A nocturnal cold season mountain wave heavy precipitation event over the lee slopes of the Wind River Mountains of Wyoming

Widespread precipitation occurred across the western mountains of Wyoming on 02 December 2005 between 0000 UTC and 1800 UTC in moist warm southwest flow ahead of a Pacific low pressure trough. During the event, a dry, cold, shallow continental polar air mass was trapped in the Wind River Basin, east of the Continental Divide and the Wind River Mountains. The majority of the precipitation was distributed orographically increasing with elevation, with the highest precipitation amounts observed on south and west facing slopes and along mountain ridges along and west of the Continental Divide. The majority of precipitation amounts over 20 mm occurred at elevations above 2400 meters. However, the heaviest precipitation (40 to 50 mm) and the greatest precipitation intensity (5 to 7 mm per hour) were observed in the extreme southern end of the Wind River Mountains east of the Continental Divide from Deer Park to South Pass. The eastern edge of the heavier precipitation extended to the foot of the Wind River Mountains in Red Canyon, between South Pass and the city of Lander, down to 1700 meters elevation. The precipitation ended along the edge of the warmer, moist maritime air mass and the colder, dry continental polar air mass in the Wind River Basin. The majority of the precipitation that fell across western Wyoming during this event was in the form of snow. However, in Red Canyon the snow transitioned to rain, with a narrow strip (~ 2 kilometers wide) of freezing rain resulting in clear ice accumulation of 10 mm along the border between the two air masses. The freezing rain occurred with strong downslope winds of 10 to 15 m/s with gusts as high as 25 m/s. Analysis of KRIW WSR-88D reflectivity and velocity products indicated that convective cells formed along and downwind of the Continental Divide in the southern Wind River Mountains. The convective cells formed as part of a quasi-stationary saturated mountain wave.

The aim of this research is to: A) show how standing mountain waves in the southern Wind River Mountains are a significant mechanism for the redistribution of precipitation from west of the Continental Divide to drainages east of the Continental Divide; B) show how this particular mountain wave produced a rare convective freezing rain event in a downslope wind storm; and C) investigate the ability of operational mesoscale models run locally at the Weather Forecast Office to forecast such mountain wave events. This research also aims to improve local high resolution (2.5 km) grid forecasts during mountain wave events and possibly assist in ongoing weather modification efforts to increase snowfall in the Wind River Mountains.