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Chinook winds: how a warm, dry wind changes the snow
What a Chinook (foehn) wind is, how to recognize one in the forecast, which ski areas see them, and what they do to snow quality, melt and the snowpack.
By SkiBuds · Updated
A Chinook is a warm, dry wind that blows down the lee side of a mountain range. You can recognize one in a forecast by a sharp temperature rise over a few hours, a fall in humidity, strong sustained wind, and a wind direction coming down off the range. For skiers it usually means softening, then a firm refreeze, and fresh snow that loses quality much faster than the temperature alone suggests.
Why the wind arrives warm and dry
Air pushed up the windward side of a range cools as it rises. Once it is saturated it drops snow or rain on that side and keeps cooling, but more slowly, because condensation releases heat. When the now-drier air descends the far side, it warms by compression at the faster dry rate, about 5.5 °F for every 1,000 feet. It reaches the lee valleys warmer and much drier than it started. The same effect is called a foehn in the Alps.
How to spot one in a forecast
The SkiBuds forecast engine uses four checks, all of which must hold together over about three hours of hourly data:
- Rapid warming. Temperature rises by more than 10 °F.
- Drying. Relative humidity drops by at least 10 points and falls below 30%.
- Sustained wind. Wind stays above 20 mph in every hour, not just in gusts.
- Downslope direction. Wind comes from the west, west-northwest, west-southwest, northwest or southwest, down off the range.
When the checks pass, the engine rates its confidence higher for a bigger temperature jump, drier air, stronger wind and a longer event. You can apply the same checks yourself to an hourly forecast.
Where Chinooks matter for skiers
The engine treats three regions as Chinook-prone when the wind is from one of those westerly directions:
- Colorado's Front Range, including Eldora, Winter Park, Loveland, Arapahoe Basin and Berthoud Pass.
- The Alberta foothills and Rockies, including Sunshine Village, Lake Louise, Nakiska and Castle Mountain, near Banff and Canmore.
- Eastern Washington and Oregon, including Mission Ridge, White Pass, Bluewood and Mt. Spokane.
The Colorado mountain guide and the resort directory have forecasts for these mountains.
What a Chinook does to the snow
Warm, dry, fast-moving air removes snow in two ways. It melts it, and it sublimates it, turning ice straight into water vapor. Wind makes both faster.
Melt. The engine estimates a melt rate from how much warmer than an 8 °F rise the air gets, how far the wind exceeds 18 mph, and how far humidity falls below 35%. Its estimates range from a few hundredths of an inch an hour in a mild event up to a cap of 2.4 inches an hour in an extreme one.
Sublimation risk. Very dry air and strong wind together strip snow fastest:
| Lowest humidity | Average wind | Sublimation risk |
|---|---|---|
| Below 15% | 30 mph or more | Extreme |
| Below 22% | 25 mph or more | High |
| Below 30% | 20 mph or more | Moderate |
Recovery. The engine grades the likely damage to the snow surface and estimates how long conditions take to recover. The starting estimates are 18 hours for minor damage, 36 hours for moderate, 60 for severe and 96 for the most extreme events, lengthened for longer, higher-confidence events.
What to do on a Chinook day
- Minor event: look for shaded and wind-sheltered terrain, where snow keeps its quality longest.
- Moderate event: expect variable surfaces and less soft snow than the morning forecast suggested.
- Severe event: expect major snow-quality loss and a firm refreeze afterward, and put off plans that depend on good snow until it recovers.
- Timing: surfaces soften during the warm wind and set up hard once it stops and temperatures fall overnight. The morning after can be firm.
Rapid warming can also change avalanche conditions. If you ride outside resort boundaries, check your local avalanche center's forecast through Avalanche.org before you go.
Why snow forecasts disagree explains how much to trust any single forecast, and snow-to-liquid ratio explained covers how temperature and wind shape new snow. The Chinook checks described here run inside the SkiBuds forecast engine for the rebuilt iOS and Android app, which is coming soon.