National Weather Service Weather Forecast Office

A very windy pattern set up this weekend as a ridge over the eastern Pacific shifted inland bringing with it a 150+ knot upper level Jet. The high pressure ridge pressed up against a low pressure system dropping south into the Dakotas from the Canadian plains...increasing the surface gradient. The orientation of the Jet streak forced the high winds aloft to the surface where it was then enhanced by mountain terrain. Extreme winds near mountains are typically due to something called "gap flow" where the strong winds are funnelled through favorably oriented valleys. The extreme winds during this event were partially to blame on gap flow, others were due to phenomena known as "breaking mountain waves." This occurs when strong winds flow across a mountan range, the mountains create turbulence and the winds become erratic and crash to the ground, often at extreme speeds and often creating hazardous conditions for those on the ground as well as in the air. There were numerous pilot reports of severe turbulence and mountain wave activity throughout the 24 hour period.

The image above is from the GOES West Water Vapor Product with the surface pressure analysis overlaid in yellow. Peak recorded wind gusts are overlaid as well. The image is remarkable because it depicts the "downslope drying" or Chinook effects that occur on the lee slopes of mountain ranges during high winds; these areas are shaded red. Even though no recorded wind gusts in the Bighorn Mountains met warning criteria, the presence of high winds is noted by the downslope drying that ocurred on the lee side of the range.