Weather Blog

Does the Convergence Zone really make it rain more in Everett?

Does the Convergence Zone really make it rain more in Everett?
Naval Station Everett (Wikimedia Commons photo by emersb)

 An e-mail from a Mukilteo resident wondered: "It seems like there has been a constant and persistent convergence zone around here all spring and now into the early summer" and as a fellow Snohomish County resident myself I would agree -- as wet and gray as the spring has been, it's seemed even wetter and grayer in up there as the dominant pattern has been conducive to forming these zones.

He wanted to compare Seattle to Everett to see if the stats do confirm the perception. It turns out the answer is: yes and no.

First a quick synopsis of how the Puget Sound Convergence Zone is formed. When we have a northwesterly flow, the air splits around the Olympic Mountains as it comes inland off the coast. One branch of wind goes east down the Strait of Juan de Fuca, while another branch goes south through the Chehalis Gap.

The winds then approach the Cascade Mountains and begin to turn -- the northern branch that goes down the Strait turns south toward Seattle while the southern branch turns north toward Seattle. When the winds collide, they are forced up where rising air creates condensation, clouds, and, eventually, rain or snow or thunderstorms or all of the above.

The collision zone is typically right over Snohomish County and northern King County, as any longtime resident can attest.

So with several zones this year, Everett has to have been wetter than Seattle, right?

Let's see:

Since March 1, Everett has reported 15.36 inches of rain.

Seattle? 15.42 inches.

So it's essentially a tie.

But here is the rub: It's rained on 83 days since March 1 in Everett, but only 72 in Seattle. So that is 11 days when the zone has been drenching Everett while Seattle remains dry.

How is that possible? Snohomish County has a counter-balance in that they are also on the fringe of the Olympic Rain Shadow -- that's the deal that keeps Sequim so dry.

When we get southwesterly flow -- the usual drill on our wet, stormy days -- the Olympic Mountains act as a big umbrella to the northeastern Olympic Peninsula, but some of that shield extends east into western Snohomish and Skagit Counties. Usaully what you notice there is on really stormy days, Everett gets quite a bit less rain than Seattle.

That's not quite an even trade off then -- it still means you're getting wet in Everett, just not *as* wet as Seattle.

For example, on April 1, Seattle had 1.13" of rain while Everett "only" had 0.56". During our wet storm on May 14-15, Seattle got 1.78" of rain while Everett had 0.92" -- but then on the 16th when the storm passed and a big Convergence Zone formed, Everett reported 0.37" of rain (quite a bit!) while Seattle had... zero.

I've placed an Excel chart comparing each day from Mar 1-July 10 here.

So yes, Everett, you're wetter than Seattle, even if the rain gauge doesn't think so :)