Most Read: Local

Posted at 02:00 PM ET, 12/06/2011

Heavy rain to likely end as snow Wednesday night; sloppy accumulations far N & W suburbs possible

The snow threat remains for Wednesday night but, for most of the region, the event will probably be a minor one. This system, a rapidly developing area of low pressure, is going to bring lots of rain (perhaps over 1”) before any changeover to snow. In fact, most of the precipitation will be in the form of rain during the day on Wednesday into Wednesday night.

First call - snow potential for Wednesday night. Most accumulation limited to grassy areas (except high elevations N & W).
The changeover to snow will start in the west, probably between 8 and 11 p.m. Wednesday night, as cold air filters in and will shift eastward through the area, to the District perhaps just before midnight.

Despite the differences in time of year, this system has a similarity with the October 29 event as there is not very much cold air around to tap to help facilitate the changeover from rain to snow. Like that October case, the most likely areas to see accumulating snow will be those well west and northwest of the city and up in elevation. Any snow is likely to end before the Thursday morning commute, but a few slick spots may form with temperatures falling to near freezing, especially west and northwest of the beltway.

Computer model overview

Last night, the models struggled with how quickly to bring cold air into the area. The two NAM model runs and the one this morning averaged giving Reagan National (DCA) around 0.40”of liquid equivalent falling as snow (around 4 inches assuming a 10-1 snow to liquid ratio which is overly optimistic given the warm surface temperatures; you might cut the ratios in half for this event, so an upper limit for snow totals might be 2”), while the GFS model only averaged 0.10” (equivalent to 0.5” of snow for this storm, maybe).

The Euro model was warmer than the NAM and little farther north with its track. It only forecast only a couple of tenths of an inch of snow over the District and about an inch in Rockville.

This morning’s NAM continues to be in the same ballpark as the earlier runs while the GFS ends up a little snowier than last night’s run (it might produce 1” of snow).

The differences in model solutions in how fast the cold air will move into the area have prompted a number of blogs to put out vastly differing forecasts depending on which model grouping the forecaster favors. Right now, I’m inclined towards the more conservative forecast. Precipitation probably will end with a period of snow around the city but any with any accumulations limited to the grass.

The best chance for accumulation snow is to the northwest and west especially in places with some elevation. I could picture the snowfall amounts being very similar to those during the October event earlier this year. However, in a case like this, small changes in track and timing of the cold air moving in can make a big difference in the forecast. Therefore, this forecast is far from certain.

Computer model analysis

Temperatures and pressure at 850 mb (5000 feet) at 10 p.m. Wed and 1 a.m. Thurs in the GFS and NAM models. (NOAA)
You can get a sense for the differences in the model forecasts by looking at forecasts from the GFS and NAM of the 850 mb (about 5000 ft) heights, winds and temperatures. Note that the GFS is a little faster in moving the 850 low northward than the NAM though both still have the freezing line west of us at 10 p.m. Wednesday night.

The GFS track of the 850 low is problematic for snow as it tracks it right over D.C. at around 7 p.m.Wednesday. Usually for a good snowstorm you want the circulation center to track south and east of the city by around 90 nautical miles. Even the forecast track from the colder NAM suggests most of the precipitation will be rain. By 1 a.m. Thursday morning, both the GFS and the NAM shift the freezing line (purple line) east of the city suggesting that any changeover will occur sometime between 11:00 PM and midnight in and around D.C., but progressively earlier to the west.

GFS vs NAM precipitation forecast between 10 p.m. and 1 a.m. (Wed night/Thurs morning)
Unfortunately, the models differ on how much precipitation will fall during the period when the temperatures start falling rapidly and the 850 mb zero (freezing) line presses east of the city. The three hour accumulated precipitation for the 10 p.m. to 1 a.m. period is shown for the GFS and NAM. Note that the NAM has more than twice the precipitation during the period. Therefore, it forecasts a little more than twice as much snowfall for the D.C. than the 12Z GFS (0.22” liquid). This is another case where the surface temperatures are warm so unless the snow is falling heavily, it will be wet and will have a hard time sticking.

The NAM is definitely more bullish than any of the other models or guidance for snowfall. The Short Range Ensemble Forecast (SREF) system has no members showing any snow at DCA throughout the storm (see image below). Its lower resolution grid spacing may hinder it from resolving the surface temperature but they still are providing a signal to be cautious about forecasting heavier snow amounts around the city.

SREF model run depicting the precipitation amounts and type for each of its ensemble members.
The bottom line is there still is plenty of uncertainty in the forecast. The most likely scenario is for there to be some accumulations in the normally colder western suburbs especially where there is elevation. Closer in to the city, both the GFS and NAM suggests some snow will fall and that looks more likely than the SREF no snow at all scenario. However, with the surface temperatures still expected to be hanging around 34 as the snow is falling around the city, accumulations should be light.

By  |  02:00 PM ET, 12/06/2011

Categories:  Latest, Winter Storms

Read what others are saying

    © 2011 The Washington Post Company