Monday, June 15, 2020

Fierce Convergence Zone Clouds and Torrential Rain

Sometimes clouds are just plain scary.  Dark, ominous and threatening.

Just like it was on Saturday night around 7 PM from Seattle northward.

I was at Magnuson Park in north Seattle just before 8 PM and a saw this line of low-hanging clouds and it was clearly raining....really hard...behind them.


My dog started to pull at the leash.  She wanted to go home.  Now.

Now I am a meteorologist and, of course, I have a weather radar app (Radarscope) on my smartphone.  I pulled it out, and I concluding my dog knew what she was talking/barking about.  Wow...there was a line of strong echos extended from Seattle to the northeast.  And my location (the blue circle) was right in front of it.  Time to run. Fast.


The Seattle SpaceNeedle Pano cam showed the line approaching from a more southern vantage point, starting at 7:20 PM and ending around 8:20 PM.   Pretty scary.




Torrential downpours, with small hail mixed in, accompanied the passage of this ominous cloud line, with some places getting a half-inch to an inch of rain in 30-45 minutes.  As the line passed, the winds turned from southerly to northerly and temperatures dropped 5-10F.

Here are the precipitation totals from 4 to 11 PM Saturday.  The precipitation cut off just south of downtown Seattle with around .4 inches in most locations but as much as 1.77 inches near Lake Stevens.  Impressive.


So what was going on?

The answer:   a strong example of our old friend the Puget Sound convergence zone.  Westerly (from the west) air from off the Pacific split around the Pacific and the came together (or converged) over Puget Sound.  This low-level convergence resulted in pushing air upwards, releasing convective showers.  Interestingly, with all the intense rain, there was no lightning and thunder.   We need to explain that.

The 7 PM winds showed the convergence zone clearly, with southerly flow in the south Sound and northerly winds to the north.


The visible satellite picture just after 8 PM shows the convergence zone clouds...tall enough to have shadow on its eastern side.


Why no lightning?  It turns out that although there was heavy rain, the clouds were not that high, with the echo tops only getting to 15-20 thousand feet.  East Coast folks would laugh at such wimpy showers.  But there was intense low level convergence, strong upward motion with the convergence zone and a moist air mass....plenty to give us heavy rain.


9 comments:

  1. I was in Everett working on my boat when the rain came. But I was disappointed in the there was no lightning, as you say.

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  2. Absolutely stunning sky - quite unusual even for a convergence zone. I stopped and took some time to admire the beauty this one..

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  3. I live in the Magnuson Park area and got caught in some showers during an earlier walk on Saturday but thankfully was inside by the time these hit. I was wondering if we had broken a rain record for the day but saw nothing. I am guessing we got at least an inch total on Saturday but it looks like it was localized enough to not get down to Sea-Tac.

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  4. I noticed the interesting bouts of rain on Saturday night, so I appreciate your explanation of what was happening. Thank you.

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  5. At my north Bellevue house I could clearly see low dark clouds moving quickly to the northwest and very dark clouds (like a squall line) moving southeast also quickly and knew a brisk convergence zone was happening.

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  6. We were watching that from Sinclair Inlet. I should have taken a picture, it looked like a huge thunder cell, ominously funnel shaped. Surface wind was coming variously from about every direction. I checked windy.com, and the radar and satellite loops, not nearly as exciting as out the window.

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  7. The maximum temperature on 6/12 at my location in NW Bellingham was 56.7F - tied with 5/30 for the lowest daily maximum temperature since 4/22.

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  8. Why do meteorologists use directional adverbs for wind flow differently from the usual English language? e.g., "southerly flow" means winds flowing to the north, whereas traveling in a "southerly" direction means going south. Just askin' I am being humorous here. Thanks for the great blog posts.

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  9. Nooooo! Please tell me it's not true. Two weather sites have just reneged on this weekend's sunny forecast! Walter Kelly promised! Are we foiled again!?

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