November 21, 2012

Intense Convective Line

Sometimes a weather feature comes through that grabs your attention, providing a sobering reminder of the power of the natural world.  Yesterday, we got hit by such a feature--an intense convective line that brought strong winds, intense rain, and a cloud feature that was nearly biblical in appearance.

First, a picture, graciously provided to us by Peter Loron, of the line as it was approaching downtown Seattle (time: 1:30 PM Thursday).  Looks like the end of the world.   

Picture courtesy of Peter Loron.  Looking towards W. Seattle from downtown.
Would have been fun to be on the big white wheel! Or scary.
 Another view of the line was provided by Paul Gockel, taken mid-span on the 520 bridge:

Picture courtesy of Paul Gockel
You see that linear appendage hanging below the main cloud base? Those threatening clouds (also known as shelf clouds) are found along the leading edge of the gust front;  cool air descends out of the cumulonimbus cloud and forces air in front of it to rise, producing the cloud  (see schematic below).

Schematic of gust front and shelf cloud
 For your viewing pleasure, here is another dramatic shelf cloud.

We could see the convection (big line of cumulonimbus clouds) in the Camano Island radar image at nearly the same time (yellow is heavy rain, red is VERY heavy rain or hail).

There was one lone lightning strike (over north Seattle as this feature went through).

As it went past the UW, the winds gusted to 30 kts, the wind directed shifted to southwesterly, heavy rain fell, pressure jumped, and temperature fell (look just past 21z on the UW rooftop observation plot below).

Or you could view the system from the cam on the roof of my building.  Here are images before, during, and after passage of this line:

One interesting aspect of this feature is one could follow it from offshore, using the Langley Hill radar.   Here is an image at 10:48 AM Tuesday...clearly apparent offshore.

Hopefully, during the next year we can extend Seattle RainWatch to include the Langley radar to provide an automated heads up for features such as this one.


  1. This radar is helping BIG time. Thanks for all your hard work in supporting the effort. Justin

  2. Any idea where in North Seattle that lightning strike hit? I was in the Green Lake area, and though I didn't see the lightning, the thunder was most impressive.

  3. Very interesting. We used to see features like these shelf clouds in front of down blasts associated with thunderheads in the Gulf of Mexico. Now I know more about them. Thanks.

  4. I saw the flash from the lightning when I was on the Burke Gilman Trail just east of the U-Village. It looked like it was to the west of the UW, and it seemed to take about four seconds before I heard the thunder.

    I then hoofed it to a coffee shop where I waited out the rain with a cup a coffee and a newspaper.

  5. That updraft feeding the shelf clouds is strong & very smooth - we "surfed" the P3 for 138 seconds (holding altitude at zero power) before Dr Brad Smull asked us to look at the storm's structure elsewhere... the Vortex Project many years ago... funny how one remembers such details...

  6. I was at home when the lighning struck somewhere close to my house near 6th Ave & 85th St NW. I heard a clicking sound (inside my house) immediately followed by a very intense light that nearly blinded me and the near simultaneous thunder clap that made my house shake like there was an earthquake, with pictures rattling on the walls. And I was about to have an afternoon nap!


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