Summer's Last Stand?

Thursday, September 4, 2014
4:14 pm

Custer's Last Stand - Mort Kunstler

I had to do some refreshing on my American history to remember what Custer's Last Stand was. I did some research, and it turns out that this Custer guy really hated Indians, so he decided to fight some (specifically the Lakota, Northern Cheyenne, and Arapaho). He died. In fact, much of his puny cavalry died, as did his family. It was truly his Last Stand.

Do I think that Friday and Saturday will be weekend's last stand? No. Even though they will be extraordinarily nice, I've got a feeling that September as a whole going to be warm and sunny.

Let's take a look at our setup right now, at 8 pm (03z). The map below is very hard to see at the given size, so click on it to increase the resolution. The lines drawn are called isobars, and delineate lines of constant pressure. You can see a ridge of high pressure in the Eastern Pacific and a thermal trough centered over the Central Valley of California. Winds are generally coming from the north-northwest off the coast and from the north in the interior. This is a very typical summertime setup for the Pacific Northwest.

Retrieved from "surface analysis" @ www.weather.gov/forecastmaps

Here's a look at the UW's WRF-GFS model 9 hours out from the 12z initialization this morning. Since it is so close to the initialization time, the large scale features are very similar to the analysis above. For all practical purposes, it just shows an alternative view of our setup.

In addition to showing the isobars, the above chart shows "thickness" contours. When the atmosphere is warmer, it becomes less dense, and the height between two levels of atmospheric pressure increases. This chart uses decameters and measures the thickness between 1000 and 500 mb. The larger the thickness, the less dense the atmosphere (and therefore the higher average temperature). As a side note, I often use thicknesses as a predictor of whether it is cold enough to snow in Seattle. I only start to seriously consider a snow when thicknesses drop below 522, and I'd rather have them in the 510s. There are exceptions of course (a relatively warm upper atmosphere with a cold lower atmosphere allowing thicknesses to be relatively high yet snow to still occur), but the 522 line is something respected by most meteorologists in the area.

As we go into the weekend, we are expected to warm up as the ridge to our west gets nudged to the southwest, weakens, and any onshore flow gets the boot. In its place, the thermal trough to our south will extend northward and open the door for warm, offshore flow to take hold over the Pacific Northwest.

Valid 08:00 pm PDT, Fri 05 Sep 2014 - 3hr Fcst   Retrieved from UW mm5rt Modeling Website

Sunday will be our warmest day, with highs reaching the mid to upper 80s. Highs will be even warmer near the foothills due to the adiabatic compression of air as sinks off the Cascades. In other words, the total amount of heat energy in the air does not change, but since a given mass of air takes up less space near sea level than at elevation, this energy is compressed into a smaller parcel of air. Therefore, no energy is exchanged from any separate parcels, but the air still changes in temperature. Pretty cool. Also, when air rises, it expands and the temperature cools, but the amount of energy stays the same

After Sunday, we will get a brief respite from the heat. I say "respite" because this 2-month period of July and August was the hottest in history, and I've not become acclimated to the heat; if anything, I've just been weakened by it. We could even see a little rain on Tuesday, but things clear up Wednesday, and a new ridge is back in full force by Thursday.

Valid 11:00 pm PDT, Thu 11 Sep 2014 - 150hr Fcst   Retrieved from UW mm5rt Modeling Website

This ridge won't extend quite as far up north, and the heights over our area will be slightly lower (567 instead of 570), meaning our temperatures will be lower as well (lower 80s if this scenario plays out). We should still have blazing sunshine though. For all intents and purposes, it will feel like our record breaking summer again.

Charlie