Weather Blog

'Lee trough' makes for windy day in North Sound

'Lee trough' makes for windy day in North Sound

Oh, those rascally Olympic Mountains. They bring rain forests and rain shadows, and a snow belt to the Hood Canal area from time to time.

But Friday? They dabbled in the wind business.

A glance across the current conditions of Western Washington Thursday night and Friday might have brought a few scratches of the head: A calm Thursday night and breezy Friday for most areas, but one spot stuck out like a sore thumb: Paine Field in Everett, where wind gusts over 35 mph were relentless for several hours -- even reaching 45 mph at times! Seattle? About 8-13 mph while on the other sides, Friday Harbor and Port Angeles spent much of Friday in single digit wind speeds.

How can one area be blowing as hard as a tropical storm while others just 25-50 miles away would barely invert a tropical drink's umbrella? Blame the Olympics and their dreaded "lee low."

Ever see a fast moving river as it interacts with a rock in the middle of the stream. Note the swirls in the lee side of the flow of the water. The dynamics are somewhat similar.

In our case, the gusts are caused when strong winds roll over the top of the Olympic Mountains. Friday, a warm front stalled to the north was bringing a strong, steady stream of winds in the upper levels from about 2,500-10,000 feet were gusting out of the southwest at about 30-60 knots at times.

When those winds sink down on the other side of the mountains, it causes a localized area of low pressure to form on the lee side of the mountain -- thus the term "lee trough" or "lee low". This low can cause localized strong winds as it can make for a vast difference in pressure over short distances.

Here is a surface map from 1 p.m. Friday when Everett's Paine Field gusted to 45 mph:



If you follow the wind barbs, you can see the general cyclonic flow right around that NE Olympics area. Here is the same map with a red arrow to highlight the general flow:



(Note: 'Hur52' is Hurricane Ridge -- high above the surface effects of the low, thus the southerly wind.) The green numbers indicate sea level pressure, and that too illustrates the lower pressure in the vicinity, but to help pick them out, here are some readings in milibars:

Port Townsend: 1023.9
Port Angeles: 1023.8

To the north:

Oak Harbor: 1024.7
Friday Harbor: 1024.8
Everett: 1026.1

To the south:

Boeing Field: 1028.2
Tacoma Narrows: 1028.3

To the west:

Forks: 1026.0
Destruction Island: 1026.5

A 2.2 milibar difference as it was over 25 miles between Everett and Port Townsend is fairly intense, thus the 40 mph gusts there. The wind has been steady as the fronts stall offshore and the overall wind flow remained steady.

Lee-side lows have been a factor in some other localized but major windstorms -- namely the 1979 storm that sank the Hood Canal Bridge and the October 2003 storm that struck Mukilteo and damaged the Ivar's restaurant on the waterfront.



Time lapse of day from Hansville

Here is a time lapse video of the windy day from SkunkBayWeather.com that even included a 53 mph gust: