Winter Storm Warning DC North and West 4AM Sunday to 4AM Monday for 1-2 inches of snow/sleet accumulation and 1/4 to 1/2" of ice.

Freezing Rain Advisory DC South and East 4AM Sunday to 4AM Monday. Up to 1/5" ice accumulation possible.

After a fine, but chilly, Saturday afternoon, clouds will begin to increase from the west overnight as a developing storm system rotates into the upper Mid West. Latest visible satellite imagery reveals a large expanse of cloud cover in the heart of the Continental United States and a clearly defined cold frontal boundary slicing through eastern Oklahoma and Texas.

All of this is headed eastward, being steered by a 100-110 mph jet stream.

The forecast over the next 48 to 54 hours is incredibly tricky--just like the majority of winter storms this season.

Precip onset:
The North American Mesoscale model brings isentropic precipitation (ie, overrunning precip) into the DC metro region shortly after 7AM as does the GFS. So, I am confident that a wintry-mix of freezing rain, sleet, and snow overspreads the District between 7 and 10 AM.

Precip Type:
Probably the most uncertain aspect of this storm. It appears that cold air will not be quite as strong as it was during the last winter storm, as high pressure is situated in the Hudson Bay in Canada. Nonetheless, a strong Cold Air Damming signal is present in forecast models as cold air bottles up east of the Appalachian Mountains.

The image to the right is a skew-t plot of temperatures and dewpoints. It's basically a vertical slice of the atmosphere, but the isotherms (lines of equal temperatures) are skewed from right to left. By 7PM Sunday evening, the GFS is clearly the warmer of the two models. If the GFS were to verify at this time, all of the frozen precipitation in the District will have turned over to plain rain. The NAM, on the other hand, is much colder, and still barely supports frozen precipitation.

I believe most of the precipitation (say, 80-90%) will be frozen, that is, either freezing rain, sleet, snow, or a mixture of all three. It will start as as snow with possibly a little sleet mixed in. By Sunday evening, enough warm air has intruded into the upper atmosphere to turn everything over to freezing rain.

This may turn out to be a fairly significant ice storm for the region, primarily north and west of the District. Places in St. Mary's county or southern PG county will likely go above freezing Sunday evening and Monday morning, turning everything over to plain rain, limiting ice accumulations.

The morning rush on Monday may be significantly impacted by this winter storm.