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."


Back to Alberta Clippers
Home