Some Snow in the DC Metro Likely This Weekend

6:19 PM, Feb 14, 2013   |    comments
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As we approach the end of winter, snow-lovers in the DC Metro are becoming increasingly depressed by our lack of winter weather-both this season and last year. But there are some indications that winter is poised to make its presence known in our area this weekend! The forecast model data holds the answer... or at least, the possible solution. 

First of all, before we look at the model data for this weekend, I just have to say that I don't see us reaching into the 50s on Friday. 850mb temps remain below freezing on all model data, and the clouds start rolling in at about noon. Can a southwest breeze really fight off the other factors, and warm us into the 50s tomorrow afternoon? I'm not convinced.

Okay, back to the approaching cold front and its associated snow. The Thursday afternoon surface map, below, doesn't look very impressive; there are no organized storms in sight. It's hard to pick out which small area of precipitation might be responsible for our upcoming snowfall, but I believe it is the blob just to the east of the Rockies in Colorado, Kansas and Nebraska. 

On the NAM model, you can see the vorticity maximum associated with this area of snow and rain. The first map is the model's projection for the 500mb heights and vorticity at 7pm tonight:

Thursday night


And the following maps show where the NAM predicts that same vorticity max will move over the next couple days: 

Friday night


Saturday night

As you can see, this relatively small feature could easily get lost in the midst of the large, 500mb trough that is forecast to dig into the eastern USA this weekend! That trough will bring much colder air to the DC Metro on Friday night into Saturday morning:

Saturday morning

 As you can probably tell, the blues are for temperatures that are below freezing, and the purples are even colder. These are not surface temperatures, but instead what the temperature is predicted to be approximately 1 mile above the surface.

The most impressive feature I saw on the model data was the vertical velocity, which helps us find areas where the air is rising quickly, and is therefore unstable. The vertical velocity map indicates that there will be a small area of fast-rising air right on top of us at about 7pm tomorrow:

Friday night

This certainly explains the concentrated area of precipitation that shows up on the surface forecast map for precipiation. Also notice the yellow lines, indicating thickness values (you can learn more about thickness here). The solid yellow line for critical thickness is well to the south of DC. So, the precipitation would all fall as snow, but with temperatures well above freezing on Friday, the nighttime snow will probably melt at first before accumulating. Overall, this appears to be another small storm, with no more than 3" of accumulation in the District. Here's a 6-hour precipiation projection on the NAM:

Liquid equivalents approach .5" in that 6-hour period on this projection. With vigorous melting at the onset, we definitely can't expect a typical snow/rain ratio (10:1) from this system. One thing is sure... any snow that sticks around by Saturday morning will still be here until President's Day Monday. The cold advection continues on Saturday behind the front with the snow, and high temperatures will struggle to reach the freezing mark on Sunday. The cold weather and a bit of fresh snow should we welcome news for skiers on the last holiday weekend of the winter!


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