There are many people out there wondering, however, why the wind weakened abruptly as one moved westward away from the Wasatch Mountains. This weakening has to do with the structure of high-amplitude mountain waves. In such a wave, high momentum air from aloft descends rapidly and accelerates on the lee (in this case west) side of the mountain range. Once in the lee, the flow frequently exhibits a hydraulic jump in which the flow ascends rapidly. A rotor, or an area of reversed flow at the surface, is typically found downstream of the hydraulic jump.
|Source: Whiteman (2000)|
|Source: GFDL/Wikipedia Commons|
At the surface, the transition from strong downslope flow to return flow in the rotor can be incredibly abrupt. Check out the MesoWest plot below, which shows strong easterly flow near the Wasatch Mountains, but three stations along I-15 with westerly flow. In particular, there is a 55 knot sustained east wind at Farmington, but two stations just to the west in the surrounding area report west winds.