Heavy snow threat for D.C. area Wednesday night and Thursday, but mix, rain also possible

crystal_ballA strong Nor’easter is expected to develop along the coast Wednesday night into Thursday bringing the possibility of significant snowfall to the area.  However, especially close to the I-95 corridor and points east, temperatures may warm enough for snow to change to mixed precipitation and rain.  Locations remaining all snow could see double digit totals.

Probability of 1, 4, and 8 inches in D.C. area Wednesday night into Thursday, issued 1:45 p.m. Monday

Probability of 1, 4, and 8 inches in D.C. area Wednesday night into Thursday, issued 1:45 p.m. Monday.  Note the 8″ probabilities, especially, err on the conservative side, taking into account the storm is still about 60 hours away.

This is a classic Miller A-type storm with the primary storm (surface low pressure center) tracking along the Gulf coast and then up the Atlantic coast. Many of today’s computer models (NAM, GFS, UKMET, and Canadian) all slam the city with heavy snow. Last night’s and today’s European model also forecast heavy precipitation but suggest snow will change to sleet and then rain after several inches of accumulation near the District.

This is one of those storms where small changes in the storm track can lead to huge differences in how much snow falls on the city. A track too far to the east would mean just light precipitation, whereas a track too far west would draw in milder air changing snow to mixed precipitation and rain.

Unfortunately, the models  differ enough on the track to make a definitive call on how much snow might accumulate over any specific place around the city almost impossible this early in the game. However, even the warmer westerly (more inland) track favored by the European model would still favor a whopper of a snowstorm for areas from Leesburg, Va. to Frederick, Md and points west.

Any way you slice it, this is shaping up to be a major storm with the snow probably starting Wednesday night before midnight in the southern suburbs and the wee hours of Thursday morning for locations north of the city. Rush hour Thursday morning could be a mess if the snowier models are correct.

The track uncertainty

Historically, Nor’easters stay mostly snow across the city when they have a track near Hatteras but east of Norfolk with the low pressure center staying off the coast. A track west of Norfolk and then over the Delmarva Peninsula ends up bringing too much warming to stay all snow. Keeping the track offshore is especially critical with this storm as the high pressure system to the north supplying cold will be shifting off the coast. The pressure difference between this high and the developing low will determine our low level wind direction and how much warming takes place. The models during the last 12 hours have been spread between the all snow track and the inland track that would ultimately lead to a changeover (to rain or mixed precipitation). Right now, more models are in the mostly snow camp but the most skillful model (the European) and the majority of its ensemble brethren are in the changeover camp.

tracksThe figure to the right illustrates uncertainty of the models concerning the track. Each line on the figure is a Short Range Ensemble Forecast (SREF) simulation of the storm track. The simulations have either slight differences in their initial conditions or minor differences in their physics. The idea for using ensembles is to try to get a handle on the uncertainty of the forecast since some days storms are much easier to predict than others. The models are in agreement that an East Coast storm will develop but suggest there is still quite a bit of uncertainty concerning the track.

The spread of the simulations pretty much mimic the spread between the model runs of the main GFS, NAM, and European model. Last night’s (see the yellow dots on the figure below) and today’s European model had the western-most track of the envelope of solutions. Last night’s GFS track was on the eastern edge of the envelope encompassing all the tracks. However, GFS has slowly edged west with its track the last several runs. The latest GFS is now far enough to the west (the green dots) to support a significant snowstorm. The 06Z and 12Z NAM both favored heavy snow with similar track keeping the low off the coast (the red track), both just far enough east to keep the city in snow throughout the storm. Any track west of the NAM track would likely introduce mixing problems.

Last night’s Canadian and UKMET models were generally on the same page as the NAM forecast and today’s UKMET and Canadian models stayed the course and would still argue for heavy snow. The European model is mostly by itself except that it has support from its ensemble mean from last night.

Today’s precipitation type products from this morning’s NAM and GFS (see below) really illustrate how critical the track is likely to be. Both have tracks favorable for producing a significant snowstorm for the immediate metro area. However, despite such a track each spread mixed precipitation (red area) into the eastern suburbs. Shift that track 30 to 60 miles west and DC would likely mix with sleet and then eventually change to rain. But as of now, both models now predict significant accumulations across the city.

(WeatherBell.com)

(WeatherBell.com)

Today’s European model (below) and last night’s ensemble mean both had such a westward shift to the track. Note how with the low track farther to the west, the freezing line is significantly farther to the west than on the GFS or NAM model forecast valid the same time. By 7 a.m. Thursday, the model also predicts the temperature at around 5000 ft will be above freezing over the city. The European model sharpens the approaching upper system a little quicker than the NAM or GFS and that sharpening helps pull the low more to the west than the other models. However, despite the changeover, last night’s and today’s European model still would argue for accumulating snow.

Today's European model fdorecast of surface pressure, precipitation and 32 degree line (dark blue) and 35 degree line (light blue) at 7 a.m. Monday.

Today’s European model fdorecast of surface pressure, precipitation and 32 degree line (dark blue) and 35 degree line (light blue) at 7 a.m. Monday.

Conclusions

Someone in the Washington area has a good chance of being smacked by a significant snowstorm – possibly one of those rare double digit behemoths. All of us are likely to see some accumulating snow though how much will depend on whether the snow mixes with or changes to sleet and rain. Areas north and west of the city have the highest probability of getting all snow and ending up with a major snowstorm. However, even areas inside the beltway could end up being primarily snow. This is one of those cases to really follow forecasts during the next two days.

The storm is expected to arrive in the area during the wee hours on Thursday morning in and around the city and the precipitation should continue through much of the day. Even if the precipitation mixes or changes to sleet or freezing rain, it might change back to snow before ending as the deformation band (comma head) works eastward over us as the upper level storm center crosses the region. Rush hour on Thursday morning is likely to be a mess especially west of the city but has the potential to be bad everywhere if the non-European models are correct with their snowy solutions.

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