December 21, 2018

The December 20 Windstorm and the Start of Winter

Yesterday, we had another moderate windstorm over western Washington, one that had even greater effects over British Columbia.   Gusts reached 70-75 mph in "favored" locations and about 300,000 customers lost power in the region.  Even my home went dark.

The National Weather Service surface analysis for 10 AM yesterday showed a fairly deep system (978 hPa) moving northeast into Vancouver Island.

This hand analysis does not accurately show the detailed pressure structure of the storm.  A far better view comes from a short-term, and thus highly accurate, model forecast shown below (6-h prediction by the UW high-resolution WRF predictions).  The solid lines are isobars, lines of constant pressure, and you will note an intense change or gradient of these lines south and west of the low center. 
Classic for a strong marine cyclone.   Strong pressure gradients suggest strong winds.  And that area of intense pressure gradient moved across northwestern WA during the subsequent hours, producing the powerful winds.

How strong?

Gusts reaching 78 mph at Destruction Is along the central WA coast and 70 mph at Race Rocks in the Strait of Juan de Fuca.  Similar winds from a downslope mountain wave on the Olympics.    And gusts to around 50 mph for exposed locations in central Puget Sound.

But lets look at the max gusts for a far smaller area....around Seattle. Around 50 mph for exposed locations (e.g., West Point or in the center of the Evergreen Pt. floating bridge), but locations with poor exposure, bad instrument placement, or poor instruments had far less wind (less than 20 mph).

Winds are a very difficult measurement and unfortunately many wind observations are unreliable. 

Surface winds should be measured at 10 meters (about 30 feet) in open area.  Not on a house.  Not with trees nearby.  Many sensors do not have such exposure, although the official airport stations should all be reliable.  A colleague of mine, Professor Bob Fovell of University of Albany and student Alex Gallagher, just published a very nice paper showing that many cooperative wind observation networks are nearly useless of determined winds during Thomas Fire  (a Santa Ana event).

Today is the start of winter and I have some good big storms in the forecast! 
No rain and partly cloudy today.   Perfect weather for cleaning up debris and doing that last minute holiday shopping.

And remember, we are in an El Nino winter and the impacts of the warm Pacific water (lesser storms, warmer temps) tends to hit after New Years.


  1. The 10 m. specification for wind speed measurement calls into question the agricultural weather station networks of the West. CIMIS (CA) anemometers are 2 m. AgriMet's at 3 m. Is there a reasonably reliable fudge factor that enables use of anemometers at lower heights above ground level?

  2. Sure blew through the Kitsap peninsula with a vengeance! At just after 11am gusts caused multiple trees down and wiped out all the power on Bainbridge where I was. Kudos to PSE for getting our power back by 9pm last night

  3. Wind gusts of 119 mph at the top of Mission Ridge Ski Area at 11:00 pm Thursday night

  4. I had 43 high wind gust in North Marysville. My wind anemometer is 8' above my roof line. I wish I have an open field to get an accurate reading.

  5. I'm one of those stations with poor wind exposure even though my anemometer is 30 feet off the ground, so yes you need to add about 15 mph or so to the measurements but I'll take that over not having power (My Gen is still in the original box) for days or a 120 ft Douglas Fir in my bedroom.

  6. My wife and I took advantage of our location in NE Seattle and drove down to Magnuson Park at the peak of the storm. From the shore one could view the development of the storm with unimpeded sight lines. The white caps on Lake Washington were impressive, but I've definitely seem them more intimidating. The wind and waves were a magnet for Kite and Sail Board pros. There were a half dozen of them flying across the lake, literally in the case of the wings which were effectively used to lift the rider several meters off the water when they decided to. One question about the forecast, Cliff. I just read the 3:15 AM UW Washington State Forecast Discussion and it states that the models show the probability of wind and rain on Sunday, albeit not nearly as strong as previous activity, then improving through most of Christmas Day. Is it your opinion this is now less likely?

  7. I was watching the building storm out the back windows of my house while enjoying a late breakfast. Suddenly a huge gust hit, and one of those free-standing patio canopies flew up out of one neighbors yard, vaulted over a six foot fence, and crashed into the yard of the house next door, leaving a ruinous pile of flapping fabric and metal struts. Sort of cool and horrible at the same time. Time to call FEMA!

  8. Any way to get a higher-res image of your second exhibit (the colour synoptic chart)?
    Thanks, Andrew

  9. Interesting that we had another mountain wave along the middle of the Strait. Temps got near 58 near Sequim briefly in late morning, but not enough wind with this one to cause problems around Bell Hill. Humidity dropped briefly into the low 40’s before returning to 60’s as the wind backed around to the west.

    Last friday, it was 63 degrees with mid-20’s humidty during the wave. So this one was milder.

    Of course, our Blue Hole exists because of this descending air, and that’s very common, but normally with much more modest changes in wind, temp, humidity.

    I don’t recall having such intense, warm, dry, windy downslopes in previous years. Not sure if that’s a failure of my perceptions and memory, or if this is an uusual year for Olympic mountain waves.

    I will say that last night was the most dramatic Moon I’ve seen all year. Clear sky with a hint of mist at ground level made everything bright and glowing silver. Very different than a summer Moon as the post storm air was crystal overhead, but cold ground mist down below was glowing so bright that it made it special.

    My dogs and I sat covered in blankets on the deck at 3am and enjoyed the show. I think they enjoyed it as much as I did.

  10. Our ski group got hit by a few 35+ mph gusts today on the ridges above Cutthroat lake off of the closed hwy 20. Est. at 6800 feet.

    From our perch we could observe two climax Avalanches. One on the north-facing slope under the Corniced saddle which Trends southerly towards Blue Peak.

    The second climax Avalanche was observed on the north-facing slope just below the rock walls that make up the North Face of Blue Peak. From our distance we were unable to observe debris piles.

    Both slopes are popular ski lines, especially for the local helicopter ski operation.(they commandeer that area during they're operating season)

    For those that don't know a climax slide is a an Avalanche exposes the ground, IE the ground is bed surface or sliding surface.

    If you read my report here at the end of November,I mentioned air spaces at the snow and ground interface.

    That is where what are known as basal facets (or Base layer facets)often reside, especially when we have early season snows in the mountains which sit there for a bit in a thin snow pack.

    There were also some Crown Avalanches observed which most likely slide on the 12-9 weak layer which consists of a thin crust (south-facing aspect est 5800' and 6400' elevation) four feet down with facets (snow grain types most likely formed as surface Frost during the previous arctic cold spell) on top of the crust and facets growing beneath the crust.

    That weak layer combo is now buried four feet below the surface. Total snow depth at 5800' is approximately 5 feet 1 inch.

    Chris H
    Heli-free North Cascades

  11. Forecast for Olympia--at the airport--is a low of 39 degrees. However it is already 32 degrees and has been since around 7:00pm. It is not unusual for the forecast and the actual temperature to be off by five or more degrees. Is this this a problem for all NWS forecasts or is there something unusual about the forecasts for the Olympia airport?

    Fortunately the temperature at our house is at our house so the lemon tree is ok. I am bringing it in for the night though.

  12. Thanks for your post Cliff. I prefer these kinds of analysis, vs debating settled science re the dramatic change in climate caused by humans. Re wind, it's okay to have microscale measurements vs just regional observations - such as Beacon Hill (Seattle) or Indian Hill (NE Tacoma). Placement variety provides a more complete view of the power and impacts of wind storms. Considering that the "cyclone" was forecasted to hit the center of mountainous Vancouver Island, I didn't think the impacts would be much in Edmonds. In fact, we lost nary a big tree branch. However, in the open terrain of Paine Field (Everett) - a reported gust of ~ 66 mph, at ~ 11 a.m. on 12/20, temporarily knocked-out the power in that area

  13. "Moderate" ?!? Cliff ... this wind storm ( preceded by & accompanied by driving rain) hit Whidbey Island with a "force" that knocked out the power at approx. 11 a.m. - picked-up speed - toppled trees which knocked down power lines - blocked roads ( including SR 525 - SR 20 ) - fell on homes & other buildings ( including the Greenbank Progressive Club clubhouse - where a tree severed the roof like a knife )... most of Whidbey Island was sans power until Friday afternoon ... and there are pockets NOW that are still without power - PSE sez they will not have power until Christmas Eve Day. Along the west side of Whidbey Island there was extensive damage done by the wind-driven storm surges of the high tide.

  14. My weather station is about 4-5 metres (15-20 feet) above my house, which is in a relatively open and unprotected by trees area. Is this reliable? If so, at my house in Port Townsend, the wind reached a maximum gust of 64 km/h (40 mph).

  15. Perhaps that Blob out there in the Pacific might get churned out.

  16. Cliff,

    The Davis Vantage Pro2 on the Cama Beach State Park boathouse recorded a high gust of 78MPH. It's on a roof but highly exposed, see the photos at the bottom of the CWOP page )

    The data didn't get uploaded to Weather Underground because the internet was out.


  17. Still waiting for an official NOAA call that we are in El Nino:

    Seems odd given that November and December have been pretty warm. Any alternative explanations for the warmth or is it just normal variation or Madden-Julian Oscillation (MJO)?

    (Oddly, our witch hazel starting blooming. We did it plant it last spring, so perhaps the plant is confused by the transplant and not the weather.)

  18. My Anemometer is on my roof, 25' extension pole, secured with guy-wires. I get almost the exact same wind reading as SeaTac airport about 1.7 miles east of my location.

    So if you install on a roof and can get over 10' above the roof line, you will likely have very accurate readings as long as you don't have other obstacles in the way. Get high enough of your roof and wind will not be influenced by the roofline.

  19. Rangers at Larrabee State Park at Chuckanut Drive near Bellingham reported 60 mph wind gusts. Next day preparing for the winter solstice walk, this ~110 year Douglas fir was down. Skagit Valley college has a popular trail also with a row of largest second growth Dougfirs down.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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