Weather 101 Answer:

Northwest winds in the upper atmosphere become split by the Olympic Mountains, then re-converge over Puget Sound, causing updrafts. Those updrafts can lead to convection and then rain showers or more active weather.

Weather 401 Answer :

The Puget Sound Convergence Zone (PSCZ) works best when there is a northwest flow in the upper atmosphere, and unstable air. They tend to be most frequent following a storm system, as usually winds blow from the northwest following a cold front passage.

Those northwest winds will collide with the Olympic Mountains. Part of the air flow will be deflected east down the Strait of Juan de Fuca, while the other part will be deflected down the western side of the Olympics.

When the northern branch reaches the I-5 Corridor and the Cascade Mountains, it will then be forced to the south.

Meanwhile, when the southern branch reaches the I-5 corridor and Cascade Mountains past the southern side of the Olympics, it will then turn to the north.
 

Eventually, the south-flowing branch and the north-flowing branch will converge. When that happens, the air has nowhere to go but up.

Rising air will lead to convection. That will lead to cloud and storm development.
 

Most Convergence Zones are strong enough to produce rain, but depending on how strong the winds are, and if the atmosphere is unstable, you can get strong thunderstorms, hail, or on cold days, heavy snow. It will also bring very heavy snows to the Cascades wherever it sets up -- usually between Stevens and Snoqualmie Pass.

(Instability occurs when you have much colder air moving into the upper atmosphere -- it makes it easier for clouds to form since the warm air from the surface can rise higher and faster. Usually also occurs with cold air moving in after a cold front passage.)

The Convergence Zone's favorite spot tends to be an east-west line that extends over the central and south Snohomish County area (Lynnwood, Edmonds, and Everett are the prime spots.) However, just to make forecasting a challenge, the Zone can move, depending on the strength of each wind component. If the south component becomes stronger, it will push the Zone further north, and vice versa.
 

Sometimes, the wind components can vary, in which case you can have a Zone move north and south like a flag flapping in the wind. The Zone can go as far north as Northern Skagit County, and as far south as northern Pierce County. Many times the Zone will begin in the usual spot, then race south over the Seattle/Bellevue metro area just before fizzling out.

Just outside the zone on either side, the weather tends to be rather calm but breezy. Many times in Seattle, the sun will be out, but you can see the dark ominous clouds to the north.

There tends to be a calm zone on either side because once those winds shoot up inside the Zone, they tend to subside on either side (much like a fountain -- the water shoots up the middle, then comes down on the sides). As air sinks, it tends to dry out, thus usually breaking up the cloud cover around the Zone.

Do It Yourself Forecasting

It's tricky to explain what to look for in the models for what would make a good Convergence Zone, but there are some tricks you can do to get a good short-term forecast of a Convergence Zone:

First of all, primo conditions would require about a 10-15 knot wind out of the Northwest (compass direction 300-330) at Hoquiam Airport. You also want strong west winds down the Strait of Juan de Fuca (check Port Angeles' weather.) The stronger the winds down the Strait, the more active the Zone can become (the "Fuel" if you will).

Check the current conditions here

You can tell a Convergence Zone is likely happening in the usual Edmonds spot. If you have North winds at Paine Field in Everett and south winds in Seattle. Sometimes, you can see the zone shift north and south of Everett and Seattle by watching the wind shift suddenly from north to south. If you live in the Zone areas like Lynnwood/Edmonds, watch the winds, and if it's pouring rain and then you get a sharp wind shift from south to north, the Zone just passed you and is heading south.