Sunday, December 19, 2010

Major Easterly Wind Event

Wind damage near Enumclaw
Picture Courtesy of the Seattle Times

Strong winds over western Washington and Oregon are almost always out of the south, the product of low pressure centers moving up the coast and/or making landfall to our north. On rare occasions (like on Nov 22 with the cold outbreak), we can get fairly strong northerly winds as cold air pushes southward during an "arctic blast" situation. But there is another direction that can bring very strong winds, but only for certain foothills locations and very specific situations---easterly winds. And yesterday such an event occurred.

Winds gusted to 70-80 mph at foothills locations like Enumclaw and Black Diamond, and a swath of 50-60 mph gusts extended westward over southern King County. North Bend and Snoqualmie Summit are other windy locations. Lesser, but damaging easterly winds also occurred in parts of Seattle. Roughly 100k customers lost power in the Puget Sound region, and power outages also occurred near Portland due to flow accelerating out the Columbia Gorge.

This is probably a 2-5 year easterly wind event. Not the biggest by any means. The big kahuna event was December 24, 1983. Winds gusted to over 120 mph in Enumclaw and the damage was extraordinary--roofs off hundreds of buildings.

Easterly winds are relatively rare in our region for a few reasons...but the major one is that we have a major mountain barrier...the Cascades..that block flow from the east for most situations. However, if you look at a good terrain map of the region, it is clear that Cascades are not uniform--there is a major sea level gap in the Columbia Gorge and a weakness in the central Cascades called Stampede Gap (my book has some good topographic maps and talks about all this if you have it).

Under the right conditions air can push through Stampede Gap and accelerate as it descends into the lowlands...late Friday and Saturday were such a day. You want a large pressure difference across the Cascades, strong flow striking the Cascades from the SE, and for reasons I will explain, a wind reversal above the mountain crest and often a layer of stable air there as well.

Here is a surface chart for 4 AM on Saturday. You can why we had this event....a low center moving northward off the coast and high pressure inland. The lines are isobars....lines of equal pressure. There is a huge pressure difference across the Cascades and this configuration also produces strong SE winds at crest level.

At the height of the event there was over 10 mb of pressure difference across the Cascades (this is quite large). Research has shown that the strong winds along the eastern slopes of the Cascades are not simply winds accelerating from high to low pressure. In addition, with a wind reversal or layer of stable air above crest level, the development of a high-amplitude mountain wave adds more "juice" to these events. I don't have the space to explain this in the current blog, but think about high amplitude versions of the waves in the lee of mountain barrier that produce lenticular clouds or the water descending down a dam.

Current generation high-resolution computer models do a pretty good job with such events and the National Weather Service did an excellent job putting out a high wind warning before the event. I should note by the way, that there were also strong winds in the western Strait of Juan de Fuca and along part of the coast, and the warnings included that.

Here is a sample of a computer forecast made early Friday, verifying at 7 AM Saturday. Wind gusts are shown by the shading. You can see the strong winds in southern King County and the accelerating winds in the Strait of Juan de Fuca and the strong winds offshore.

Finally, I should note this was also a major wind event in the Gorge and winds in the Portland area accelerated to 40-50 mph. This winter is one that keeps on giving...at least meteorologically.

9 comments:

yolandapadilla said...

The forecast missed it for east Kitsap. Our SE facing waterfront on Bainbridge Island got hit by strong winds, driving rains. Lots of tree branches and debris. Parts of the island lost power. Our Davis weather station measured 31 mph even though the weather station is sheltered by the neighbor's house when the wind is from the E or SE. The house shook and we could hear wind whistling in.

I read the NWS forecast discussion Friday night and nothing serious was predicted. High winds were predicted for "east puget sound lowlands" and the large pressure differential was discussed, but no warnings or watches for our area in the forecast, forecast discussion or marine forecast. A wind advisory was posted after the strong winds had started and were below what we were experiencing. Fortunately, this event occurred at a time when boaters and kayakers were not out. The conditions were quite fierce.

The neighborhood beach had large wave action because of fetch. The wind direction was across a long (for the Sound) distance of water that we are exposed to. Other area beaches were likely not hit so hard.

I read the forecast discussion daily and try to learn from such events for when I'm out on the water. I *think* I learned: 1) continue watching pressure differentials closely, 2) don't just skip down to advisories and watches in the marine forecast -- read the full forecast discussion 3) have internet access on the boat because marine band radio does not give much info. Thoughts from the community?

amy_mor said...

Well that was a little bit of a windstorm to top off the year: rainstorm, snowstorm, windstorm. But it didn't quite hit all of western Washington. Still waiting for that one.
Dr. Mass, could you tell us where we can access that surface map information in real time? I have been looking on the NWS website for a looped version but haven't found one.
Thank you!

Ferdi said...

Just to underscore how local these winds can be: I went to Anacortes Friday, a choppy boat ride but not bad. However, in Anacortes the wind was blowing a good 25+ mph. This was outflow from the Skagit River Valley. On the way back in the afternoon it was really blowing good down the Guemes Channel between Guemes Island to the north and Anacortes. I was quite concerned about making it back to Sinclair Island safely. Sinclair lies a couple of miles north of Guemes Island. Boy, was I surprised to discover the waters north of Guemes were flat calm, glassy smooth. So this river of air flowing out of the valley was just that, a river.

That was a well done forecast.

wavelength said...

Your Sunday discussion says: "Here is a sample of a computer forecast made early Friday, verifying at 7 AM Saturday. Wind gusts are shown by the shading. You can see the strong winds in southern King County and the accelerating winds in the Strait of Juan de Fuca and the strong winds offshore."


It does not help to hear about tomorrow's forecast two days later.

JewelyaZ said...

When I read the weather report calling for super-duper extreme lowland winds with gusts to 80 mph, I kinda said "baloney" to myself and came here to check out what you were saying. Nothing. I therefore didn't believe the doom and gloom forecast, which caught me out. My station recorded a 48 mph gust!

This wind storm made me realize that I've become too dependent on the combination of your blog and Probcast for my weather outlook. You didn't post anything before the event and Probcast doesn't do anything with/about wind. Time to add the NWS discussions back to my reading list... LOL

Oogoo said...

No watches or warnings for us either here on the Long Beach Peninsula. We lost power for about 6 hours on the north end and had some really good gusts off the bay.

This was wind that got down low, not the usual "high up in the tree's" type.

WX said...

If you don't like reading NWS discussions (I didn't either) you can read my "automagically" trnaslated version. Every once in a while, something gets screwed up, but usually it is right on.
http://home.comcast.net/~olneytj/discuss.html
and it is always easier to read.

I also collect and redistribute as one page various wind forecasts for around the state
here The "spots" collected are near hang/paragliding sites.
TJ

Mike (Orchards 255\') said...

This reminded me so much of the December 2003 event here in east Clark County, WA.

Josh said...

First California had dryer than normal weather in a El Niño year. Now they have a wetter than normal in a La Niña year (which usually brings wildfires in Southern Cal throughout the winter. Whats up.... So long short term climate prediction.