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Final storm assessment

Jason Samenow @ 1:40 AM

A winter storm warning is in effect for the entire DC area. Computer model guidance has converged and painted a pretty consistent picture of what will happen tomorrow. I continue to forecast about six inches of snow from this storm for our area, beginning between 8 and 10am. Having said that, some important uncertainties remain that could affect totals. Here's my latest accumulation map and storm assessment:

Overall storm assessment
Storm Impact:

Travelcast (Saturday):

Schoolcast (for Monday):
The changes to the snowfall map from yesterday include increased snow amounts in eastern and central Pennsylvania and increase in precipitation amounts to our east and southwest. The immediate DC area outlook has not changed.

Now let's talk about uncertainties. There are factors that could increase or decrease the amount of snow we get.

Factors that could decrease the amount of snow we get include:
  • Dry air at the onset could cause several hours of virga (snow that doesn't reach the ground). Dewpoints will be quite low initially.
  • A dry slot may form over the area as the initial low transfers its energy to the coastal low.
  • Warm air aloft could change the snow to sleet and/or freezing rain for a period which would cut down accumulations.
  • The storm track--if the low pressure system goes a bit further north than forecast, that would cut down snowfall amounts
Factors that may increase the amount of snow we get include:
  • Possibility of convective snow or thundersnow. It's not out of the question and snowfall rates could reach 2-3"/hour for a time
  • Possibility precipitation stays all snow-- with model guidance showing 0.8-1" of melted precip, that converts to 10-15" or so of snow
  • The storm track-- if the track is a bit south of forecast, then amounts would increase.
Taking into account all of the above, I present one final probabilistic forecast:

I expect the snow to begin between 8am and 10am tomorrow and begin accumulating quickly due to cold ground temperatures. By 1pm, several inches are possible. Snow may be heavy in the early afternoon and may mix with and/or change to sleet/ice (especially south and east of town) late in the afternoon. The worst time for traveling tomorrow should be between noon and 5pm. The steady precipitation will rapidly begin tapering off from west to east after 5 or 6pm. This storm will likely be a fast mover.

There is some possibility that some wrap around snow could occur overnight Saturday into Sunday morning. Any additional accumulation would be light.

Please post what's happening where you live and snowfall totals using the comments link below.

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