Cold Front Aloft Model Comments

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Cold Front Aloft Model Comments

• The lower portions of Pacific weather systems are
  destroyed as they cross the Rockies.
• The upper short-wave and its associated thermal
  structure can pass across the barrier.
• Troughing on the eastern side can occur on two
  scales
• small mesoscale locked to the terrain associated
  with downslope warming.
• QG leeside cyclogenesis

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CFA

• The low level leeside troughing cannot move
  eastward as fast as the upper trough and a
  natural forward tilt develops.
• With upward motion leading the upper trough,
  precipitation natural leads the slow-moving lower
  trough. Often a potentially unstable environment
  exists ahead of the trough, leading to convective
  precipitation.
• Eventually cold air is able to move into the
  eastward moving trough, producing a more
  classical cold frontal structure.

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CFA

• Another way to look at this. There is a natural
  tendency for normal wind shear with height to
  shear apart systems, changing the phasing between
  lower and upper levels.
• Naturally occurs even without mountains
  decrease in slope with time.
• The mutual support of upper and lower structures
  and interacting vertical motions tends to delay
  this decoupling. (Palmen figure)
• Shearing off the lower portion of a system, and
  establishing a new trough on the eastern side
  that is not dynamically coupled to the system
  aloft facilitates the rapid development of a
  forward tilting system. (e.g., no low level cold
  air advection that supports structure aloft)

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Locations of Lee Trough/Cyclogenesis

• Two major foci Alberta (Alberta Clippers) and
  Colorado/New Mexico (Colorado Lows).
• A map reveals why

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