** Winter storm watch Wednesday night and Thursday **

2:30 p.m. update: Here’s a look at the snowfall forecast from the National Weather Service.  It’s close to ours (see below, from noon), but a little more aggressive on snow totals.

National Weather Service snowfall forecast issued at 1:45 p.m.

Given some of the latest models which more support the low-end of our forecast and signal some uncertainty in the high-end amounts, I think a wider snowfall range is probably best for now. This could go either way.

1:20 p.m. update: The European model tracks the storm farther west than its run from last night, which suggests snow switching over to some mixed precipitation especially along and east of I-95 during the day Thursday with the heaviest snowfall totals in D.C.’s western suburbs, where less (or no) mixing occurs. The Canadian model has a similar idea (see below). So, we continue to think the NAM model – which shows the heaviest snow along and east of I-95 – is an outlier.

Canadian model shows snow/sleet/rain changeover line straddling D.C. at around 10 a.m. Thursday (Environment Canada)

From noon: The snowstorm Wednesday night and Thursday is still roughly 36 hours away, which can be an eternity when a 30 mile shift in the storm track can be the difference between a moderate snowfall and paralyzing dump.  That said, we’ve analyzed all of the latest data and offer our first estimate of snow totals for the region.

Capital Weather Gang snowfall forecast, issued noon, Tuesday, 2/11.

Generally, we’re forecasting 5-14 inches of snow for the region, with the highest amounts most likely in our colder north and west suburbs (8-14″), which are least likely to mix with sleet and rain.  The lowest amounts are forecast for our southeast suburbs into southern Maryland, where less than 6 inches is most likely, due to strong possibility of a changeover from snow to sleet and/or rain.

GFS model shows freezing line (dark purple) cutting through D.C. at 1 p.m. Thursday. The temperature at 5,000 feet (850 mb) – a rough rain-snow line – is just east of D.C. (StormVistaWxModels.com)

There is a wrinkle in the forecast for eastern areas, however.  Some of the modeling actually predicts the heaviest snowfall there, even exceeding a foot of snow.  Those models assume low pressure will track far enough off the coast so that temperatures will remain sufficiently cold for snow.  See, for example, the NAM model snow forecast below.

High resolution NAM model snowfall forecast (WeatherBell.com)

We don’t completely want to dismiss the idea of the heaviest snow along and east of I-95.  However, some other models (GFS and European) track the low farther west, drawing milder air inland, which would change snow to sleet and/or rain along and east of I-95 – at least for a few hours.  Historically, this kind of changeover happens in many storms tracking up the East Coast, and – usually – areas north and west of I-95 – which don’t changeover – end up with more snow.  For now, we favor the idea of the heaviest snow west of the District, but are prepared to make adjustments east if needed.

Additional key points about this storm:

When does it start? Most likely between about 7 and 10 p.m. Wednesday night, south to north.  However, adjustments to this timing may be necessary and we cannot rule some snow starting in southern zones towards the end of Wednesday evening’s rush hour.

When does it end? Most likely between Thursday afternoon and Thursday evening. We’ll fine tune this projection as we get closer to the storm.

Will the snow stick? Yes – at least during the first half of the storm.  Temperatures will be at or below freezing when the snow begins and will be mostly between 25-30 degrees Wednesday night into Thursday morning.  By dawn Thursday, 4-6″ of snow are a good bet across much of the metro region.

When will conditions be worst?  Most likely between late Wednesday night into Thursday morning, when snow could fall heavily. Hazardous travel is likely for Thursday morning’s rush hour.

Will conditions improve during the day Thursday? This will depend on how much temperatures warm up, and whether the intensity of precipitation diminishes. Some modeling is gung-ho about heavy snow continuing and temperatures remaining below freezing for the duration of the storm – well into Thursday afternoon. Other models suggest temperatures creep up to above freezing and that the intensity of snow and/or mixed precipitation (especially along and east of I-95) will diminish some. It’s these competing factors that result in the uncertainty in forecast amounts.

Will schools and governments close? Unless there is a significant change in the forecast, there is a good possibility of school and government closings. We’ll issue SchoolCast and FedCast (for Thursday) tomorrow.

Will flights be affected? Yes – many delays and some cancellations are a good bet, especially early Thursday morning, possibly extending into the afternoon if snow continues at a good clip.

How excited should snow lovers be about this storm? I would be cautiously optimistic but guard against irrational exuberance; i.e. celebrating models spitting out double digit amounts.  There is still bust potential here given the possibility of milder air moving in during the day Thursday and shifts in the storm track.

Programming note: CWG’s winter weather expert Wes Junker will take an in-depth look at how this storm could BOOM or BUST in an update early-to-mid afternoon.

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