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.


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