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.

Are Weekends Wetter than Weekdays?

I have been hearing from a lot of folks suggesting that our weekends have been wetter than our weekdays, and asking me whether this was true.

So is it true?  Are the weather gods preferentially punishing Puget Sound residents over the weekend?

Are the weather gods punishing us?

Let's find out! 

To do this, I secured the daily precipitation statistics at Seattle-Tacoma Airport for May and June so far--43 days in all.

For the full period, there was 4.04 inches at Sea-Tac for .09 inches per day.

For the 31 weekdays, there was 1.72 inches in the rain gauge, or .05 inches per day

But for the 12 weekend days, there was 2.32 inches or .19 inches per day.

Yikes! It is true!  The weekends have been wetter, with nearly FOUR TIMES MORE PRECIPITATION PER DAY THAN DURING THE WEEKDAYS.

Good reason to complain.  But is this a real effect or just chance?

There have been a number of investigations of this serious topic, a recent one led by one of my ex-students:  Professor David Schultz of the University of Manchester.  Their conclusions based on examining the data from 219 stations across the U.S for the period 1951-1992 was:

"neither the occurrence nor amount of precipitation significantly depends on the day of the week"

A conclusion illustrated by this figure from their paper:

Other earlier papers have produced contradictory results.  Some have suggested that weekly variations in pollutants from industrial processes, aviation, and driving would influence precipitation processes, thus producing a weak weekly precipitation cycle.  But lets face it, such effects would be greatly minimized in this period of COVID-19 shutdowns and the protest-related fires did not start until late in May.  A plot of the concentration of particulates over Seattle does not suggest a weekly signal at all, with relatively clean air over Puget Sound for the period (see below, 5-10 is clean air).

I suspect strongly that this precipitation pattern is just bad luck, an unfortunately roll of the meteorological dice and will not continue into the summer.  Yes, it is raining today.  But according to the excellent weather.com forecast, tomorrow (Sunday) should be mainly dry, showers will be around Monday and Tuesday, and next weekend should be dry.

But providing an offering to your favorite weather god might not be such a bad idea, something meteorologists like myself are well practiced at.

In my department, we instruct the uninitiated how to appease the weather gods

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Good news to report.  Seattle parks and recreation has reopened the tennis courts, the athletic fields (for up to five), grills (up to five), and will open all the boat launches by June 20th.