Satellite photos did a great job in capturing a Convergence Zone that set up over southern Snohomish County on Friday afternoon.
The zone is formed when you get colliding winds -- the northern part from winds racing east through the Strait of Juan de Fuca; the southern winds wrapping around the Olympics and coming north through Seattle.
When the winds collide, the air is forced upward, where it cools, condenses, and can turn into the Wild West of weather while areas just a few miles outside the zone are basking in sunshine, and the radar sometimes captures this beautifully.
But zones are quite variable as it depends on the strength of the winds and which branch is stronger than the other. Sometimes the zone can be quite wide; some times it's barely a couple miles in width.
Check out this image from 12:49 p.m.:
That's, what, about 3-4 miles in spots?
But what is many times not known about a convergence zone is it often creates fair weather on either side of the zone -- as marked in this example by blazing sunshine in Seattle as this zone dropped heavy rain just 15 miles to the north. While the winds collide inside the zone, the air then sinks back on either side of the zone. Sinking air has the opposite effect of drying out the rain and can leave clearing spots immediately north and south of the zone.
And on Friday, we got a perfect satellite shot showing this exact example: