** Winter Weather Advisory Tuesday for snow over entire region **

Power outages | The Grid: Tweets, Instagrams from storm | Maryland pavement temperatures | Snow/ice amount reports | Local weather photo gallery

Update, 2:35 pm.:  The National Weather Service has taken down the winter storm watch for the region, and issued a winter weather advisory for 3-5″ of snow – more or less consistent with our forecast.

From 1:58 p.m.: Snow and/or sleet is expected to commence tomorrow morning around or just before rush hour. There is the potential for a stripe of 2 to 5 inch of snow across the region – which would have a major impact on the morning commute.

We can’t even completely rule out pockets of 5 inches or more snow, while less than 2 inches of snow is also possible, if the storm tracks too far south.

The precipitation will arrive first in the western suburbs probably around 4 or 5 a.m. and then will spread rapidly eastward across the region  Temperatures are expected to start out  around freezing but then drop a degree or two as the precipitation begins in earnest.  They should be low enough to support accumulating snow.

Falling snow could at times restrict visibilities and lead to slick spots on major arteries. Untreated secondary roads could become covered if a period of moderate to heavy snow develops.  Because of the storm’s timing, this storm has the potential to cause commuting woes and lead to school closures.   There is some potential for 1-2 inch per hour snowfall rates (not a guarantee) during rush hour – which would result in gridlock if roads are crowded.

Given the possibility of a hazardous commuting situation,  the National Weather Service has issued a winter storm watch for much of the area.  The National Weather Service’s Weather Prediction Center earlier this morning assessed the probability of getting over 4 inches of snow to be around 30%. That assessment seems about right to us.

Detailed storm timeline

Tuesday morning: Snow develops around the beginning of rush hour around 5 a.m. or so.  A pretty good snow burst is possible from 7-11 a.m., generally from southwest to northeast.   Temperatures fall from near freezing to around 30.  Visibilities could be restricted in the falling snow and roads could become hazardous.

Tuesday afternoonThe storm will be a fast mover and probably will only last for about 6 hours.  Most of the snow will have exited the region by mid-afternoon. Right now it looks like most accumulations will be in the 2-5 inch range with locally higher and lower amounts possible.

Tuesday evening:  The snow should be over and major arteries should be wet rather than snow covered.  However, untreated secondary roads could still be slick.

Likely impacts of the storm

1)  The storm is likely to negatively impact the morning rush hour – leading to delays and, in a worst case scenario, crippling travel. As the pavement may still be wet in areas from the storm Sunday into this morning, the combination of falling temperatures and snow could make  roads particularly icy. Allow plenty of extra time for commuting or, if it looks bad when you get up, don’t venture out.

2)  Due to the storm’s timing, there is a good chance that schools will be cancelled in some/many jurisdictions.

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

(2.5 apples) Better than 50/50 chance of no school. Good chance of at least a delay. Do your homework now, so you don’t have to sweat it later.

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.

3)  If/where ice remains on trees and if heavy snow falls on top of it, damage to tree limbs could occur along with additional power outages.

4) Expect minor to major delays at the airports in the morning.

Tuesday TravelCast

(2.5 airplanes) Minor-major airport delays expected, mainly in the morning.

The storm’s evolution

The GFS model simulations, shown below, show the storms projected movement and timing.


Note that by 7 a.m. tomorrow morning the GFS already has snow spreading into D.C. The heaviest snow looks like it would fall from around 7 a.m. through 11 a.m. with the entire storm only lasting around 6 hours.  The quick hitting storm will exit the region rapidly by mid-afternoon.

Note, in the figure above, how quickly the weak surface low races from North Carolina to well off the coast into the Atlantic by 1 p.m. tomorrow.  That quick movement is why the storm will end so quickly.  The image also gives an idea how the temperatures are cooling as the snow is falling.

The timing of the NAM model is slightly slower but its forecast is very similar to the forecast shown above.

Several factors make this a tricky forecast (technical discussion)

The storm is being produced by a weak surface low along a front to our south (see figure above). If the wave were to remain a little flatter than currently forecast (keeping it farther south) that would cut down on the precipitation amounts and keep us on the northern fringe of the precipitation.  That is what the European model has been showing and would suggest only around 2″ of snow in the area.

The Short Range Ensemble Forecast System (SREF) model also simulates less snow than the GFS and the NAM models. Remember that ensemble members are model runs with the initial conditions or model physics tweaked a bit to try to estimate how predictable the pattern is.  The ensembles members suggest there is less certainty about getting a good solid shot of precipitation than with Sunday’s storm.  

Note that in the simulation below, after today’s light rain, a few lines – showing the amount of precipitation with time in different simulations –  are flat  suggesting they simulate the storm mainly missing us to the south (the colors indicate precipitation type – the transition from green to light blue to blue indicative of rain changing to sleet to snow).  Those simulations (or ensemble members) showing a miss are in the distinct minority so we’re leaning towards a compromise between the European model and two American models.

Plume diagram showing precipitation accumulation over time from different model simulations

Of more concern on the diagram above is the number of members that hold onto sleet (light blue color) long enough to keep accumulations down.  The GFS and NAM models suggest any period of mixed precipitation will be short-lived but there are a few SREF ensemble members suggesting that the sleet could hold on long enough to mess up our snow forecasts.

Also, surface temperatures across the region are expected to be only a degree of two below freezing. If temperatures are degree or two warmer than forecast, that’s yet another factor that could keep accumulations down.

Finally, the heaviest snow often ends up occurring in a rather narrow band. Where that banding sets up is often almost impossible to pinpoint ahead of time.  Within such a band,   localized amounts exceeding 5 inches could materialize.  On Sunday, the heavy band developed north of model predictions leading to surprise snowstorm from Frederick, Md. to Philadelphia (and into New Jersey).  There’s no knowing exactly where the heaviest band will set up tomorrow.

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