* Winter weather advisory north and west of the District Tuesday from 4 a.m. to 1 p.m. *

GFS model shows strong storm producing heavy rain over the region Tuesday night (WeatherBell.com) GFS model shows strong storm producing heavy rain over the region Tuesday night (WeatherBell.com)

A powerful, and poorly-timed East Coast storm promises a messy couple of days leading into Thanksgiving in the D.C. area.

The storm’s first act offers a chance of a wintry mix Tuesday morning, mainly in the colder suburbs.

The main act – rain and lots of it – spans Tuesday afternoon through midday Wednesday, everywhere.

The final act – though it may decide not to show up – is a possible round of snow and/or sleet Wednesday afternoon and evening.

The payoff? Thanksgiving day itself is dry, though very cold.

Let’s break down each act

Act I: Tuesday morning ice potential

Yes…it’s cold out there. And, as moisture may sneak into the region very late tonight (after 3 a.m.) with cold air still in place, there’s a small chance of some mixed precipitation (snow, sleet and freezing rain).  The air is not likely to be cold enough at high altitudes for accumulating snow.  If there is snow, it will quickly changeover to sleet/freezing rain. The risk of any accumulating mixed precipitation is mainly west of the District – in western Prince William, northern Fauquier, western Fairfax, western Montgomery, western Howard, Loudoun and Frederick counties.

The high resolution NAM model nicely highlights (in brown) where some light frozen precipitation could occur between 4 and 7 a.m.

NAM model simulation of precipitation type at 7 a.m. Tuesday morning (WeatherBell.com)

By 10 a.m., this same model, shows precipitation changing over to mostly rain in this region.

NAM model simulation of precipitation type at 10 a.m. Tuesday morning (WeatherBell.com)

If you don’t like ice and slippery roads, the good news is that some of the latest model data holds the onset of the precipitation until mid-to-late morning, by which time temperatures should be above freezing in most areas, meaning mostly plain rain would just fall.

Graphic showing winter weather advisory details for D.C. region (National Weather Service)

Pay attention to forecast updates this evening and Tuesday morning. When you head out Tuesday, check the temperature. If it is precipitating and below freezing, beware of possible black ice on sidewalks, side roads, ramps, bridges, and overpasses. I would expect main roads to be OK.

Tuesday SchoolCast:

Loudoun, Frederick, and Fauquier counties

50/50 chance of school. Good chance of a delayed opening.

Howard, Fairfax, Prince William, and Montgomery counties:

50 percent chance of a delayed opening.

Everywhere else:

Do your homework. You’re having school, and probably on time.

Act II: The deluge

Steady rain moves into the entire region by late Tuesday afternoon.  The rain is heavy at times Tuesday evening and overnight as a deep feed of tropical moisture is ingested into the storm.

Models are fairly consistent in simulating 1.5-3 inches of rain for this event.

The European model from last night, shown below, spits out an impressive 3 inches for much of the region.


Total rainfall simulated by the European model from Tuesday morning through Wednesday night – click to enlarge (WeatherBell.com)

Localized flooding of basements and streams cannot be ruled out.  The National Weather Service expects to issue a flood watch for the region.

The heaviest rain is over by mid-morning Wednesday.  Precipitation during late Wednesday morning and into the afternoon should be lighter and more scattered.

Act III: A closing bout of snow and sleet?

As the storm departs, it will wrap cold air into the region.  Rain Wednesday afternoon and evening may transition to a period of sleet and/or snow showers from west to east.

GFS model simulates some light snow on the storm’s back side Wednesday afternoon/evening (WeatherBell.com)

The chance of snow is around 30-50 percent (highest in the western suburbs, lowest east) and the snow is unlikely to stick to the roads.  However, there’s a chance it could briefly reduce visibilities and coat grass and car tops.

When does it end?

Any lingering precipitation – which should be light – winds down late Wednesday afternoon into early Wednesday evening.  Thanksgiving Day is dry, but cold, with highs in the 30s.

Travel concerns

Yes, this storm will be a nuisance for holiday travel but should not prove crippling. It is mostly a rain storm and the worst of the rain is Tuesday night.  In the D.C. area, you will be able to get to airports and train stations, but anticipate traffic and delays.

Any iciness on the storm’s front end (Tuesday a.m.) and any  snow on the back-end should not stop air/rail travel.  I would not advise  changing flights and train tickets on account of the weather but I would allow extra time to get to airports and rail stations.  You also should check the status of your favored mode of transportation before heading out.

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