Update: Significant winter storm Sunday night and Monday

* Winter storm watch Sunday night-Monday *

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

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

Overview: We’re now about 24 hours away from the beginning of the winter storm expected to impact the area Sunday night into Monday.  Following a fairly mild day on Sunday, with highs potentially nearing 60 degrees in parts of the region, the storm should start off with rain showers transitioning to a wintry mix of freezing rain and sleet. However, very cold air crashes south as we get through the night and into Monday, ultimately changing everything over to snow across the area.

Related: Today’s detailed update on the storm, including a timeline of how it’s anticipated to progress

When all is said and done, we currently anticipate around 5-9 inches of snow for most spots locally, perhaps less in our far south and east suburbs.  The trend today in modeling has been a southward one with the heaviest snow risk on Monday, so our forecast may end up needing some tweaks if those trends continue.  Tonight’s model runs might help give a hint at any future shifts in the forecast, but the details here should not be considered a forecast. We’ll update thoughts as necessary once the early group of models is in around 11 p.m.

11:15 p.m update: The bottom line from tonight’s model runs: not much has changed compared to our forecast thoughts from earlier in the day. There has been a bit of a southward shift in the maximum snow area compared to our map, and we may need to address that in tomorrow’s updates should it persist. Overall, tonight’s modeling continues to point to a significant winter storm across the whole area.

First up, we get a little taste of warmer air tomorrow, with highs heading toward the 50s if not higher. Showers may begin to approach as early as 1-3 p.m. out west, with more working across as we get toward and through sunset. Temperatures then begin their nosedive overnight, with a wintry mix transitioning to snow before sunrise many spots as temperatures fall into the 20s. Snow continues for much of Monday before departing during the afternoon or evening. Extra tricky travel is anticipated along with very cold conditions.

That’s all for tonight. We’ll have more tomorrow throughout the day on Sunday. Read below for earlier model updates…

10:45 p.m. update: At worst, the GFS held compared to earlier runs, and really it increased precipitation totals on the north end a little, which indicates that perhaps the south shift has ceased (despite the NAM being further south than other models at this point). Below is a comparison of total precipitation from the new run of the GFS and the last one. Snow totals in the D.C. area are about 6-8 inches or so on this run, and overall it looks very much like our forecast above, but maybe with the heaviest axis a bit south. Again, it changes mixed precipitation over to snow in the early morning hours Monday, and snow then continues much of the day before tapering in the afternoon.

This evening's GFS (left) compared to this afternoon's GFS (right). The new run increased precipitation on the northern edge. (Weatherbell.com)

This evening’s GFS (left) compared to this afternoon’s GFS (right). The new run increased precipitation on the northern edge. (Weatherbell.com)

10:35 p.m. update: The GFS is further north than the NAM and in line with earlier thoughts. A changeover to snow occurs around D.C. before sunrise, with most of the area seeing snow by 7 a.m.

GFS precipitation type forecast at 7 a.m. Monday (Weatherbell.com)

GFS precipitation type forecast at 7 a.m. Monday (Weatherbell.com)

10:10 p.m. update: The Canadian GEM still looks quite snowy, and suggests an event close to our forecast is still in the making. Step back from the ledge snow lovers. :) GFS soon…

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10:00 p.m. update: While we wait for the GFS to come out — it should be in range by 10:30 p.m. or thereabouts — let’s all keep in mind that average March snow in D.C. is 1.3 inches, and at Dulles it’s 1.9 inches. A storm of 6 inches or more in March in D.C. would rank close to top 10 all time for the month. The NAM run may be a bit disappointing for snow lovers, but it’s just one model, and there’s still plenty of reason to believe a snowy situation is on tap for Monday. The model itself still suggests more than a few inches locally.

Top snowfalls in D.C. in March. (Ian Livingston)

Top snow events and snow months in D.C. in March. (Ian Livingston)

9:45 p.m update: A high resolution version of the NAM also shifted the heaviest axis of precipitation to the south of the immediate area as seen in the precipitation and snowfall maps below.

0z 4km NAM (Weatherbell.com)

0z 4km NAM (Weatherbell.com)

0z 4km NAM (Weatherbell.com)

0z 4km NAM (Weatherbell.com)

9:25 p.m. update: The NAM shifted south again, a trend we’ve seen for the last day or so. It remains a pretty solid event for the immediate D.C. area and south in particular, but the northern edge of the heavier activity cuts through the area. Either way, the run still spits out about 1 inch of liquid equivalent precipitation D.C. and south, with at least half of that snow (again, 10:1 snow to rain is a general idea but it’s more complex than that). Here’s a comparison of snow maps from the current run (left) and the last run (right) — click for larger:

New NAM (left) and last NAM (right) snow maps. (Instantweathermaps.com)

New NAM (left) and last NAM (right) snow maps. (Instantweathermaps.com)

9:15 p.m. update: Some questions about ice potential on the front end of the system. Overall, this doesn’t look like a big ice storm or anything. The new NAM run shows the risk for freezing rain fairly short lived — something we’ve seen on most guidance in the last day or so. A lengthier period of sleet is possible though, as the cold is initially somewhat shallow with some warm air just off the surface.

NAM precipitation type at 1 a.m. Monday morning. Mostly sleet across the area at this point, but transitioning to snow. (Weatherbell.com)

NAM precipitation type at 1 a.m. Monday morning. Mostly sleet across the area at this point, but transitioning to snow. (Weatherbell.com)

9:05 p.m. update: The NAM is running, but not far enough out to get much detail on the storm itself. My first thought is it looks pretty cold fairly early in the evolution of the event. Most of the region is at or below freezing by late Sunday evening (10-11 p.m. tomorrow night):

Mar 2 0z NAM temperature forecast for 10 p.m. Sunday night. (Weatherbell.com)

Mar 2 0z NAM temperature forecast for 10 p.m. Sunday night. (Weatherbell.com)

8:50 p.m. update: The first main model of the night is the NAM, but before it runs the Short Range Ensemble Forecast (SREF) comes out. The new (21z) version of SREF more or less matches up with most of the modeling today. In short, it shows a substantial winter storm across the area. Below is the maximum 24 hour precipitation panel, showing over 1 inch of liquid equivalent precipitation across the area (a simple conversion is 10 inches of snow to 1 inch of rain, though that’s oversimplified in this case). Not all snow, but a lot of it is snow. It fits in well with our forecast:

21z SREF precipitation forecast, showing the region under the maximum axis. (NCEP)

21z SREF precipitation forecast, showing the region under the maximum axis. (NCEP)

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Storm update: Heavy snow and crashing temperatures likely Monday; starts as rain to wintry mix late Sunday