Weather Blog

Windy Monday, thanks to 'lee-side trough'

Windy Monday, thanks to 'lee-side trough'
With all the focus on cold air this week, Mother Nature gave us a not-so-gentle reminder that November is better known for wind than snow.

A seemingly innocuous cold front swept through on Monday, but then boom, as the afternoon turned to evening, breezy winds turned to full-on gales.

Gusts reached 52 mph at Alki Beach, and 46 mph along the 520 Bridge and at the West Point Lighthouse station on Magnolia. The University of Washington gusted to 41 mph, while Seattle and Renton had gusts near 40 mph.

Meanwhile, up north where you would suspect really strong winds during November wind storms? Barely noticeable. At 5 p.m., Everett reported a paltry 11 mph. Orcas Island? 15 mph. The perpetual wind champ of Western Washington -- the Naval Station on Whidbey Island? 25 mph. Over on the coast? Gusts to 25-30 mph.

Why so windy in such a localized area? We're blaming what's called a "lee side trough" or also known as a "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. It's sort of like that.

In our case, it's caused when strong winds roll over the top of the Olympic Mountains. When they sink down on the other side, 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.

In this case, at 5 p.m. on Monday, the lee low looks like it set up right over North Seattle. The pressure reading at the University of Washington and over to the west at West Point Lighthouse (Magnolia) was 1010.7 milibars, or 29.85" of mercury. Just down the road, it was 1012.3 mb at Boeing Field (29.89") and 1014.2 mb at Sea-Tac Airport (29.95") Having 3.5 milibars of pressure difference over roughly 15 miles is very large, and thus, the extreme winds in that zone.

Lee-side lows have also 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.