Forecasting the Movement of Alberta Clippers
The departure of the lee trough from the slopes of the Rocky
Mountains is vital for determining the timing and movement of
an Alberta Clipper. Historically, the numerical forecast models have
handled lee cyclogenesis poorly; they tend to forecast the cylones
too far north and west. Without proper placement of the cyclones,
the models cannot accurately forecast their departure. Meteorologists
have been able to reveal some key features to look for in order to
better forecast the departure of lee cyclones.
One way to possible forecast a cyclone departure is to look for signs
of
chinook winds. Alberta is known for being particularly
susceptible to these winds. The onset of these downslope winds leads
to the imminent departure of the lee cyclone.
Another way to help forecast departure is to track the location of
the zero isallobar (lines of constant pressure tendencies).
Isallobars are useful in examining the magnitude and location of sea
level pressure changes. In several cases, the arrival of the zero isallobar in Alberta was
coincident with departure.
A third way to get an idea of the timing of departure is to track
upper-level features. Probably the most consistent feature that
signals the departure of a lee cyclone is the presence of an
upper-level vorticity maximum. One way to locate vorticity maxima is
to analyze the wind and thermal structures in the upper atmosphere.
This is best done using upper-air soundings, however, soundings are
only available every 12 hours.
Meteorologists suspect that tracking pressure mimima at the surface
can also be used to locate vorticity maxima. The traveling pressure
minimum is "realated to forcing from the mobile vorticity maximum
aloft"
Schultz
and Doswell. By watching for these pressure minima in time
series at upstream stations, meteorologists were often able to
anticipate lee trough departure hours in advance.
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