 7 a.m. Monday…

Forecast temperatures form the NAM model at 7 p.m. Monday. (WeatherBELL)

Now this seems too aggressive, as we think it may take until later in the day Monday for temperatures locally to dip all the way down near 20. But at the very least the temperature drop depicted above is indicative of the intensity with which cold air is expected to blast in from the north.

Technical analysis by CWG winter weather expert Wes Junker

The Monday morning commute is likely to be a mess. Whether the problems will be caused by sleet, snow or some combination of the two may depend on where you live relative to the city and what time you’ll be on the road. The precipitation is likely to change to snow during the morning. The operational models having seemingly reached a consensus that Monday is likely to be snowy with most now signaling the possibility of significant accumulations.

The last several GFS runs have been bullish on the idea that the low will track far enough south to give the area a cold and snowy Monday. Last night’s European model was in the same camp and would predict a significant snowstorm for the area if it were right, and today’s European model is not much different. Today’s NAM has shifted from a warmer look last night to a look similar to that of last night’s GFS and European models. The run is much colder, snowier and holds onto the precipitation much longer than last night’s version. The southern stream system should be accompanied by abundant moisture. Therefore, the NAM’s forecast that hammers the area with a significant snowstorm is not too surprising.

Right now I think that’s the way to lean as it has support from the GFS and European models. The combination of a low tracking to our south at the same time a massive arctic high pressure system is located to the northwest is the most common combination associated with significant D.C. snowstorms. The arctic air mass will flex its muscles Monday with temperatures during the day probably falling through the 20s into the teens. This is one of those rare March storms when snow can accumulate on the roads during the daylight hours despite the increasing sun angle.

Is a major storm yet a certainty? No. The 09Z SREF guidance still had a wide range of solutions so some uncertainty remains despite the growing consensus of the operational models. Still, it’s hard to ignore the bullish forecasts of the higher resolution, more skillful models. They should have an advantage handling the low level cold air than the lower resolution ensemble runs. The threat of a significant snowstorm has risen significantly since yesterday.