Ice risk Tuesday night into Wednesday; significant storm possible this weekend

(NWS Baltimore Washington)

(NWS Baltimore Washington)

3:10 update: The National Weather Service has issued a Winter Storm Watch for locations mainly north and west of the immediate area, but including Frederick and Carroll counties as well as counties to the west and east of those locations. It runs from Tuesday evening until Wednesday afternoon. The watch is for the potential of greater than 0.25″ of ice accumulation tomorrow night into Wednesday.

From 1:00 p.m…

cwg_JunkerMore winter weather threats are on tap this week, but none are clear cut “no-brainer” forecast situations.

A possible freezing rain changing to rain event is on tap for late tomorrow night into Wednesday morning. It could lead to a messy morning commute, especially in the colder locations north and west of the city.

Additional winter weather threats are possible sometime between Saturday through Monday. The timing of these events — and whether they might bring snow, freezing rain, or a mix — is still very much up for grabs.

Late Tuesday into Wednesday event

The next storm in this active pattern is slated to arrive late Tuesday night into Wednesday morning.

A low pressure system is expected to track to the Ohio Valley, with just enough cold air to support the potential for freezing rain in the region. Such a track promises to keep temperatures aloft warm enough to preclude much in the way of snow (some locations, especially north of the immediate area, could briefly see snow or sleet to start).

Despite warmer air aloft, the cold air trapped at the surface may end up at or below freezing across portions of the area, especially those typically colder areas west and north of the city. Below is the latest NAM forecast. The freezing line runs right through the city overnight on this particular run.

(Author adaptation of NCEP graphics)

(Author adaptation of NCEP graphics)

Most of the SREF and GFS ensemble members keep the city just warm enough to stave off freezing rain. However, last night’s European model, and today’s NAM and GFS, all suggested there could be a brief period of freezing rain for the city, and a more extended period of icing north and west of the city.

The higher resolution models like the NAM are quite often better at resolving low level temperatures during cold air damming events (also called a wedge, since cold air from the north gets “stuck” east of the Appalachians).

What’s the bottom line?

Freezing rain is a good bet tomorrow night into Wednesday morning across portions of the area. The colder western and northern suburbs could end up with enough freezing rain to cause travel concerns. Some spots well north and west, mainly outside our area, could see enough ice to cause power issues.

It’s another case where a few degrees one way or another can matter quite a bit, and we’ll need to monitor as the event closes in tomorrow. If you see your temperatures falling below freezing as precipitation arrives, slick spots and other hazards could develop.

Watching the weekend

The models, including the GEFS ensemble members, are showing significant differences from run-to-run, suggesting that while above normal precipitation is likely, other details about individual storms are unclear.

Below is the 06Z GEFS plume diagram. Each of the grey lines is the forecast from one of the GEFS model runs. The only difference between them is having slightly different initial conditions.  Each line in the graphic reflects different simulations of liquid equivalent precipitation over time; when lines rise steeply, heavy precipitation is mounting, when they fall back to zero again, no precipitation is falling.

(Weatherbell.com)

GEFS plume diagram for Dulles. Times are in GMT. (Weatherbell.com)

Clearly, no consensus shows up about when a storm might hit us. Some of the runs bring precipitation in as quickly as Saturday the 8th, while others hold off until late next Monday. A majority of the ensemble members however do suggest that a storm in the weekend time-frame could have lots of precipitation with it.

The handling of any threat in that period is further complicated by the operational models. For instance, last night’s European model brought a wintry mix to rain threat on Saturday, and a more significant ice to rain threat to close the weekend.

At the same time, the last couple GFS runs have leaned toward bringing the initial low pressure system to the Ohio Valley, and then reforming it off the East Coast. That’s usually not a favorable evolution for an all snow event in and around the city, but one that can offer accumulating snow before a changeover to freezing rain or rain. Sometimes it can also produce a significant snowstorm for our north and west counties.

Finally, last night’s Canadian model would argue that the storm could even track to our south.

With so much uncertainty about the track of any storm, all I can say for certain is that there is potential for a significant precipitation event that could bring rain, snow or a mix to the area sometime toward the weekend.

Also on Capital Weather Gang

D.C. area forecast: Mix ends this afternoon; iciness possible Tuesday night, weekend storm threat