A downslope windstorm over central Oregon

Wednesday morning brought a major windstorm to the eastern slopes of the central Oregon Cascades.   

Winds on Mt. Bachelor accelerated to 105 mph and swinging lifts and falling trees closed the ski area!   Power outages affecting tens of thousands of people from Bend to the nearby foothill communities.  A number of roads were closed by falling trees.

And it was all associated with a relatively small area of strong winds associated with the air flow rapidly descending the eastern slopes of the Cascade barrier. 

A map of the maximum winds Wednesday AM over Oregon shows some big variations.   While winds gusted to 59 mph just outside of Bend, there were half that speed  on the western side of the Cascades.  You will also notice a few scattered high values (60 mph or more) on some scattered high areas of eastern Oregon and Washington.

A forecast of the wind gusts (in knots, color shading) and sea level pressure (red lines) at 7 AM Wednesday from output of the University of Washington high-resolution WRF modeling system shows the fascinating situation.  There was a HUGE east-west pressure difference on the upper eastern slopes of the Cascades with very distinct low pressure areas down on the slopes.  Since pressure differences cause winds to accelerate, these pressure differences produced the very strong winds, with the model going as high as 70 knots (about 80 mph) in places.  Lots of 45 kt areas (blue) are apparent.  The model simulation looks very realistic in terms of the near surface wind speeds.

The origin?  A high- amplitude mountain wave.  

When winds perpendicular to a mountain crest are sufficient large with the proper conditions aloft, air is forced to rise up the mountains and then oscillates up and down on the lee side, producing a series of cloud lines when the air goes up (see schematic below).  You notice that the air is moving downward on the immediate lee slope of the mountain, and because of that there are no clouds.

But sometime when the winds approaching the mountain are strong and the vertical variations in wind, temperature, and humidity are jut right, the wave amplitude can increases resulting in energetic descent and very strong winds on the lee side of the barrier (see schematic).  This is what happened on Wednesday.

A high-resolution satellite image Wed morning clearly shows the strong downslope flow on the eastern side of the Cascades (evident from the sharp clearing east of the Cascade crest and the hint of many mountain wave clouds downstream (the rippling cloud lines).

The big wind, downslope windstorm event was initiated by the development of very strong incoming (westerly) winds near crest level of the Cascades--something illustrated by the 850 hPa (about 5000 ft) heights (like pressure at 4 AM Wednesday) below (wind vector asl shown as are temperatures).  A low center was moving into northern WA and the resulting very large north-south height gradient (think of it as a pressure gradient) resulted in strong western flow hitting the Oregon Cascades.  The strong winds was the key in producing this event...they helped produced the strong downslope flow reaching the surface on the eastern slopes.

Another example of how good our weather forecasting technology has become.  And you really have to love the complex meteorology of our region, with intense local circulations and big changes over space and time because of major terrain barriers, gaps, passes, and land-water contrasts.

from Cliff Mass Weather and Climate Blog http://bit.ly/2WjHLrV