Weather Blog

Olympic Rain Shadow struts its stuff again

Olympic Rain Shadow struts its stuff again

It was a pretty wet day across Western Washington Sunday (and will be again Monday) but inaugural autumn storms like these give me a great chance to revisit the Olympic Rain Shadow and just how much the Olympic Mountains affect our local climate.

A short recap for the newbies to the area: The mountains stand as a barrier to the moist, southwest winds from incoming storms. As the winds approach the southwestern slopes of the Olympics, the air is forced to rise. Rising air causes condensation and, eventually, enhanced rainfall -- thus the rain forests on that side of the mountains that get over 200 inches of rain a year.

But on the other, northeastern side, the air sinks and dries, in essence eating a hole in the radar and creating a banana belt over the Northeastern Olympic Peninsula -- areas like Sequim, Port Townsend and Chimacum are in the heart of this belt, with other spots like the San Juan Islands, Whidbey Island and Camano Island getting some fringe effects.

It can make for a wide range in rainfall over a short distance.

For example on Sunday, Sequim is just 55 miles northwest of Gig Harbor as the crow flies. But Gig Harbor had 1.52" of rain while Sequim just had 0.12". Forks, just 75 miles to the west of Sequim and had over 2.5"! In fact, as annual rainfall goes, for every mile you go west of Sequim, you add about an inch of rain to your annual total.

Here is more proof in the Sunday rainfall totals:

Midnight to Midnight tallies:

Seattle: 0.65"
Olympia: 0.68"
Forks: 2.62" -- Daily record. Old record 2.02"

CoCoRahs spotter totals (7am Sunday to 7 am Monday)

Montesano: 2.61"
Ocean Shores: 2.08"
Gig Harbor: 1.52"
Port Orchard: 1.43"
Shelton: 1.43"
Aberdeen: 1.41"
Tacoma: 1.02"
Bremerton: 0.95"
North Bend: 0.89"
Ferndale: 0.86"
Bainbridge Island: 0.83"
Issaquah: 0.74"
Federal Way: 0.72"
Friday Harbor: 0.70"
Port Angeles: 0.67"
Lake Stevens: 0.66"
Bellevue: 0.65"
Auburn: 0.65"
Renton: 0.64"
Kent: 0.61"
Redmond: 0.57"
Sammamish: 0.50"
Everett: 0.47"
Woodinville: 0.47"
Bellingham: 0.40"
Marysville: 0.38"
Camano Island: 0.35"
Shoreline: 0.29"
Mukilteo: 0.24"
Oak Harbor: 0.16"
Port Townsend: 0.15"
Sequim: 0.12"