Potential Downslope Windstorm

Many residents of the northern Wasatch Front recall the major downslope windstorm that occurred on December 1st, 2011, when wind gusts reached more than 100 mph near Centerville.  Discussion of the event from previous Wasatch Weather Weenies posts is available here, here, here, and here.  The latter includes some videos, including a humorous one of some of our students attempting to launch a weather balloon.

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)

Most of us, especially whitewater kayakers, have seen similar phenomenon in rivers and streams where the flow moves over a rock, accelerates on the downstream side, and then rises abruptly in the hydraulic jump.  In the atmosphere, as in the stream below, considerable turbulence can be found where the wave is breaking.

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 leads to a period where strong easterly flow is able to plunge down the west face of the Wasatch Mountains.  The cross section cuts across northern Utah and southwest Wyoming at 1200 UTC (0600 AM MDT) tomorrow.  Note the layer strong flow in excess of 25 m/s (50 knots) indicated by orange-red color fill that extends from southwest Wyoming into northern Utah, and then down the western face of the crudely resolved model Wasatch Mountains.  

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.