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Posted at 01:19 PM ET, 01/20/2012

Winter weather advisory for entire D.C. metro region, frequently asked questions


Winter weather advisory covers area in purple. Winter Storm warning covers area in pink.
The National Weather Service has issued a winter weather advisory for the entire metro region, effective 11 p.m. this evening through 1 p.m. Saturday.

The stage is set for an icy mix of weather in the metro region starting tonight and lasting into Saturday.

Very cold, dry air is steadily bleeding into the metro region. How cold? The temperatures to our northwest are only in the teens and 20s. Around here, many spots are below freezing west and north of town and will stay there through the afternoon.

Our last post detailed the expected storm timeline and impacts by zone while providing a snowfall accumulation map.

Now, here are some frequently asked questions...

FREQUENT QUESTIONS

Where is the precipitation now? Follow the progress of the storm on national radar, regional radar and on our Weather Wall. At the moment, it’s snowing in Chicago from this same storm, and 5-8” are expected there.

How confident are you in your forecast? We have fairly high confidence a good part of the metro region will experience frozen precipitation and slick travel tonight. Determining the precipitation type at particular times and the timing of transitions is another matter.

For example, temperatures aloft will only be marginally cold enough for snow and for a brief time, so it’s possible we get mostly sleet and freezing rain and not much snow at all.

As another example, it’s difficult to say - especially right around the District, exactly when temperatures will rise above freezing. It could be as early as 6 or 7 a.m., or it may take until 9 or 10 a.m., if even then. (The farther north and west you go we’re increasingly confident temperatures will remain below freezing)

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Could the storm produce a lot more snow than forecast? Probably not. Models are consistent in bringing in a layer of warm air that will melt the falling snow flakes before they refreeze into sleet or turn into ice on contact with the cold ground.

Could the storm produce more significant icing than forecast? Yes. The real risk in this forecast is that temperatures stay colder longer than forecast over a larger area. This sometimes happens with these kinds of events. So there is a possibility (30% chance), for example, precipitation remains in the frozen form for a good part of the storm even into the immediate metro area.

Could the storm produce less ice/snow than forecast? Yes. In fact, one of the models simulates (the Euro) very modest precipitation amounts. But just a little ice can cause hazardous travel.

Where will conditions be worst? Definitely the farther north and northwest you go, where temperatures may hold in the 20s for the entire storm, leading to the potential for more significant icing.

When will conditions be worst? An initial thump of snow this evening between 10 and 1 a.m., may create hazardous travel. Then a second wave of steady freezing rain may produce particularly icy conditions between 7 and 11 a.m. Saturday morning, but mainly in the colder areas well north and west of the beltway towards Loudoun and Frederick county (Zone 4).


Probabilities of different amounts of freezing rain from NOAA. These are probably a bit conservative. (NOAA)

How much ice are we talking about? Around D.C. and to the south and east, generally a light glaze of 0.1” or less (Zone 1 and 2). In D.C.’s immediate north and west suburbs 0.1-0.2” of ice (Zone 3), and from Leesburg to Frederick and to the northwest (Zone 4) about 0.2” of ice or maybe a bit more.

Related link: Zone map

Will there be power outages? This does not appear to be a crippling ice storm with widespread power outages. However, in some of the far north and northwest areas like Frederick, Leesburg, Hagerstown and Winchester (Zone 4), there may be enough ice accretion for isolated to scattered power outages.

How much snow before the transition to sleet, freezing rain, and rain? See our previous post.

When does the precipitation start and end? See our previous post.

What does Wes Junker, CWG’s winter weather expert think?

The models have continued to trend colder from yesterday’s model runs. Right now we’re leaning towards the NAM temperature forecast because of its better vertical resolution (which should be an advantage in cold air damming events such as this) and towards the GFS precipitation amounts.

While the models still differ on how quickly the cold air modifies across the area and also differ on how much precipitation we get, they both now start the precipitation as snow towards midnight. They each now suggest there could be an inch or two of accumulation especially north and west of the city before changing to sleet and then freezing rain.

Like most of these events, the changeover will probably be later to the north and west of the city and quickest for points to the south and east.

The big question is how long the freezing rain might last across the city itself.

Right now it looks like the precipitation will probably change to rain sometime in the morning after 7 a.m.. However, low level cold air can be more stubborn than forecast by the models so even people who live inside the beltway will need to monitor temperatures and conditions tomorrow morning.

By  |  01:19 PM ET, 01/20/2012

Categories:  Latest, Winter Storms

 
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