wpostServer: http://css.washingtonpost.com/wpost

The Post Most: Local

Posted at 01:34 PM ET, 01/19/2012

Icy weather possible late Friday night into Saturday morning

Absent for much of the season, a bout with wintry weather may finally present itself Friday night into Saturday morning.

This does not look like the storm that snow lovers crave. Rather, it looks to be primarily a sleet to freezing rain storm that slowly transitions to rain during the day on Saturday. However, the precipitation could start as a brief period of snow late Friday night especially across the northern suburbs.

As often is the case with this type of storm, the biggest impact is expected to be north and west of the city and the smallest to the south and east where there may not be much of an icing problem. However, computer models have been trending a little colder in recent runs.

With no real strong low level winds, cold air sometimes can be stubborn and can hang around longer than forecast. Although precipitation amounts should be light, just small amounts of ice can create traveling hazards. Anyone who needs to travel late Friday night or Saturday morning should pay close attention to the forecast as well as evolving temperatures and road conditions.

POTENTIAL IMPACTS BY ZONE


Important note: these impacts may well shift some tomorrow based on new available data and model forecasts....

Zone 1: Primarily a rain event

Zone 2: A brief period of sleet of snow is possible across the northern portions of the zone followed by a period of freezing rain but the temperatures are forecast to rise to above freezing by 7:00 a.m.. However, places like Bowie could see temperatures remaining below freezing at 7 a.m. so a few slick spots could develop. The southern section near Fredericksburg should primarily get rain.

Zone 3: This is the trickiest zone to forecast as the precipitation around and north of Baltimore could stay snow longer than areas farther to the south and west. Some ice accumulations are possible and the roads could be slick with the precipitation possibly remaining freezing rain past 7 a.m. in some areas north and west of the city.

Zone 4: Icy conditions are expected to last past 7 a.m. and could last through most of the event. Some ice accumulations are likely and roads will probably be slick.

MODEL DISCUSSION

The models differ a little on how quickly they warm the surface temperatures and also on how quickly the precipitation gets into the area. This morning’s NAM is a little colder at the surface than the GFS but paradoxically a little warmer at around 5000 feet.


This morning’s NAM surface and precipitation forecast. The middle green shade indicates more than .10 liquid equivalent during the six hours ending at the time of the map. (NOAA)
The NAM forecast to the right illustrates the very slow northward creep of the freezing line (the most southern placed blue line on each panel below). I’ve chosen to show the NAM temperatures as its higher vertical resolution should give it an advantage in handling the cold air compared to the GFS. The maps show that parts of western Maryland and the West Virginia panhandle stay below freezing beyond 10 a.m. or through the period when most of the precipitation is expected to fall.

According to the model output for Frederick, Md and Leesburg, VA, the surface temperatures at 1:00 a.m. Saturday morning may only be in the mi- 20s when the precipitation starts late with temperatures only warming to around 30 at Leesburg and the upper 20s around Frederick by 7:00 a.m. Even by 10 a.m. temperatures are forecast to be hovering around freezing suggesting much of the precipitation may fall as freezing rain in the far west and northwest suburbs.

By contrast, the temperature in the city is expected to be in the upper 20s at the onset of the precipitation with the temperatures forecast to rise to just above freezing by 7 a.m.


The freezing line at 5,000 feet Saturday morning. This is often a dividing line between snow and rain or mixed precipitation.
If you are wondering why most of the precipitation is not expected to fall as snow, the NAM temperature field at 850 mb (about 5000 feet) tells the story. The temperatures at 850 mb are forecast to rise to above freezing fairly quickly (see to the right). However, the model soundings do suggest that at 4 a.m. places like Leesburg and Frederick Maryland could still be having sleet rather than freezing rain. How quickly the precipitation switches from sleet to freezing rain is still up in the air.

Just to add a wrinkle into this already complex forecast, the new Canadian model holds cold air at the upper levels around a bit longer, increasing the odds for a little snow accumulation before the precipitation transitions to a mix. On the other hand, the European model simulates only very light precipitation and transitions any frozen precipitation to rain fairly quickly (by 7 a.m.) except in the far northwestern suburbs.

By  |  01:34 PM ET, 01/19/2012

Categories:  Latest, Winter Storms

 
Read what others are saying
     

    © 2011 The Washington Post Company