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Posted at 01:43 PM ET, 02/09/2012

Another borderline chance of snow Friday night

Another tricky winter weather event is on the horizon for Friday night into early Saturday morning. Like so many of the events recently, there are still questions about how much cold air and moisture will be available to the system.

Unlike the last two chances for snow, this one will occur mostly at night which might help the chances of accumulations north and west of the city.

However, models generally have the surface temperature in the low 40s after dark on Friday night before the precipitation arrives. The temperatures then slowly drop to around 34 towards the end of the event. Where have we heard that before? Pretty much every storm this year.

For the areas inside the Beltway, the precipitation is likely to start as rain with the rain continuing through much of the event before maybe ending as snow. In the far western suburbs towards Leesburg and Frederick, the models suggest a more winter like scenario with much of the precipitation probably falling as snow giving that area some potential for accumulations. For areas between the beltway and Leesburg and Frederick, you probably see some sort of mix.

MODEL DISCUSSION

This morning’s GFS and NAM have both trended wetter with their simulations as they now take a southern stream upper level impulse and try to phase it with the northern stream trough that will be delivering our weekend cold shot. That gives this system a little more potential to tap moisture than the last one. However, the southern stream impulse is a weak one and the surface low development associated with it does not look vigorous enough to really tap into Gulf or Atlantic moisture to make this more than a nuisance type event.


NAM model simulations for 1 a.m. and 4 a.m. Saturday. Note the dark blue 32 degree line is well north and west of the District (NOAA)
The NAM surface maps shown above give a sense of how the low level temperature field is forecast to evolve during the event. Note that the freezing line (southern most blue line) stays north and west of D.C. through 4 a.m. even as a low develops off the North Carolina coast and spreads precipitation into the region.

Both the GFS and NAM now forecast over 0.20” liquid equivalent precipitation. If we had cold air in place, that might be enough to cause some excitement but with the temperatures forecast to stay above freezing, this look more like another tease than a bonafide threat except for the far western suburbs.


SREF probability of snow at 1 am. and 4 a.m. Saturday morning (see scale on right). Precipitation amounts indicated by black contours
The 09Z run of the Short Range Ensemble Forecast System (SREF) reflects the uncertainty resulting from the warm surface temperature. They forecast a 60-80 percent probability of snow across the typically colder far western and northern suburbs (Leesburg, Frederick) by 1 a.m. Saturday morning. By 4 a.m., they suggest snow will probably (80% likelihood) be falling across most of the area north and west of the city. However, at that time the probability of snow is only around 60% in the city itself. In and around the city, the surface temperatures remain a problem for getting accumulations and for getting a n eventual turnover to snow.


SREF plume diagram for Washington, D.C. The green lines indicate when rain is falling, the blue represents snow. Each line represents a different “member” or simulation of the ensemble.
The SREF plume diagrams (see above) and ensemble mean forecast suggests that only about a quarter to a third of the precipitation might be in the form of snow in the city. At Reagan National Airport (DCA), the maximum snowfall for any member of the ensemble members is only 0.11” (an inch or less) and he mean is only 0.05” (a coating). There are a few members that keep the precipitation rain through the event. That’s still a legitimate possibility as this morning’s NAM has the temperatures in the low 40s at 10 p.m. and only drop it to 36 or 37 by 1 a.m. and 34 by 4 a.m. at Reagan National.

The high snow probabilities in SREF guidance indicate the best snow potentialy as you north and west of the city towards higher terrain. However, even as far west as Dulles airport, the precipitation is likely to start as rain as the NAM temperatures at 1 a.m. are still forecast to be around 35 degrees.

In these types of weather systems, precipitation rates often are the deciding factor for determining whether snow sticks or not. If we get a period of moderate precipitation, it might help lower the temperatures enough for snow to stick at least on grass west of D.C. as far east as Sterling and Gaithersburg. From the city south and east, temperatures again look to be too warm for accumulation.

The latest European model is even less impressive than the NAM, GFS, and SREF mean, generating very light precipitation amounts mostly in the form of rain except in the far west and north suburbs.

The best chance of snow flakes around the city may come Saturday afternoon in the wake of the cold front as a weak upper level impulse zips by.

By  |  01:43 PM ET, 02/09/2012

Categories:  Latest, Winter Storms

 
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