Small Zone, Intense Convergence

Wednesday, April 10, 2013
10:30 P.M.

I don’t own an umbrella. I don’t own rain boots.The only thing that I need to protect me from the elements is my subcutaneous fat. It’s worked out pretty well so far.

But today, when I walked out of a physics lab, I saw looked out the window and saw a torrential rain shower that ranks up with the heaviest I’ve ever seen in Seattle. I wanted to head over to a different physics building that was a measly 50 feet away, but I had to sprint across the pavement to keep from getting absolutely drenched in water. I suddenly wished I was enclosed in some sort of waterproof force-field, like the ones Violet made from The Incredibles.

Source: DisneyScreenCaps

However, the rain was still nowhere near as heavy as the rain I witnessed at my house between 4 and 5 P.M. on December 14, 2006, which was the day of the Hanukkah Eve Storm. I don’t think I’ve dedicated a specific blog to urban flooding, but I will do so in the future. Kate Fleming, an award-winning audio book producer and narrator who lived in Madison Valley, died when she drowned in her basement due to rising floodwaters from the downpour, and a more effective water drainage system was built soon afterward to prevent a tragedy like this from ever happening again. Below is a Youtube video of that storm… I will never, ever forget the roar of the rain outside my house. Rest in peace.

Whereas the 2006 Seattle flooding event was associated with the most damaging windstorm to hit the Seattle area since 1962, this brief downpour was associated with a weather phenomenon Seattle natives should be quite familiar with – the Puget Sound Convergence Zone.

Retrieved from Colorado State University’s Virtual Institute for Satellite Integration Training (VISIT) “Meteorological Interpretation Blog”

The idea behind the convergence zone is pretty simple… winds coming off the ocean (typically from the northwest in the wake of a cold front) are directed to the north and south of the Olympics through the Strait of Juan de Fuca and the Chehalis Gap, respectively. These air masses curve into the Western Washington lowlands, converge, and cause clouds and precipitation. The result is a band of precipitation that generally stretches across the lowlands into the mountains and is located around the King/Snohomish county line. However, I have seen convergence zones form in Skagit and Pierce counties as well.

Let’s take a look at a radar image that shows the convergence zone we saw this afternoon.

This is a pretty typical convergence zone for the area. It is roughly perpendicular to Puget Sound and extends into the Cascades. It is a little further south than normal and doesn’t extend as far westward as some other convergence zones I’ve seen, but it’s still a pretty good example nonetheless. If you take a look really closely, you can see a tiny but intense shower embedded within the convergence zone over the UW campus. Meanwhile, places a mile to the north or south were completely dry. The Pacific Northwest may not get big supercell thunderstorms or hurricanes, but we do have some local weather features that keep things interesting around here, and the convergence zone is one of those.

If I had to estimate, I’d say rainfall rates were around or slightly over an inch per hour for the 10-second duration it took me to run from one building to another. I checked the UW atmospheric sciences building rooftop data and it said that there were 0.05 inches of rain from 2-3 PM and 0.07 inches from 3-4 PM. This just goes to show that when it comes to heavy rain, I was in the wrong place at the wrong time. Had I waited a couple more minutes to flee from one building to another, I wouldn’t have to wait an hour for my sweatshirt to completely dry. Yes, the rain was really that heavy. If you want a fantastic explanation of what a convergence zone is, check out Scott Sistek’s page on it here.

We’ll have some rain coming in Friday afternoon, but I’ll bet my life on having a strong convergence zone somewhere in the area Saturday afternoon due to a cool, moist, onshore flow splitting around the Olympics and converging somewhere along Puget Sound. If I’m still alive on Sunday, you’ll know I was right. 🙂

Charlie

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