Sunshine As Far As The Models Can Calculate

Saturday, September 15, 2012
9:56 P.M.

If you don’t recognize this sun, you obviously missed out on some great children’s ‘tele’vision shows
Folks, I have never seen Seattle be as dry for as long as it has been this past year. And although I’m only 19 years old, I bet many older folks can’t remember the last time it has been this dry. We had 48 consecutive dry days, and almost surpassed the 1951 record of 51. I was absolutely infuriated that .02 inches of rain fell and ended our near-record run and have regrets about not sneaking up to the Sea-Tac rain gauge and covering it with some plastic for those couple showers, but life goes on, and according to the weather models, sunshine does too.
Let’s take a look at our current situation. Here’s a infrared satellite shot from this evening with some 500mb height lines laid over it. It is important to note that these 500mb lines are lines are NOT isobars. Isobars delineate differences in pressure, where as these 500mb height lines delineate the contours of the altitude at which the pressure is 500mb. The two are definitely related though. High heights generally correspond to high pressure because there is more “distance” for the atmosphere to reach 500mb in pressure if the surface pressure is high. For example, if the surface pressure is 1030mb, the atmosphere has to decrease 530 mb in pressure before it reaches 500mb. If the surface pressure was only 980 mb, the atmosphere would only have to decrease 480 mb, and since 480 is less than 530, the distance it would take it would take for the pressure in both circumstances to reach 500mb would be less for a place with a surface pressure of 980 mb, and the height of the 500mb line would be lower.
However, the heights are not only dependent on pressure but on temperature as well. Places near the equator have very high heights because the air mass there is warm and not very dense, so there is a small vertical pressure gradient. On the other hand, heights at the poles are generally very low (especially in winter). There, the air is cold and dense, and the molecules are packed closer together there than an area with warm air. In this situation, the 500mb height line is low because the vertical gradient of pressure difference in the atmosphere is high compared to warmer climates
Ok, here’s your satellite shot. 🙂
High height lines over the eastern Pacific, so we can infer that there is an area of high pressure here. If you take a look further north, you can see that there is some interesting weather happening up in Alaska, but let’s be honest, there’s always interesting weather occurring somewhere in Alaska.
Now, let’s look at what UW WRF-GFS model says is happening over our area right now.
Valid 05:00 pm PDT Sat, 15 Sep 2012 – UW 36km 00z WRF-GFS 500mb thickness, SLP
Here are both the height lines and the pressure lines (isobars) on the same chart. As you can see, they line up fairly well. The biggest discrepancy in height lines and isobars in this graphic is on the west side of that Alaska storm, where the counterclockwise rotation of the low is bringing colder, denser, polar air southward.
And of course, over our area, the weather is more boring than an atheist in a church. High pressure to our west is blocking any sort of, well, anything, entering our area, and all we are left with is nothing. Hence, sunny days, with an occasional cloud making a brief appearance. 
Let’s take a look at the same 500mb SLP chart 120 hours later.
Valid 05:00 pm PDT Thu, 20 Sep 2012 – 120hr Fcst – UW 36km 00z WRF-GFS 500mb thickness, SLP
That, my friends, is a gigantic ridge of high pressure. So it looks as though our dry weather will continue for the next five days. What about beyond that?
Valid 05:00 am PDT Sun, 23 Sep 2012 – 180hr Fcst – UW 36km 00z WRF-GFS 500mb thickness, SLP
Our ridge has moved a bit to the east, but is still very intact and protecting us from any storms. 180 hours is as far as the UW WRF-GFS goes out. 
But, let’s see if we can go any further. Will we ever see rain?
Valid 05:00 am PDT Tue, 2 Oct 2012 – 384hr Fcst – NCEP GFS 1000-500mb thickness, SLP, 6 hour precip
Now, I generally don’t put any stock in these forecasts because they are practically useless by day 16, but believe it or not, this is the rainiest day in the whole 00z GFS model run. It is also the last frame. See that tiny bit of green over Western Washington that might extend into Seattle? That’s the most significant thing in the foreseeable future, even if the models aren’t very good at foreseeing it.
In the mean time, water your lawns, use sunscreen, and go swimming. Even with the days becoming shorter, the water temperatures are still pretty warm right now because we have been above average in the temperature category in addition to below average in the precipitation category. Take a look at the temperature data for the Matthews Beach over the summer.
Great for taking a dip in the lake, or, better yet, fishing. There are some big, fat, hungry perch and bass in that lake right now, and now’s the time to get them before the water gets colder and they become more sluggish.
I know that’s what I’ll be doing 🙂
Charlie Phillips
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