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.


Comments

Post a Comment

Popular posts from this blog

The Time of Year You Can See the Air Move

Image
  Most of the time, you can not see the air move.   You can feel it, but the complex, turbulent motions are invisible to you, except when it interacts with leaves and branches on trees and plants. But for a few weeks of the year, the sky is full of small, cotton-like tuffs that like a natural MRI machine reveals the complex three-dimensional motions of the atmosphere.   Cottonwood season.  And we right in the middle of it.   Being cottonwood aware is of particular value this year.  It reveals the active dispersion and movement of air that makes the outdoors essentially COVID safe, and thus should be of some comfort for those nervous about transmission outside. Cottonwood seeds are embedded in small cotton-like fibers.   A million seeds weigh about 3 pounds.  Yes, each seed and tuff weights about .000003 pounds.   And that fact, plus the cottonwood tuff, means that these seeds fall VERY, VERY, VERY slowly.   So slowly that they are excellent markers for the three-dimensional flow of the
Read more

Why so little lightning in the Pacific Northwest? And a very nice weekend ahead.

Image
During the past 24-hours, there have been a few lightning strokes over the region (see below) but nothing to write home about (see below). It turns out that the Pacific Northwest and the remainder of the West Coast have some of the lowest frequencies of thunderstorms in the entire country (see below). But why?    My podcast provides the answer. And I also give the weekend forecast....and I think most will like what I have to say. To listen to my podcast, use the link below or access it through your favorite podcast service. Some major podcast servers:     Like the podcast? Support on Patreon 
Read more

Strong Atmosphere River Heads into British Columbia and Southeast Alaska

Image
While the Northwest has been drier than normal, it has been drenching wet a few hundred miles to the north. An unusually strong atmospheric river ....a long current of moisture stretching southward to the subtropics...has been in place for days, bringing large amounts of water vapor to Alaska.   And the atmospheric river will be in place for several more days, slowly moving southward. To illustrate, the scale and beauty of the atmospheric river, here is a water vapor satellite image for 11 AM today. The lighter areas have more water vapor.  You can see the current of moisture extending from near Hawaii into central BC.  This current is the result of low pressure over the Gulf of Alaska and high pressure offshore of California.   A forecast of total water vapor content in the atmosphere for 11 PM tonight shows a healthy atmospheric river (white and blue colors indicate the higher amounts of moisture). As this atmospheric river slams into the mountainous West Coast, the air is forced to
Read more