Wednesday, September 3, 2014

A Convergence Zone to End All Convergence Zones

Wednesday, September 3, 20142:14 pm

Someday, somehow...

Sorry, I was listening to Nickelback. But that question has often graced my mind when thinking about convergence zones. I've been wondering when we'd get a "zone to end all zones," and this is the best one since the morning of December 18, 2008. I don't think anything can top that one... I got 4-6 inches of snow and school was canceled. Our house also almost got struck by lightning around 5 a.m. It was bloody fantastic. That convergence zone represented the leading edge of the arctic air mass that settled in afterwards for nearly a week and wreaked havoc over the entire city.

05:08 am PST, Thu 18 Dec 2008

This zone, while not snowy and having quite as vicious lightning, had much heavier precipitation and much more lightning to go with it. Let's take a look at some screenshots at certain times, and then some animations of the entire evolution of the convergence zone as a whole. All of these were retrieved from the UW Atmos Online Weather Data Archive.

04:40 pm PDT, Tue 02 Sep 2014

We started out with a strong squall line heading through Snohomish and Skagit Counties. It even had the signature of a "bow echo," something commonly associated with very strong squall lines and mesoscale convective systems over the Great Plains, and something that is generally indicative of very high straight-line winds.

05:25 pm PDT, Tue 02 Sep 2014

45 minutes later, the squall line turned southeast and weakened.

05:59 pm PDT, Tue 02 Sep 2014

30 minutes later, we see more of a SW-to-NE oriented convergence zone begin to develop. The area of rain begins to broaden, and it continues to move southward. Northern Seattle is getting thunder and lightning.

06:57 pm PDT, Tue 02 Sep 2014

By 7, much of downtown has gotten thunder and lightning or is getting some at the moment. The real action, however, is further east. The zone becomes more east-west oriented.

07:26 pm PDT, Tue 02 Sep 2014

By now, only a few select places west of Lake Washington are getting hammered with precip. The vast majority of precipitation is falling east of the lake.

08:57 pm PDT, Tue 02 Sep 2014

Later on that evening, a secondary zone developed over Seattle as the main zone moved further southward.

Here are some animations.

The thing about convective precipitation (precipitation driven by rising air and instability) is that it is much more localized than the stratiform precipitation we usually get around here during our fall and winter months when our great Pacific storms roll in. Take a look at Seattle's Rainwatch rainfall estimates, and notice how much the precipitation varies based on your location. This is over the past 48 hours, but keep in mind that most of this precipitation fell within the space of just a few hours. Some places got the majority of their rain in a few minutes.

Retrieved from Seattle RainWatch

Over 6 inches east of Maple Valley! Simply incredible. Meanwhile, places like Mountlake Terrace hardly got anything.  That's how thunderstorms work.

Unfortunately, I can’t show any pictures of the flooding that occurred around the area because all the ones I’ve found are from commercial websites and I don't have copyright permissions. I've emailed a site to ask where I can find some ones available for public use/if I can use some of theirs. If I get my hands on some pictures, I'll put them up!


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