Downslope windstorms are produced by high-amplitude mountain waves. In a high amplitude mountain wave, high momentum air from aloft descends rapidly and accelerates on the lee (downwind) side of a mountain range. The strongest winds typically occur near the base of the mountain, and one can often find a hydraulic jump in which the strong flow ascends rapidly just a bit further downstream. A rotor, or an area of reversed flow at the surface, is typically found downstream of the hydraulic jump.
|Source: Whiteman (2000)|
The forecast models suggest that we could see a downslope windstorm tonight along the northern Wasatch Front. As shown in the NAM forecast below, a closed 500-mb low will move across southern Utah today and tonight, with cold air moving southward into Wyoming. Eventually, a shallow layer of cold-easterly flow penetrates from Evanston westward across the Wasatch Range, where easterly flow penetrates into the northern Wasatch Front.
|NAM model forecast of 500-mb heights (black contours) and 800-mb winds (vectors) and temperature (warm-to-cool color contours every 2ºC) from 1200 UTC 8 April –1200 UTC 9 April 2013.|
This and other model forecasts indicate significant potential for a strong downslope windstorm tonight. How strong will depend on the details, including the strength and height of the inversion at the top of the cold air, details of how the wind speed and direction vary with height, etc., but it is clear that this could be a significant event. The National Weather Service as issued a High Wind Warning for the northern Wasatch Front. Take appropriate precautions and secure loose items that could turn into projectiles in strong winds.