Monday, November 18, 2013

Umbrella Treason

Monday, November 18, 2013
12:47 p.m.

Replace "Capt. John Brown" with "Umbrella," and you're set. It's not like the umbrellas have any choice.

Seattlites are not supposed to have umbrellas, let alone use them. Hoodies, ponchos, rainboots, and the like are tolerable, but umbrellas are unofficially forbidden. So imagine my surprise when I walked outside my house onto the UW campus today and saw enough umbrellas to block out the little light that peeked through the thick nimbostratus layer there was. Granted, not everybody who goes to the University of Washington hails from Western Washington, but still, after living here for even two months, they should be able to catch on. I'm seriously considering making some obnoxious chartreuse signs and trying to pitch my message in the middle of Red Square. I'll make the Mormons look like chirping crickets.

Why the sudden influx of umbrellas? Rain, of course! Let's take a look at our current radar to see what exactly is happening out there.

As this 3:37 p.m. PDT radar image shows, there's nothing too exciting about our rain. No squall lines, no thunderstorms, no strange shapes. Just some globs of rain coming in off the Pacific. The snow levels are above the major passes with this system, but there are still Winter Weather Advisories in the Washington Cascades for 6 to 16 inches of snow above 4,000 feet. As far as driving conditions go, it's a good thing that the snow levels are above the passes, because just look at how effectively the Cascades are orographically enhancing the precipitation, and look at the rain shadows northeast of the Olympics and east of the Cascades.

When air rises and precipitation is enhanced, some air also has to sink to "balance out" the situation. I'll learn more quantitative methods of explaining this as my atmospheric science education advances, but let me paint a picture in your head for a second. You drive on over to Archee McPhee's and decide to place a Whoopee Cushion under the seat of a girl you are taking out for a first date to dinner at a decently nice restaurant... like a Red Lobster or something. For the record... I've tried this... it doesn't work. When the air in the Whoopee Cushion is subject to an external force (in this case, your date's bottom), it seeks the path of least resistance to equalize the pressure. In our weather case, when the air is forced upward by topography, this path of least resistance is downward, and downward movement warms and compresses the air adiabatically (meaning no heat is exchanged with the surrounding environment), meaning that the relative humidity becomes lower and clouds/precipitation are less likely to form. This is why the windward slopes of the Cascades/Olympics are wet and the leeward slopes are dry.

Diagram of a Rain Shadow. Created and Released into the Public Domain by Wikipedia Contributor "Bariot." Retrieved from Wikimedia Commons.

The 4/3 km resolution WRF-GFS model gives a great idea of how the rain shadow effect is manifested in our area by our local topography. Check out the graphic below, which is the 24 hour forecast ending at 4 a.m. Tuesday.

Valid 04:00 am PST, Tue 19 Nov 2013 - 24hr Fcst:

Look at how the precipitation seems to be a mold of the topography, with higher amounts where there is higher elevation on the windward slopes and then close to nothing as you transition into Central Washington. Amazing.

But it's not just this macroscopic shadow-ism that is at play. There are lots of local effects going on, especially in the Cascades. In the far eastern Skagit County Cascades due east of Mt. Vernon, a group of mountains is expected to pick up 5 to 10 inches of rain, while some valley a couple miles north of them might get a half inch. That's a HUGE spread. Pretty amazing.

Things look to be calming down for the rest of the week, and I don't see any storms in sight. In fact, I don't see much of anything in sight. So, for all you weather enthusiasts, now's the time to put away your phermometers and study your physics.

Enjoy the short days!!! They are only gonna get shorter!

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