Why Such a Busy December?
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Busy/Active Weather Pattern/Set-Up
in December 2008

A Weather Pattern has set up over the region where just about every other day (give or take 12 hours), Central PA is getting another round of mixed precipitation. But many ask -- WHY?

The reasons for this progression of storms include:

  • A Nearly West-to-East Upper Air Flow (above 10,000 Feet)
  • A Stalled (or at least very persistent) Frontal Boundary that is staying right over or just to the South of the region.
  • Waves of Low pressure that ripple along in the steering flow aloft, following each other on a similar path over the Ohio Valley, Great Lakes and Mid-Atlantic Regions.
  • As each wave rolls along, it first bubbles warm/moist air up and over the boundary, creating (mixed) precipitation.
  • After the wave has passed to the east, it draws cold air back down. But, this push of cold air is not strong enough to move the storm track any farther south.
  • The timing of each (larger) wave is about 48 to 60 hours apart right now.
Map of relevant weather features in December 2008
Map of relevant weather features in December 2008

What factors determine the type of precipitation we get?

  • Precipitation type is dependent on the temperatures the precipitation falls through as it falls from the clouds down to the ground.
  • In winter, most of our precipitation starts out as ice crystals (snow).
  • If the precipitation does not melt -- it stays as snow all the way to the ground.
  • If it melts a little (due to a thin/brief layer of warm air aloft), it will be either sleet (re-frozen rain drops) or freezing rain (rain which freezes as it hits the ground).
  • If it melts, and reaches the ground where it is warm enough, it will be plain old rain.
  • In general, the farther you are away from the surface warm front, the better your prospects for snow.
  • Vice-versa -- the closer you are to the warm front, the better the chance you will have sleet or freezing rain (or plain rain).
Precip-Type Diagram (courtesy Brooks Martner)
Precip-Type Diagram (courtesy Brooks Martner)

Our Upper Level Winds are nearly west-to-east due to a weak upper level ridge over the Southeastern U.S. and Bahamas, coupled with a deep polar vortex (low) over Hudson Bay.

Map of Upper Level Geopotential Heights (wind direction) in Mid-December 2008
Map of Upper Level Geopotential Heights (wind direction) in Mid-December 2008

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Page Last Modified: 18 December 2008 03:04:03 UTC
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