Chinook Winds and Their Effects on Southern Alberta

Chinook or “foehn” winds slide down the eastern slopes of the Rocky Mountains and can be responsible for significant local changes to winds and temperatures, leading to impacts on road transportation. The effects of Chinook winds are felt from southern New Mexico through central Alberta. Areas prone to Chinook winds can experience intense and potentially damaging wind gusts and temperature fluctuations over a short period of time, leading to the sudden melting of packed snow on roadways. Once the Chinook winds subside or the winds shift, the sudden subsequent drop in temperatures can lead to a flash freeze, the rapid freezing of standing water on roadways and other surfaces. Chinook winds can occur anytime during the year in Alberta but happen most frequently in the winter months where downsloping winds happen as often as every three days in southwest Alberta and a Chinook pattern can last as long as 10 days. Chinook winds typically blow from the southwest to the west and can reach speeds as high as 150 km/h (93 mph). While Chinook winds can occur as far east as western Saskatchewan and as far north as Edmonton, their effects are most significantly felt over far southwestern Alberta, especially downwind of the Crowsnest Pass.

The process of the development of a foehn wind is a bit complex but is a phenomenon that an experienced meteorologist should be able to identify, using a combination of surface and upper air model data. The meteorologist must first understand the phenomena of a Chinook wind. As an air parcel ascends a barrier (in this case, the Rocky Mountains), the water vapor condenses and releases heat into the atmosphere. This release of heat limits the cooling rate of the parcel. Once it descends the mountain, the parcel warms by compression at a rate that is twice the cooling rate. It is typical for the temperatures on the windward side of the Rockies in a Chinook wind setup to rise as much as 10⁰C or more in such scenarios. A synoptic weather pattern that is common with Chinook wind setups is one where a 500-700 hPa ridge extends from the northwest U.S., northward through the western border of Alberta, creating an upper level westerly flow over British Columbia. At the surface, the ridge is displaced a few hundred kilometers to the west, over the eastern valleys of British Columbia. A “lee trough” develops downwind of the Rockies and over the eastern foothills of Alberta. The lee trough separates the ascending air to the west and descending air to the east. The trough development yields a tightening of the pressure gradient from it to the surface ridge over eastern British Columbia.

Accurately forecasting Chinook winds presents a challenge to forecasters but techniques have been developed to assist with this process. Once such technique that has been developed by the National Weather Service is a regression equation that considers factors such as a constant wind gust variable, 700 hPa height differences across two locations through the area of interest and upstream to the north as well as the 700 hPa height change upstream and differences in the three-hour pressure change of nearby locations to the east and west. Since the development of this technique, new high resolution model data such as the WRF (Weather Research and Forecasting) model and the GEM-LAM (Global Environmental Multiscale - Limited Area Model), has assisted meteorologists with the preparation of road transportation crews for the possibility of rapidly rising temperatures, leading to the sudden melting of snowpack and subsequent refreezing of standing water on roadways as well as advising motorists, especially tractor trailer operators of the possibility of dangerous winds gusts along their route.