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Posted at 02:05 PM ET, 03/03/2013

Major winter storm eyes Washington, D.C. and Mid-Atlantic for midweek: Snowquester

The likelihood of a significant snowfall occurring in the Washington, D.C. region Wednesday is increasing as a very dynamic storm moves in.

How much snow falls in the city and close surrounding areas is less certain and still depends on the storm track, intensity of the precipitation and the surface temperatures which are likely to be in the low-to-mid 30s during the storm.


Storm timing: Tuesday night to Wednesday night, March 5-7
Chance of 1” or more in D.C.:
70 percent
Chance of 5” or more in D.C.:
45 percent
Chance of 10” or more in D.C.: 10 percent

(note: these odds will be updated tomorrow)

The warmest temperatures are likely to be from I-95 eastward to the Chesapeake Bay and the coldest out towards the Blue Ridge. Therefore, the highest probability of getting significant snowfall with this storm, like many in March, will be west of the city in locations with elevation. Places like Charlottesville, Harrisonburg, and Winchester stand a very strong chance of receiving at least 5 inches of snow.

Even Washington, D.C. could see snow accumulations exceeding 5 inches if the snow falls heavily enough.

Power outages are possible where the greatest totals occur because it will be a heavy wet snow.

Programming note: Visit this blog tonight starting around 9 p.m. for a live blogging session of the evening’s models.

Scenarios and probabilities

Scenario 1: High impact, probability 35%

Precipitation overspreads the area Tuesday evening with the precipitation possibly starting as rain but changing to heavy wet snow during the night and continuing through most of the day on Wednesday and not ending until Wednesday night. Temperatures fall into the low 30s by Wednesday morning and only rise to around 34 in the city and stay a degree or two colder in the western suburbs. Intense snowfall rates would lead to heavy accumulations even in the city and would likely produce double digit accumulations within 60 miles of the city. The heavy wet snow could bring down tree limbs and power lines leading to power outages. Thundersnow could occur.


Scenario 2: Moderate impact, probability 35%

Precipitation overspreads the area Tuesday evening but temperatures only fall to to around 34 by Wednesday morning and then stay steady or rise a degree or two during the day. Periods of moderate to heavy precipitation produce snowfall that accumulates 1 to 4 inches in and around the city but only an inch or two at National Airport. However during periods of lighter precipitation the snow could mix or change to rain especially in the city and to the east. Well west of the city, especially areas with elevation, accumulations top 6 inches. Places to the southwest with elevation that are closer to the storm like Charlottesville and Culpeper still garner double digit snowfall totals. Areas receiving over 6 inches of snow could experience isolated power outages.

Scenario 3: Lower impact, probability 30%

The storm track is a little too far south to spread intense precipitation rates into the area for an extended period. Temperatures don’t cool quite as much as in either of the other two scenarios. The weaker precipitation rates lead to periods of mixed precipitation or even rain. Any snowfall accumulations around the city would be limited to an inch or so on grassy surfaces. For most of the area, any snow would be of the nuisance variety, more wet than slippery. However, higher elevation locations especially to our southwest closer to the storm might still receive 6 inches or more.

Technical discussion

The question of how much snow will fall in city and whether it will accumulate is still very much in question. Most models suggest a period of snow but differ on whether the surface temperatures would be cold enough to allow accumulation. This morning’s NAM is a worst case scenario with the model predicting over 2.00 inches of liquid equivalent in and around the city, most of it in the form of snow, with the temperature remaining in the 33 to 34 degree range during the day on Wednesday. That would lead to double digit accumulations. The good news (assuming you don’t like snow) is that the NAM at at such time ranges isn’t that good and often forecasts too much precipitation.

However, this morning’s GFS also plastered us with almost 1.75 and has a very similar evolution to the NAM. The models suggest the air mass is plenty cold enough for snow except right at the surface where temperatures climb to around 34 or 35 depending over the city with the GFS being slightly warmer than the NAM. Still the forecast sounding would argue for snow. This is a very dynamic system and the strong dynamics should prevent too much warming as long as we remain in the band of heavy precipitation.


(StormVistaWxModels.com)

With such good agreement between the GFS and NAM concerning the storm track and snow possibilities, some might question why we are still toying with the possibility of this storm producing little or no accumulations in the least snowy of our three scenarios. This morning’s Short Range Ensemble Forecast system gives an idea of the uncertainty about the precipitation falling as snow versus rain.

At 7 a.m. Wednesday the SREF guidance (see below) is suggesting that there is a greater than 80 percent chance that any precipitation falling would be snow around the city (and points west). Even east of the city, odds of snow are at least 60 percent at that time. But during the day, around 40 percent of the members change the snow to rain across the city, and any mixing or changeover to rain would limit accumulations. (West of the city towards Sterling, Va., there’s an 80 percent probability the precipitation stay snow.)


Also, the SREF plume diagram (below) showing the precipitation types and amounts for Reagan National Airport from this morning’s run shows that getting heavy precipitation - despite the seeming model consensus between the GFS and NAM - is still far from certain. One member only predicts 0.04 inches of liquid, essentially a whiff. Both the GFS and especially the NAM are on the high end of the spectrum of solutions. However, the mean snowfall from all the member is 1.05 inches of liquid equivalent.


That much precipitation as snowfall would argue for some accumulations. For that reason our forecast for accumulating snow in and around the city is 70%, quite high this far in advance of storm. We could still be fringed by the storm, but that possibility is decreasing and the odds of getting accumulating snow continue to increase. The forecasts from the GFS, NAM and the heavier members of the SREF guidance argue that there is potential not just for accumulations but that someone could really get hammered by a major late season snowstorm.

Conclusion

A strong storm is expected to be located somewhere near the North Carolina coast by Wednesday morning. Some or many parts of the metro region would get a significant snowstorm that could result in power outages if the worst case scenario plays out.

This system is a very dynamic one that the models right now are aiming squarely at our area. Surface temperatures are expected to be right at the cusp between being cold enough for accumulations or being just a little too high to support accumulations in the city. This is a storm to monitor closely. It has the most potential since the Commutegeddon storm of January 26, 2011.

By  |  02:05 PM ET, 03/03/2013

Categories:  Latest, Winter Storms

 
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