Winter storm warning on Tuesday; moderate to heavy snow a good bet

** Winter Storm Warning most of the region 3 a.m. until 2 p.m Tuesday **
** Dense Fog Advisory through 2 a.m. **

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Updated snowfall forecast as of 10:40 p.m.

Updated snowfall forecast as of 10:40 p.m.

10:40 p.m. update: This evening’s American models are in, and they generally agree with our forecast thoughts. The map above has been updated to expand both the 2-5″ zone and the T-2″ zone south.  We expect snow to begin to arrive over western parts of the area before dawn, with bands of moderate to heavy snow (possibly mixed with sleet in spots) pushing through the area during the morning. 7 a.m. through 11 a.m. remains the preferred time for the heaviest locally.  Snow winds down by early afternoon, with temperatures remaining near freezing.

One FAQ worth addressing here…

Is there any way this could be a bust? Yes, there is one scenario in which the heavier precipitation misses the metro area to the south, temperatures don’t cool down enough and we end up with some sleet and closer to an inch of snow. We think the chances of this scenario happening are on the low side, but not impossible.

We’re going to sign off with updates for the night, but we’ll be back by 5 a.m. to cover the storm.

10:15 p.m. update: We’ve made some small tweaks to SchoolCast, but are leaving FedCast as is from earlier. In either case, travel during the morning appears it will be extremely difficult.  Keep track of what’s closed.

Tuesday SchoolCast (area-wide, excluding St. Mary’s county):

(3 apples) Lots of school closings are likely.  Some, like Fairfax County, have already announced such.

Tuesday FedCast

capitol-black.jpgcapitol-black.jpgcapitol-gray.jpgcapitol-gray.jpg (2.5 domes) Better than 50/50 chance of unscheduled leave policy and/or a delay. 25 percent chance shut down.

We’ll have a quick forecast update in the next 20-30 minutes or so…

Hi resolution outputs from the North American Mesoscale model hi-res run this evening.  On left we see strong bands of snow moving into the area. On right, total precipitation through the day.  Note a max along I-95. (Weatherbell.com)

Hi resolution outputs from the North American Mesoscale model hi-res run this evening. On left we see strong bands of snow moving into the area. On right, total precipitation through the day. Note a max along I-95. (Weatherbell.com)

9:45 p.m. update: Not much change to the early models this evening. The image above shows the high resolution NAM. It indicates moderate to heavy snow moving into the region around 5 a.m. out west, and really going nuts in the 7 to 10 a.m. range (7 a.m. panel shown at left). This is just one model, but it more or less matches our timeline and thoughts from earlier. Of potential interest is a precipitation maximum of about 0.50″ liquid (snow ratios of 10:1 snow to liquid are common) along I-95. We’ll have more soon, plus an updated SchoolCast and FedCast…

Most of the local area has been upgraded to winter storm warnings. (National Weather Service)

Most of the local area has been upgraded to winter storm warnings. (National Weather Service)

9:00 p.m.: The National Weather Service has upgraded the I-95 corridor in our area (most of the region) to a Winter Storm Warning. They expect 3-6″ of snow across the zone.

Our snow accumulation forecast of 2-5″ is similar and we are assessing new data this evening. We will update this post with any new thoughts between 10 and 10:30 p.m.  Below is the text of the winter storm warning.  Of particular note: 1-2″ per hour snowfall rate potential at times.  That’s the type of stuff plows can’t keep up with as it falls.

… Winter Storm Warning in effect from 3 am to 2 PM EST Tuesday…

The National Weather Service in Baltimore MD/Washington has
issued a Winter Storm Warning for snow… which is in effect from
3 am to 2 PM EST Tuesday. The Winter Weather Advisory is no longer
in effect.

* Precipitation type… snow… heavy at times. Precipitation may
start as a period of snow and sleet before dawn changing to all
snow after daybreak.

* Accumulations… 3 to 6 inches.

* Timing… around daybreak Tuesday lasting through early Tuesday
afternoon… including the Tuesday morning rush hour.

* Temperatures… upper 20s to lower 30s.

* Winds… northwest 5 to 10 mph.

* Impacts… hazardous travel conditions are expected from heavy
snow. The snow will impact the morning rush Tuesday with the
potential for accumulation rates of 1-2 inches per hour.

Dan Stillman and Jason Samenow contributed to this post.

Also on Capital Weather Gang

PM Update: Mostly calm tonight before Tuesday snow; snow drought ending?