1. 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

2. 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.

3. 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)

4. Locations of Lee Trough/Cyclogenesis Two major foci: Alberta (Alberta Clippers) and Colorado/New Mexico (Colorado Lows). A map reveals why…