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Sympathy for the Devil: Winter, the Remix

A. Camden Walker @ 6:11 PM

[POSTED FOR Steve Scolnik due to technical difficulties]
A low pressure area tracking eastward from the Texas-Oklahoma panhandle area this afternoon presents an interesting forecast challenge for the mid-Atlantic region tomorrow. The main U.S. forecast models have been relatively consistent in handling the evolution of this system, but as is so often the case in this area, the devil is in the details.

Bottom line: We expect rain or mixed precipitation to begin during the morning on Tuesday. Mixed precipitation will change to snow during the afternoon, mainly in the higher elevations to the north and west of the District. (You can get a rough estimate of the altitude for most of DC and nearby Virginia with the USGS map at Unisys. For Montgomery and Prince Georges County, scroll north or east.) Accumulations of up to 1"-2" will be mainly on grassy areas, but some untreated roads may become slushy or even coated in spots, especially bridges or in any heavier bursts of precipitation. For details on the timeline and accumulation map, see Jason's post below. Stay tuned as conditions develop; the team will keep you updated through the evening and during the day on Tuesday.

The Devil Plays 3-D Chess (above): North America surface weather map at 1pm today from HPC/NCEP/NWS

Background Details

Here's the current rationale:
  • Temperatures: This afternoon's temperatures, despite a sun filtered by high overcast, are in the mid to upper 40s, with some 50s on the southern fringes of the region. Considering the lateness of the season, this would not be very favorable for frozen precipitation, but the low will be moving eastward through southern Virginia and North Carolina. This means that we will be on the cold side of the storm. There is a big fat cold high pressure area parked west of Hudson's Bay, but early this afternoon, there was hardly a single reading below freezing south of the Canadian border and east of the Mississippi. I was very skeptical of this at first, but the models have been very insistent on lowering our temperatures nearly 20 degrees by mid morning tomorrow, to the upper 20s. How is that possible? Well, the air over us right now is very dry (dewpoints in the teens to low 20s). As the precipitation begins to form at higher levels and it falls through this drier air, it will evaporate for a while, cooling the air at the surface, but probably not as drastically as the models are implying. If the temperature does drop that much, we would have to raise our snow accumulations.

  • Moisture: Unlike the pitifully dry system late last week, this system has lots of moisture to work with; rain in Texas yesterday was quite heavy.

  • Upper-level energy: There is an area of strong upper-level energy associated with this system, but it will be weakening as it moves eastward. This means that the secondary low pressure area which forms off the coast will track eastward, rather than moving up the coast, limiting the amount of precipitation we will get.

  • Surface circulation: This is expected to remain strong as it moves eastward to southwestern Virginia, then transfer much of its energy to a new area off the North Carolina coast.

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