A Well-Defined Front

Thursday, October 18, 2012

6:52 P.M.
I just got done with a physics midterm, my first midterm of the autumn 2012 quarter! It went decently well, although there was one question involving a inclined plane, two weighted objects, tension, kinetic friction, and a whole bunch of gnarly stuff where I was supposed to find the angle (θ) of the inclined plane.
I’m pretty sure I got the wrong answer… I didn’t have enough time to completely go through and figure it out. But now that I think about it, there’s no wonder I didn’t get the right answer. The test question was flawed. They weren’t specific enough, and they left a lot of crucial information up to individual interpretation. They didn’t specify what material the plane was made out of. What if the plane was made out of slick memory foam? The angle would decrease the closer the block got to the angle. What if the whole plane-block system was actually 20,000 leagues under the sea? This would strongly counteract the force of gravity, add immense pressure, and would involve making sophisticated calculations we haven’t learned yet. What if the figurative incline, is, as I am talking, slowly falling into the supermassive black hole at the center of the Milky Way? This would stretch out the plane, so the angle that students would have calculated would have been different depending on what time during the test they completed the problem, since the angle would have been a function of time. The angle would have been larger if the student did the calculation early in the test, and it would have been smaller if they had rushed an answer at the last minute, since the plane would be more oblong. What if the kinetic friction by the block on the plane was so great that it caused the plane, now hypothetically made out of balsa wood, to catch on fire? Then there wouldn’t be an angle to calculate! 
Alright, enough of the flippant excuses. I’ve got more midterms to study for. But infinitely more important, I have weather to talk about.
Valid 18:50 PDT, Thu 18 Oct 2012
Take a look at how well-defined that front is! This front is the leading edge of a system that will dump a brief dose of heavy rain over our area. How much rain are we talking here? Quite a bit, actually, anywhere from 0.5 to 1 inch in the lowlands from 5 P.M. Thursday to 5 P.M. Friday. The picture below is from UW’s ultra-high resolution 4/3 km WRF-GFS model. I love this model because it allows me to see how much the rainfall varies with our topography.
Valid 05:00 pm PDT Fri, 19 Oct 2012 – 36hr Fcst – UW 4/3km 12z WRF-GFS 24-hour precip
Just for a comparison, look at the 12km resolution chart from the same model run and time below.

Valid 05:00 pm PDT Fri, 19 Oct 2012 – 36hr Fcst – UW 4/3km 12z WRF-GFS 24-hour precip

It gets the general idea, but it isn’t as precise as the 4/3 km version, and it’s a whole hell of a lot less pretty to look at. For a true meteorological buff, the UW’s 4/3 WRF-GFS is not just a great tool for forecasting, but it’s eye candy too.

Let’s take a look at the timeline for this rain. As I write this sentence, it is 7:47 P.M. Let’s see what this morning’s 4/3 WRF-GFS model run predicts for one-hour rainfall from 7-8 P.M. tonight.

Valid 08:00 pm PDT Thu, 18 Oct 2012 – 15hr Fcst – UW 4/3km 12z WRF-GFS 1-hour precip

The rain that’s passing through here right now (see above) will be out of here by 11 P.M (see below).

Valid 11:00 pm PDT Thu, 18 Oct 2012 – 18hr Fcst – UW 4/3km 12z WRF-GFS 1-hour precip
Another shot of rain comes in Friday morning. This one will be even briefer, lasting only an hour or two.
 Valid 11:00 am PDT Fri, 19 Oct 2012 – 30hr Fcst – UW 4/3km 12z WRF-GFS 1-hour precip
After this wave passes through, we will get cool and showery. A large upper-level trough in the jet stream will slide into our area, and direct cool, unstable, northwesterly flow over our area. We won’t have much in the way of lift, so I wouldn’t expect any convective activity to be too widespread, but the environmental lapse rates will be high (meaning there is a sharp decrease in temperature with altitude), so we will see some convection.
Valid 05:00 am PDT Sat, 20 Oct 2012 – 48hr Fcst – UW 36km 12z WRF-GFS 500mb vorticity, heights
The awesome thing about this trough is that it will direct cooler air into our region, and all the mountain passes, even Snoqualmie, will see their first snow of the season. In fact, the National Weather Service just issued a “Special Weather Statement” addressing the first mountain snowfall of the season. Nothing major, but hopefully the first snow of a very snowy season for the ski resorts.
Peace and love.

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