wpostServer: http://css.washingtonpost.com/wpost

The Post Most: Local

Posted at 04:45 PM ET, 02/18/2012

Sunday snow probably won’t be much, but minor forecast changes still possible

4:45 p.m. update: This afternoon’s model runs have us continuing to live on the edge in terms of snow chances, mainly tomorrow afternoon and evening. The latest NAM run would suggest 1-2” of wet snow could fall, especially from the District and points just to the south. But the GFS would give much of the area no snow. The bottom line is nothing has really changed from earlier updates. The best chance of seeing a little accumulating snow will be from D.C. south, with chances of getting shut out increasing to the north. But depending on the exact northern edge of the steady precipitation, the entire area could get shut out or end up with light amounts of accumulating snow. Next update around 11 p.m. tonight.

From 1:55 p.m.: The winter that wasn’t... still isn’t today, as plenty of sun helps afternoon highs well into the 50s. But what about tomorrow?

We’ve been highlighting for several days now the significant chance that tomorrow’s storm tracks too far south to deliver much snow to the area, and as of this afternoon that continues to be the case. The models have mostly now locked in on a storm track that would give the D.C. area 1-2” inches of wet snow at most, with no snow (or at least no accumulating snow) a possibility as well, especially from D.C. to the north.


Sunday accumulation potential. No change from map issued at 11 p.m. 2/17.
The map to the right shows our latest accumulation forecast for tomorrow into tomorrow night.

What’s the timeline? A few sprinkles or flurries are possible late Sunday morning into mid-afternoon, especially south and southwest of D.C., as highs reach the low 40s most spots. The best chance of a steady period of snow, or rain changing to snow, looks to be mid-to-late afternoon (after 2 p.m.) through late evening. Odds of seeing a steady period of snow decrease the further north you are, especially for counties north of D.C. (e.g., Montgomery, Frederick, Howard).

Will any snow stick? Could the forecast still change? Keep reading for answers to those questions and more, plus a model discussion. Also, see our full forecast into the coming work week.

Will any snow accumulate? If snow is heavy enough - and that’s still a big ‘if’ - then we could see minor accumulations especially on grass, sidewalks, car tops and other elevated surfaces. Our “pre-heated” ground limits the potential for accumulation on roadways, but some is possible if and where snow is heavy enough, especially after the sun goes down. Again, odds of accumulation are highest south of D.C. where precipitation is expected to be steadiest/heaviest.

Could the forecast still change? We’ve tried to account for potential model changes in this forecast. But the reality is there’s still time for things to change such that we’d see a little more or less snow than our map shows above. A shift to a major storm with, say 4 inches or more of snow, seems unlikely.

Where will it really snow? Well southwest of the D.C. metro area in southwest Virginia appears to be the sweet spot for this storm. Areas near I-81 from around or just southwest of Charlottesville and Harrisonburg southwest to around Roanoke and Blacksburg could see several inches of snow tomorrow into tomorrow night.

What does CWG’s Winter Weather Expert Wes Junker say?

The models continue to waffle back and forth with the various runs giving us anything from a whiff like last night’s 00Z NAM to around the 0.30” of QPF that this morning’s NAM is now giving us. However, its precipitation is spread over a 12 hour period and with marginal temperatures such light precipitation still might have a hard time sticking.

The range of model forecast fluctuations with regard to precipitation seems to be narrowing as we approach the event with the most likely amounts somewhere in the .10” to .25” range.

The European and GFS models from last night and this morning would argue for little or no accumulation with the precipitation possibly starting as rain, while this morning’s NAM and RGEM models arguing for the band to come northward enough to produce some albeit probably minor accumulations.

Right now I’d favor the GFS and Euro solutions over that of the NAM, suggesting the most likely snowfall accumulation would be in the trace to 1 inch range. The very tight gradient within the precipitation field still makes this storm worth watching as small storm track changes could alter how much precipitation falls across the area.

Model Discussion

NAM

After blanking D.C. and north with no snow in last night’s run, this morning’s NAM increased precipitation a bit along the storm’s northern fringe and now gives the area about 0.3” of liquid equivalent. Taking into account marginal surface temperatures, that might translate to 1-2” of snow at most. The NAM, though, has been wildly inconsistent on this storm. (You may remember at one point it spit out 1.0” liquid equivalent with temps likely cold enough for several inches of snow). NAM does typically do much better inside of 48 hours, so its more modest 0.3” can’t be completely discarded.
NAM grid interpolation from this morning (12Z) shows .32” of precipitation while 2-meter temps are 31-32F. (Texas A&M University)

GFS


24-hour precipitation forecast from this morning’s (12Z) GFS for 4 a.m. Sunday through 4 a.m. Monday. (NOAA)
This morning’s GFS gives D.C. only .05” of liquid equivalent. That’s the third consecutive run the model has spit out .08” or less. Some of that would probably be rain, and any that’s snow might not be heavy enough to stick. GFS has been much more consistent than NAM and is probably the better suited model for this particular storm, although the NAM’s higher resolution may be catching some banding on the storm’s northern fringe that the GFS isn’t.

Euro

The Euro model was the first to indicate early in the week a potential snowstorm for the mid-Atlantic and maybe into the Northeast. More recently, the Euro has consistently suggested the storm will skirt mainly south of the D.C. area and then out to sea, and that trend continues with this morning's run (released around 1 p.m. this afternoon) which gives the area around 0.1” of precip.

Bottom line, as far as the models are concerned, is that while they could still shift north over the next 24 hours or so, it’s unlikely they would shift enough north to dramatically change our current accumulation forecast for the D.C. area. To get anything more than around 2” - and even that much seems unlikely as of now - we’d need an extended period of at least moderate snow, heavy enough to overcome those warm surface temps. And the only model hinting at that is the NAM, which has been the most inconsistent of the bunch.

By  |  04:45 PM ET, 02/18/2012

Categories:  Latest, Winter Storms

 
Read what others are saying
     

    © 2011 The Washington Post Company