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DC: Difficult Call

Steve Scolnik @ 5:45 PM


It's been a sunny and cold December afternoon in the Washington metro area, but there's been a flurry of activity among the snow lovers and haters alike. It doesn't look like much on this afternoon's weather map, but a weak low pressure area moving eastward from the lower Ohio Valley has strong support from energy at higher levels in the atmosphere. The models have been consistent in strengthening the low as it moves eastward, bringing precipitation to the mid-Atlantic area. The forecasting problem centers around how much precipitation, and especially what type, will fall at any given location. As so frequently happens in winter, the DC area is expected to be right on the borderline of frozen vs. liquid.

See Josh's post below for the time line.

Let's review the factors that make this such a "yes, but" kind of situation:
  • Cold air: It's quite cold in the region now. Temperatures have struggled to make it to the low 30s today. On the other hand, the strong cold high pressure area which is now over us is already slipping away off the coast, and there is nothing in the upper-air circulation pattern which would keep it in place or provide more cold air during the storm. Local surface winds, although weak, have already veered into the east and even south. This should continue overnight. Since it's still so early in the season, Atlantic water temperatures are quite warm: 52 degrees just off Fenwick Island, Delaware, and further south, 71 degrees at Frying Pan Shoals off Cape Fear, North Carolina. Any air flow coming in off the ocean will certainly favor above-freezing temperatures.
  • Moisture: There seems to be plenty of moisture available for this system from both the Gulf of Mexico and the Atlantic.
  • Low pressure: As we noted before, the low pressure area is still quite weak, but it is expected to strengthen. The key factor, however, is just where that strengthening occurs. If it remains to the north and west of our area, we will have more ice and rain vs. snow. If it develops to the south and east, we will have more snow. The models today have been favoring the warmer of these 2 scenarios. One way you can play along at home is to watch where pressures are falling at the surface to see where the low is strengthening and/or moving. The 3-hour pressure changes have been the most negative in the Ohio Valley this afternoon, again favoring the warmer scenario.
  • Energy: There is a supply of energy at middle levels of the atmosphere sufficient to support the vertical motion necessary for precipitation. However, this energetic area is moving rather quickly eastward and the models keep it north of the DC area. This would support a shorter period of precipitation and a warmer scenario locally.
As you can see, there are several factors which favor limiting the amount of pure snow in much of the Washington area. For now, CapitalWeather will be sticking with its forecast accumulation. If any changes are necessary later, however, I expect that they are more likely to be lowering accumulations instead of increasing them. Howard B of Channel 9 stopped by earlier and offered his forecast map, which looks quite similar to ours. If you have any ideas, pro or con, you can let us know in the Comments section. Please try to give some data or rational explanation, however, something other than "because it's Friday and I need a day off!"

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