Thursday, January 24, 2019

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.


8 comments:

  1. Thank you for this good discussion. It illustrates the complexity of wind prediction in the PNW.

    This is a similar result but different mechanism from the strong downslope winds in Port Angeles in December that you also analyzed:

    http://cliffmass.blogspot.com/2018/12/heat-wave-hits-port-angeles-while.html

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  2. Just a fascinating natural event - thank you for the explanation Cliff. It's especially interesting to see things unfold from high above, given how clear our satellite images are becoming.

    Question for Cliff.. you mention the strong winds at about 5000', which isn't very high. Is this a typical ingredient for strong mountain waves to occur, ie, strong lower to mid-level winds? It seems like it makes sense - the strong winds would need to be at levels BELOW the mountain tops in order for the waves to get going.

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  3. It's very interesting to read your analysis of how differing factors come together to create specific events.

    I continue to understand more and more why the mountains' role in our regions weather is one of your favorite topics. Of course, I have a lot more to learn and hope you continue to break events into details a lay person can understand.

    Also, it would be fun to read a compare/contrast analysis of factors' roles in creating two specific weather events where appropriate. I am particularly interested in factors that create a wide degree of variability in weather events.

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  4. We live down slope from Mt. Bachelor, near Sunriver, OR, and experienced that sudden windstorm on Wednesday morning. There was no heads-up from the weather forecasters until it was happening. The wind gusts were really impressive, electric power was blinking on and off and one tree was downed in our neighborhood. Thanks for the explanation - we were wondering what could have caused that wind event!

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  5. After 30+ years in the Seattle area, we recently moved to a place between Sisters and Redmond (OR) and that was indeed a wild, unexpected, morning wind storm the other day. We had a couple junipers fall over.

    I will say, without qualification: I desperately miss Seattle area forecasts, which in the 1-5 day range have become incredibly awesome, even including micro-climates.

    The ones for central Oregon (Bend valley in particular) suck. That's a big, unqualified, disaster-laden SUCK. Pure wrongness. Brutally bad. As bad as I recall Seattle forecasts from 30 years ago.

    We get rain or snow when the forecast is sun. And vice versa. We'll get a freezing day when the forecast high is 50F. And vice versa.

    Yes, there are micro-climates here too. But Puget Sound area forecasts have dramatically improved over the years; why not here?

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  6. WinterWinter For for the Columbia Basin in eastern Washington are also terrible. We have no local radar station so our radar comes from Spokane. The problem is it doesn't read precipitation on the surface but rather at high altitudes, which is usually different from what we observe here on the ground. Summer weather forecasting is usually pretty simple around here. Hot and sunny with wind in the afternoon. Look I'm a weather forecaster now! Plus we have no comparable cliff mass blog over here.

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  7. The lack of a "Cliff Mass Of Bend" is (seriously) the only downfall I've found to living in these parts, compared to Seattle.

    Maybe we can convince Cliff to move here? Zero traffic, many people are remote workers, insane quantity of sunshine, this place is Outdoor Activities Mecca (you are required by law to purchase x-country skis, a mountain bike and paddle board within 18 months of moving). And the socks-with-sandles thing? Totally cool here, too!

    A billion micro-climates, plus I can watch the weather literally roll over 10 volcanic peaks out my living room window.

    And maybe if we convinced Cliff to move (or get a vacation home) near Bend, he might convince the NWS to put a weather radar close by!! Wins all around! :-)

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  8. That downslope wind blew the transparent dome right off of one of our yurts just east of Sisters, OR. Didn’t think that was possible

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