Snow, Then Ice, Then Rain and Some Combination of Everything in Between

I thought about just posting that headline and calling it a day because that's pretty much what the forecast boils down to.

There's a good chance that we'll see every possible winter precipitation type tomorrow at some point.  As to exactly when the changeover occurs and how much precipitation will be falling at what times - that's where the complexity of the forecast really lies.

A couple of things:

- The models have been trending towards less overall moisture for this storm but have also been shifting the axis of heaviest precipitation right over our region.

- The NAM has a wet bias so I wouldn't get too excited about snow amount forecasts and instead concentrate on the area modeled for maximum precipitation.

- Models in general are pretty bad at forecasting overrunning events like this one (cold air at surface hemmed in by the mountains, warm moist air aloft).


Here's the NAM clown map from this morning:

again, focus less on the amounts for now and more on that stripe of heavier precipitation that goes right over us.  This modeled local maximum in precipitation has been shifting southward with consecutive runs of the models and appears to be in pretty good position to give us some precipitation.

Now, what kind will fall?

My best guess is that the precipitation will start mid-morning in the DC area as snow and it will snow for the rest of the morning into the afternoon.  The changeover will begin from east to west, first as sleet (which could be heavy for a bit), then to freezing rain (liquid precipitation that freezes on contact with the surface) then to plain rain as temperatures even at the surface rise above freezing.

I don't think temperatures get much higher than 32 tomorrow or so - maybe less if enough snow and sleet fall in the morning.

By Monday morning, it's all rain and then we'll have to watch and see what happens.  Some models suggest a continuation of the moisture flow into our region and with the cold air firmly in place by Tuesday, we could have another shot at some snow then.

This graphic (called a meteogram, tells some of the story):

Since this is built off of the NAM run, we should probably take the precipitation totals with a grain or two of salt (if this verified, for instance, it suggests 6-8 inches of snow by Tuesday night).  But check out the precipitation types and the difficulty the model has in resolving them.  Blue bars with an "S" caption is snow, red bars with a "P" caption is sleet, orange bars with a "Z" caption is freezing rain and the green bars with an "R" caption is plain rain.

You can see the secondary burst of snow Tuesday morning over there on the right of the graph.

At this point, I think a Winter Storm Warning will be needed for points north and west of DC and a Winter Weather Advisory for DC and points east. 

This should be the type of storm which will keep you inside on a Sunday and, since the Redskins are 3-9 and playing a 9-3 team, should keep attendance at FedEx field on Sunday to about a dozen of Daniel Snyder's family members.

More updates if the afternoon model runs show anything different.