We've all heard that if you don't like the weather in the Northwest, just wait a few minutes and it will change. In the Puget Sound region...
We’ve all heard that if you don’t like the weather in the Northwest, just wait a few minutes and it will change. In the Puget Sound region, there’s a variation: If you don’t like the weather where you’re at, just travel a few miles to find some sunshine.
Frequently, the area that has locally heavy rain and sometimes thunder, lightning, snow and hail, is north King and South Snohomish counties. The reason is this area’s position relative to the Olympic Mountains.
This range, together with the Cascades to the east, the Willapa Hills to the south and the Vancouver Island mountain range to the north, work together to produce a unique weather pattern called the Puget Sound Convergence Zone. That’s its formal name; forecasters and local residents often just refer to it as “The Headache,” with a capital “T” and a capital “H.”
I was kayaking with a friend from Seattle to Port Townsend on what began as a sunny morning about this time last year. We set out from Shilshole Bay midmorning with southwesterly winds at our back, only to abruptly run into strong northwesterly winds a few hours later near Point-No Point.
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Ominous dark clouds began to build, producing thunder and lightning just south of us as we fought our way into the teeth of a rising gale. That raised an urgent question: Was our best choice to head for shore and try to find shelter immediately, head back to Shilshole or continue north toward Port Townsend? After watching the pattern develop, I suggested we continue north — a tough paddle, but one that took us away from the Convergence Zone.
Situation No. 2: One late winter afternoon, we were expecting Arctic air to move south from British Columbia. Moist air was already in place, making snow very likely as the Arctic front pushed through the Sound. Based on what seemed to be good computer projections, we forecast 2 to 6 inches of snow. That was exactly what did fall — everywhere except within the Puget Sound Convergence Zone. Up to 14 inches fell there! Small wonder forecasters call this pattern “The Headache.”
So what exactly is “The Headache,” and how does it form? In most areas of the country, significant weather tends to arrive with the approach of a front; a cold front, warm front or a sort of hybrid of these two called an occluded front. The Convergence Zone, however, develops after a front moves through the Puget Sound area and crosses the Cascades.
As winds shift to an onshore direction, that is, blowing from the Pacific Ocean onto land, they typically begin to veer from the southwest to the northwest (wind direction refers to the direction the wind is blowing from). As this ocean air moves onshore, it first encounters the Olympics, which force this flow of air to split.
The result is a westerly surge of wind through the Strait of Juan de Fuca to the north of the Olympics, and through the Chehalis gap between the south end of the Olympics and the Willapa Hills.
The Cascade Mountains block these westerly surges from continuing east, deflecting some of the flow through the Strait of Juan de Fuca southward, and some of the flow through the Chehalis gap northward. That puts these two deflected currents of air on a collision course. The zone where they collide is The Puget Sound Convergence Zone.
As this zone develops, we begin to see weather-reporting stations in places like Coupeville and Clinton on Whidbey Island, Arlington and Everett report northerly or northwesterly winds.
Stations in Seattle and Tacoma, on the other hand, typically observe south to southwesterly winds. As the opposing winds collide in the Convergence Zone, they rise. That’s the key to the poor weather found there; as that air rises, it cools, condensing moisture into clouds and precipitation.
The process is essentially the same as when you breathe out on a cold day; the warm, moist air in your breath is rapidly cooled, which condenses the water vapor into liquid water droplets. When you say “I can see my breath,” you’re actually making a tiny cloud.
The Convergence Zone operates on a much bigger scale, and the stronger the collision in “the Zone,” the taller the clouds. Taller or thicker clouds not only make precipitation more likely, they also produce heavier precipitation, occasionally accompanied by thunder and lightning.
The air that rises within the Convergence Zone eventually begins to spread out, looping back toward its original direction. For example, the air that arrived from the northwest near Everett begins to flow back toward the northwest thousands of feet above the ground or water, and then descends toward the ground.
This creates a localized area of high pressure, which is why we can find bright-blue skies and sunshine just a few miles to the north or south of the Convergence Zone.
There are a few interesting twists to this phenomenon. Studies of the Puget Sound Convergence Zone conducted by the Department of Atmospheric Sciences at the University of Washington show the greatest number of such zone events occur during the afternoon and early evening hours and during the late spring and early summer months.
“The Zone” also has a major impact on local climate. If we examine the major weather-observation stations in the Puget Sound area — Everett, Seattle, Tacoma and Olympia — only Everett tends to pick up precipitation from most Convergence Zone events.
Because the Convergence Zone tends to be most active in the spring and early summer months, Snohomish County (especially South Snohomish County) sees little decrease in monthly precipitation in April, May and June. It remains fairly wet, unlike the rest of the Puget Sound region, which typically sees a pronounced decline in rainfall. In fact, Everett receives more rainfall in these three months than Olympia, which averages 17 more inches of rain a year than Everett.
The “workings” of the Puget Sound Convergence Zone can combine with an Arctic front to substantially boost snowfall during the winter months.
As a forecaster, I work hard to determine if such a combination is likely. If it is, I’ll boost my estimates of snow in south Snohomish and north King counties.
The localized nature of the Convergence Zone is also why I elected to continue paddling north on that sea-kayaking trip to Port Townsend. I recognized that since the dark clouds producing thunder and lightning were south of us, we had to be north of the zone.
Knowing the Convergence Zone tends to remain stationary or slowly shift southward, we elected to continue paddling (very slowly) under clearing skies but gusty northwesterly winds. As our kayak (a double) finally slid up on the gravelly beach well after dark, we climbed out with sore shoulders and stiff legs but untouched by the weather that had pounded the water just to the south.
That’s why Convergence Zone residents occasionally say, “If you don’t like the weather, travel a few miles.”
I’d just suggest you use something a little faster than a sea kayak.
Jeff Renner is the KING-TV chief meteorologist.