The Formation of Alberta Clippers
Source: USA TODAY research by Chad Palmer, Graphic by Chuck Rose
http://www.usatoday.com/weather/tg/wclipper/wclipper.htm
Alberta Clippers form just east of the Rocky Mountains and generally
sweep south and eastward across southern Canada and the upper Midwest
states. They then move eastward off the Mid-Atlantic Coast. They
tend to develop east of the Rockies because air flowing eastward over
the mountains creates favorable conditions for lee cyclogenesis.
The term Alberta Clipper is much more popular in the United States than
in Canada. One study found an average of 22 Alberta Clippers per
4-month winter period, or about one system every 6 days. A more
technical description of their formation follows below.
The following information was largely gathered from the article
written by
Dave
Schultz and Charles Doswell listed on the sources page.
An Alberta Clipper was defined in one study to be "a preexisting lee
trough or cyclone in southern ALberta [that] moves eastward away from
the lee slopes of the mountains past 110ºW."
A typical Alberta low begins its life as a cyclone over the eastern
North Pacific Ocean. The system moves eastward and gradually approaches
the west coast of North America. Once the cyclone reaches the Rocky
Mountains it weakens and decelerates, due to the topography of the
mountains, and curves northward.
Redevelopment then occurs on the lee side of the mountains. "As the
tropospheric flow crossing the Rocky Mountains increases, lee
troughing occurs, fixed to the lee slopes of the mountains by warming
due to dry-adiabatic descent." This process is what creates what we
call an Alberta low.
The lee trough is then set in motion when a short wave trough or jet
streak couples with the lee trough. The system is now ready to make
its track across southern Canada and into the United States
bringing with it some snow and a
reinforcement of cold arctic air. The departure of the trough is
crucial for forecasting
the track of the Alberta Clipper system.
So, to summarize, the formation of an Alberta Clipper system depends
on "a dying, landfalling Pacific cyclone, and rebirth from the lee
trough."
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