Abstract

Damaging foehn winds, locally known as “chinook” winds, are loosely defined and generally described for the east slopes of the Montana Rockies. Three upper-level patterns associated with chinook episodes in Montana are described and illustrated. The “basin high” pattern is characterized by a strong surface high pressure system in the Great Basin, and is associated with light to moderate southwest winds and mild temperatures along the east slopes of the Rockies. The “Klondike” pattern is associated with chinook winds when slightly warmer air replaces an arctic air mass that has pushed southward over Montana. The “frontal” pattern produces the most significant and damaging chinook wind episodes and occurs when a rapidly moving Pacific cold front sweeps into Montana or southern Alberta as a ridge remains farther south over the western United States. An objective aid for forecasting the strength of wind gusts for the frontal chinook pattern at Great Falls, and other locations in the chinook belt, is described. Verification of the multiple regression equation is presented, and possible reasons for underforecasting peak wind speeds in some cases are discussed.

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