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Posted at 01:23 PM ET, 01/11/2012

Two upcoming chances for light snow: Friday morning and Sunday morning

Two minor snow threats may be in the works for this week. The first one is with the strong cold front that will be moving through our region early Friday morning. The second is from a clipper type low that is forecast to track southeast from the Midwest towards our region Sunday morning.

In terms of snowfall, neither looks like a big deal. But, in the D.C. area, timing is often more important than amounts. That’s where the Friday event gets tricky as the front will be passing through the area sometime in the early morning hours, potentially coinciding with the commute.

Thursday night/Friday morning event overview

The bulk of the precipitation late Thursday night into Friday morning is likely to be rain but could end as a little snow as the temperatures drop towards freezing from northwest (5 a.m.-ish) to southeast (latest, after 7 a.m.). That always opens the door for the possibility of slick spots developing.

But, right now, it looks like roads across most of the area will be wet rather than icy as much of precipitation falls before temperatures drop off. The D.C. area rarely gets accumulating snow with a cold front that has no surface low to our south. However, the event is still worth monitoring especially for those living north and west of the city in case the cold air moves in a little quicker than forecast.

Sunday morning event overview

The second event has more potential to produce accumulating snowfall as the temperatures on Sunday should be cold enough for any precipitation to fall as snow. Because of their northern origins, most clippers like this rarely produce more than an inch or two of snow and the accumulating snow usually falls in a relatively narrow band.

To get in the accumulating snow, the track of the surface low and associated 500 mb vorticity (spin center at around 18,000 ft) usually needs to track the south of D.C. This morning’s GFS model has such a scenario but the run is only forecasting very light precipitation amounts with the system because the surface low weakens as it crosses the mountains.

The track of the system suggests, despite the light amounts forecast by the GFS, there is still potential for an inch or two if everything falls exactly in place. This early in the game, there is also potential for the system to give us little or no snow. But for snow lovers it is at least a system to watch.

Right now, it looks like any snow would end before the the Ravens playoff game against the Texans in Baltimore at 1 p.m., but the timing could shift, so stay tuned.

Technical discussion: Thursday night/Friday morning event

The figures that follow explain part of the reason why this event is tricky.


(Left) 1 a.m. forecasts of the NAM 700 mb heights (around 10,000 feet), relative humidity and vertical motion. (Right) 850 mb heights and temperatures (around 5,000 feet) (NOAA)
To the right, the band of higher relative humidity (dark green area) moving into the District by around 1 a.m is coming through right as the temperature start to fall rapidly. Note that the freezing line at 850 mb (5,000 feet, violet line) is still well west of D.C. as the precipitation starts. By 4 a.m. the 850 mb freezing line is east of D.C. and the model still holds onto some precipitation (not shown) but the temperatures at the surface still are too warm.


Vertical profile of temperatures and dewpoints from the NAM model at 4 a.m. Friday morning.in
The forecast sounding at 4:00 a.m. Friday morning (a product that shows the vertical temperature and dewpoints as you ascend in the atmosphere) to the right illustrates the problem. The top arrow is pointing to the 850 mb temperature which is around -2 C. The bottom arrow points at the surface temperature which is around 7 C. The sounding clearly suggests the precipitation is still rain. The precipitation should end before 7 AM suggesting that most if not all of the precipitation will probably fall as rain except possibly right as the precipitation ends.

Technical discussion: Sunday morning event

Clipper events are always dicey since they are so lacking in moisture and because the stripe of accumulating snow is usually so narrow. The current GFS run only offers the Washington area a dusting which could be right. Last night’s Euro suggested that we might not get anything except possibly flurries.


GFS forecast of surface pressure and precipitation (left) and 500 mb heights and vorticity (spin at around 18,000 feet) at 7 a.m. Sunday

The figures above show how the GFS tracks a surface low from northern Kentucky at 1 a.m. Sunday morning southeastward but that the model more or less loses the circulation as it comes across the mountains. If the low were to stay a little stronger, the model solution would undoubtedly would be snowier.

On the accompanying 500 mb forecast on the right, I’ve attempted to show how little the 500 mb chart would need to change to bring the surface low a little farther north and to also have the upper center produce more precipitation over Washington as it passes by.

The clipper could offer us an inch or two of snow or could scoot by with little or no snow. Right now I’d lean towards a dusting but stay tuned.

By  |  01:23 PM ET, 01/11/2012

Categories:  Latest, Winter Storms

 
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