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Posted at 02:41 PM ET, 12/27/2012

Some Saturday snow possible, then cold



Another possible winter weather event looms on the horizon for early December 29. Yet again, the Washington area is expected again to be near the line between rain and snow.

Next chance of accumulating snow: Saturday morning

Chance of at least 1 inch in Washington, D.C.: 30 percent (50 percent in the colder north and west suburbs)

This is another tricky winter forecast and it is still too soon to say exactly where the rain/snow line will fall. But, as always, the west and north suburbs look to have the highest odds of having an all-snow event while spots south and east of the city have the lowest.

In locations north and west of the beltway - as with the events on Monday and Wednesday - some accumulation is likely. In the city, there is much more uncertainty with almost equal chances for rain and snow.

Precipitation is likely to begin around dawn or a bit before (especially west of town) and will probably be snow to start. But a changeover to sleet and/or rain is possible (especially in town and south and east). Precipitation may end rather quickly, by around midday.

Technical discussion for Saturday’s storm

Much of the difficulty in this forecast stems from the question of how strong a low becomes tracking across the Ohio Valley before weakening relative to the another low that will be tracking to our south. The stronger the northern low becomes relative to the southern one, the greater our chances of ending up with rain instead of snow. The quicker the southern stream low coming across the South develops, the better our chances of snow.

The two lows on the map below are two main players in determining how much snow the Washington area is likely to have. Having two lows almost always leads to problems in trying to forecast precipitation type across our area.

The blue line on the left hand map below from the GFS model indicates that at upper levels the temperatures are below freezing and would support snow as the precipitation overspreads the area. The right panel shows low level winds are from the east-southeast across the area as the air flows from the high pressure off the Atlantic coast to the lower pressure over West Virginia.


(Alan Huffman)

Because of the east-southeasterly winds, the surface temperatures at the onset are above freezing but still in the low-to-mid 30s across most of the region. On this model, the southern low is not yet close enough or strong enough yet to turn the flow and have it keep a northerly component to lock in the cold air.

The western areas out towards Leesburg and Frederick Maryland would undoubtedly be snow. However, inside the beltway and points south and east of the city, surface temperatures are forecast to warm into the the mid-to-upper 30s suggesting rainfall might become the dominant precipitation type especially if precipitation rates remain on the light side.

A weaker northern low and faster and deeper southern one could shift the surface freezing line southward if the winds start turning more northerly in response to a deepening low along the Carolina coast. Last night’s European model essentially painted such a picture. However, it still kept the temperatures across D.C .a little above freezing but probably cold enough for snow. The more developed southerly low and quicker development also led to a forecast of more precipitation than forecast by the GFS. That’s the forecast dilemma. Right now there is no consensus about how quickly the northern low will weaken compared to the southern one, therefore, making any precipitation type call for the city little better than a coin flip.


Chance of at least 1” of snow Saturday (HPC)
In my view, the Hydrometeorological Prediction Center (HPC) has done a pretty good job of quantifying the probability of getting one inch pf snow across the region based on the uncertainty in how the battle between the two lows plays out. It has the lowest snow probabilities to the south of the city and the highest to the north and west. It may even be a bit overdone (snow probabilities too high) in light of new information coming in.

A look ahead

Cooler than average temperatures are expected to prevail after Saturday’s storm with the weather for Sunday’s Redskins’ game looking to be chilly and breezy with temperatures probably in the 30s by game time.

During the bulk of the December 31-January 8 period, normal to cooler than average temperatures are expected to prevail.


CPC forecast for temperature and precipitation

The Climate Prediction Center suggests colder than average temperatures and that there might be an active southern storm track in the January 1-5 period. Note the large green area on the above map representing above normal precipitation probabilities. That raises the possibility that another storm could head across the South and threaten us with winter weather sometime in the January 1-5 period.

By  |  02:41 PM ET, 12/27/2012

 
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