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Posted at 12:44 PM ET, 01/16/2013

A possible tricky snow situation for Thursday’s evening rush hour around Washington, D.C.

* Winter storm watch issued by National Weather Service *

A storm passing to our south may give the area a period of snow tomorrow afternoon and evening. Precipitation could start in the morning across the far southwestern suburbs. The storm has the potential to produce shovelable snow accumulations but also has the potential to skirt us to the south giving us only light precipitation or even none at all (note: the latest European model shows NO snow for D.C. and points north).

Next Chance of Accumulating Snow: Thursday and Thursday night

Probability of Accumulating Snow (1” or more): 45 percent

The compact storm will have plenty of moisture with it. The big questions for our area are: 1) how far north will the precipitation shield get? and 2) will it extend far enough north to deliver moderate to heavy snow? If the latter occurs, accumulating snow becomes likely and roads during Thurday afternoon’s rush hour could become slick and hazardous.

The precipitation gradient (how quickly the precipitation drops off as you go from south to north) is exceptionally tight so minor variations in the storm intensity and track could have a significant impact on whether we see accumulating snow.

Surface temperatures are expected to be near but a little above freezing further complicating the forecast as the precipitation could start as rain. The heaviest precipitation and snow is likely south and southwest of the city and the lightest to the north.

There is sufficient uncertainty about the storm to offer four possible storm scenarios with our best guess at the probabilities of each one as such:

1) Precipitation starts as rain or snow late morning or tomorrow afternoon and changes to all snow with temperatures falling to near or below freezing by rush hour with snow accumulating several inches (1-3 inch accumulations being the most likely range). This is one of the rare cases where the heaviest accumulations could fall south of the city. Based on this morning’s model runs, this is the most likely scenario (probability of around 45 percent).

2) The storm skirts to our south but our precipitation rates stay low with temperatures staying above freezing giving us rain to snow but little to no accumulation (less than one inch). (35 percent)

3) The storm stays far enough south to keep the D.C. dry with only our southern suburbs seeing any precipitation. One of the two least likely scenarios. (10 percent)


The National Weather Service’s official forecast is essentially our scenario 4, just a 10 percent possibility in our mind. Can’t rule out it — amounts are more realistic - in our opnion - southwest of Stafford county. (National Weather Service)

4) The storm edges a little farther north and heavy snowfall rates allow heavy snow accumulations of 4-5 inches or more (10 percent)

Technical discussion

The plume diagram below - showing the evolution of precipitation type and amounts over time - from an overnight model run illustrates the wide range of solutions offered by the model’s various ensemble members.


How precipitation type and amounts evolve over time for different runs of a given model. Green shades indicate rain, blue shades snow...amounts increase with time between 7 a.m. and 7 p.m. Thursday.

Remember that these members have their physics and/or initial conditions tweaked to get a feel for the certainty of a forecast.

The plume diagram below displays members having drastically different solutions. Essentially the diagram is screaming that this particular forecast is a tougher than normal one.

The precipitation with the storm does not start until sometime during the day on Thursday January 17. Note that the a few members forecast almost no precipitation or just light rain while most are now calling for snow.

The members with lighter precipitation do not cool as much as the members with offering heavier amounts. Even if the precipitation were to fall as snow, if the precipitation rates were on the low side, accumulations would be difficult to achieve.

The colder, heavier precipitation members showing snow would support some accumulations. The very heaviest, would even support heavy snow.

The reason for now favoring the accumulating snow scenario in the 1-3 inch range is the similarities of this morning’s model runs. Both the GFS and NAM (see below) spread precipitation across the area with the surface temperatures at 7 p.m. forecast to be in the 33 to 34 degree range in Washington, D.C. depending on which model you look at.


The SREF model mean for snow from all ensemble members is .43 (liquid equivalent) pretty much in line with the latest forecasts from this morning’s GFS and NAM. The models appear to be coming to a consensus that there will at least be some precipitation but also that some of the precipitation will fall with temperatures being above freezing. That is our reason for no buying into a worst case scenario and leaning towards more modest amounts.

Conclusion

A compact storm is expected to track across the Carolinas and bring precipitation towards the area. Temperatures are expected to be near or a little above freezing for much of the storm but still look cold enough to support snow. Small changes in the track and intensity could lead to significant differences in how the storm impacts the area. Right now, it looks like some accumulations are likely but that for areas in and around the beltway accumulations are more likely to be in the 1-3 inch range than to exceed 5 inches. To the southwest into central Virginia (including Charlottesville), heavier amounts are more likely.

By  |  12:44 PM ET, 01/16/2013

 
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