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.
In fact, the Convergence Zone is a frequent culprit of giving its area a snow that the rest of the region misses out on. Again depending its intensity, it can turn a 39 degree day into a surprising 34 degree snow due to evaporative cooling. (More information on evaporative cooling at this link).
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, Mukilteo, Mill Creek, Mountlake Terrace, Shoreline, Bothell 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.
But during its peak, it can just park on one spot as the winds on either side fight their battle. Here are some radar animations of past convergence zones that show this effect:
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.
Note on this satellite image the intense vertical band of clouds right over the Convergence Zone's usual Snohomish County spot -- and the clear skies just to the north and south:
(And here is the radar image taken at the same time as that satellite image:)
More information on that particular Convergence Zone>>
This video is from a different zone but you can see the dramatic edge of one zone where it was pouring rain in the zone, but rapid clearing on the edges:
Watch the Convergence Zone in action
Here are some great time lapse videos that illustrate the convergence zone in action.
This video is from a convergence zone on March 2, 2013 as seen from Skunkbayweather.com in Hansville. It's a great illustration of how the zone forms then races south. Pay particular attention to the weather data on the video and note the wind shifts and dramatic drop in temperature:
(Find out more about this particular PSCZ from this weather blog entry.)
This one is from the Silverdale area looking west toward the Olympic Mountains. Watch as the winds radically shift from south to north as the zone moves south through this area:
This one shows the radically changing winds during a zone as it passed by the University of Washington. Note the low-level shear that has some clouds moving south to north and others north to south:
Convergence Zone balances Olympic Rain Shadow effects
You might think for those who live in the Convergence Zone, it's analogous to being the Eeyore of the Puget Sound region -- where the rain picks on you all the time. But while it's true that those in the Convergence Zone get more rainy days than their neighbors in Seattle and Bellevue, overall their annual rainfall is about equal.
How? It's because on other days when we get our wettest storms, the Olympic Rain Shadow coincidentally reaches over and brings a lighter rain to the convergence zone areas. So while the zone areas get more days with rain, on our widespread rain days, the zone areas will get less daily rain than non-zone areas.
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 shifts.
If you live in the Zone areas like Lynnwood/Edmonds, watch the winds, and if you get a radical shift from the south to the north, it's a sign the zone is coming and rain (or snow) is imminent. Areas to the south like Shoreline, Northgate and Seattle will also see wind shifts to the north as the zone approaches.
Other prior stories on Convergence Zones:
Surprise Convergence Zone brings stormy night to a few
The great Convergence Zone trade-off
Convergence zone brings sloppy commute to North Sound
Time lapse videos shows Convergence Zone chaos
Satellite photo shows off vigorous convergence zone over Snohomish County
Views from the edge of the Convergence Zone