Puget Sound Convergence Zone highlights how wacky the weather can get here
SEATTLE -- Saturday was either a gorgeous sunny day, or a drenchingly rainy day where it felt like the skies were wringing out an entire winter's worth of rain in a few hours.
Welcome to the wacky world of the Puget Sound Convergence Zone, where it was truly two different worlds of weather.
The Puget Sound region is known for its microclimates, and it's not unusual on a showery day to have some spots be totally dry and sunny while others get soaked by a passing shower. But the key to that phrase was "passing" shower.
Saturday, there was no such luck. The Convergence Zone formed right around 3 p.m. in southern Snohomish County and then it started to rain... and rain... and rain! Not just the typical "I don't need an umbrella" Seattle rain, but tropical-esque downpours.
And it wasn't as passing shower. It lasted for 9 hours! Paine Field in south Everett, which was stuck right in the heart of the zone, showed constant rain from 3:05 p.m. to 11:45 p.m. All told there, the gauge measured 1.55 inches of rain!
A similar story was told across much of southern Snohomish County. Monroe reported 2.08", Mukilteo reported 1.77 inches; Bothell had 1.54" and 1.27 inches in Mill Creek.
Meanwhile just down the road in Seattle? Hardly a drop. (Some rain gauges had a few hundredths in their report but it was from showers Saturday morning.) Those who went to the UW-Arizona State game at Husky Stadium -- what, like 10-12 miles south of the deluge -- were treated to a gorgeous and pleasant evening:
You can see in the radar images above how narrow the band of precipitation held for hours.
Convergence Zones are most common after a front passes, when winds behind the front typically come from the northwest.
The winds run into the Olympic Mountains, where they split into two channels -- some wind heads east down the Strait of Juan de Fuca and then curve south into the I-5 corridor, while a second wind heads along the southern edge of the Olympics and turn north in the I-5 corridor. Where those two channels collide, the air is forced upward, where it creates storm development.
The location of the zone can vary depending on the strength of the north and south component, but the most typical spot is right around the King-Snohomish County line. It can go as far north as around Marysville and as far south as about Sea-Tac (maybe sometimes even Tacoma) and the zone can float north and south as the winds ebb and flow. A typical progression is for it to hang out around Everett, then drift south into the Seattle-Bellevue corridor as it begins to falter with the overall wind shifting to a more northerly direction.
The strength of the zone is also dependent on the strength of its incoming winds to feed it. Sometimes a zone just brings a few extra clouds or drizzle; sometimes you can get ferocious thunderstorms and/or hail and snow squalls in the winter.
So you've got a narrow band of sometimes intense rain/snow that can move north or south, or stay parked for hours, and can strengthen and wane on a whim, and bring stormy conditions to one spot while leaving other spots a few miles away in total calm. If you think that is a recipe for weather forecaster heart burn, you would be right.
Luckily the more modern, higher resolution forecast models are pretty adept at picking up potential convergence zones these days so we can usually get a day or two warning that one is possible to at least let those along the King-Snohomish County line know that, yep, you're about to be picked on again!