Weather Blog

Hey Sequim, thanks for the umbrella!

Hey Sequim, thanks for the umbrella!
Those of you from about Seattle north to Everett must be wondering how in the world we're talking about extreme flooding when the roads around town are barely damp. We can ask Sequim for the answer, as this is their way of life.

Sequim boasts being the driest city in Western Washington, with just a paltry 14-16" of annual rainfall a year. That's drier than some cities in northern Arizona. (I'm not so sure about their claim of "300 days of sunshine" unless they count seeing the sun even for an instant as a "day of sunshine". Even Phoenix only has officially 211 sunny days a year.)

But today, we're stealing their thunder -- or is it, their umbrella? The reason Sequim is so dry is due to the Olympic Rain Shadow, where the Olympic Mountains block most of the rain from reaching the northeastern Olympic Peninsula. In our predominant southwesterly flow in the upper atmosphere, as that air runs into the southwestern face of the Olympics, the mountains push the air upward.

As the air lifts, it condenses and squeezes out its moisture -- think of it as the mountains acting like a sponge soaking up and then squeezing out the rain. That's the reason there are vast rain forests on the southwestern side of the Olympics. They receive over 200 inches of rain a year.

On the flip side, once the air reaches the Olympic Summit, now it's pretty much lost its moisture. As it goes over the top of the mountains and comes down the northeastern slopes, it sinks. And just like rising air condenses, sinking air dries out as it encounters warmer air near the surface. So you already have semi-dry air becoming even drier.

But with this event, the upper air flow is aligned almost due west-to-east, as the plume of moisture from the southwest is taking a turn to the due east before it reaches Washington.

That shifts the rain shadow from the area northeast of the Olympics, to areas due east of the mountains -- or basically the Seattle to Everett corridor. (The moisture is actually coming in a touch north of east, which is why the shadow is centered more North Seattle to Everett than centered right over Seattle.)

In the past 24 hours between 10 a.m. Tuesday and 10 a.m. Wednesday, Tacoma has had 2.53" of rain, while Sea-Tac Airport had 1.51" of rain.

Everett? Just 0.13".  Meanwhile, a spotter near Sequim says they had about 1.70".

You can see this clear as day on the radar image:



Here is a closer spot:



To prove it, here is the DOT camera from Shoreline -- note the bone-dry roads:



And here it is in Tacoma, a much different scenario:



The rain shadow is helping to mitigate urban flooding areas in Seattle through Everett, and over on the Kitsap Peninsula.

It looks like the rain shadow will hold through about early afternoon, then the upper level wind will shift to more of a southwest direction and bring heavy rain back into the interior. But by then, rain will only have a few hours left so it won't be too long of a period.

We should all begin drying out by later Thursday, which means the next question will be -- how to find a big enough box to ship the rain shadow back to Sequim?