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 news....no 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.

from Cliff Mass Weather and Climate Blog http://bit.ly/2V1X6wv