What a refreshing weekend! With a high of only 68 degrees, Sunday was Portland’s first sub-70 degree day since we hit 69 degrees on June 14. Our chill was brought to us via a deep upper-level trough arriving from the NW, and though we warmed up to 79 today as this trough passed east, Glacier National Park in Montana saw a dusting of snow as this trough made its way through the Intermountain West.
Webcam image is of Logan Pass at 7:51 am and was retrieved from the Missoula National Weather Service’s twitter page (@NWSMissoula)
When you see images like these, it’s tempting to say that “winter is just around the corner.” But when you take a look at the latest model runs, you realize that winter is still just a tiny speck on the horizon.
This image shows a pattern we’ve seen many, many times over this past month: a “thermal trough” of warm air west of the Cascades. The trademark signature of a thermal trough is the “inverted V” isobar signature over the Willamette Valley, as the warm air associated with this thermal trough induces lower pressure at the surface due to it being less dense than the cooler air surrounding it. Because this warm air causes surface pressures to fall, a more accurate name for a thermal trough would be a “thermally-induced trough” (TIT), but I think that might be a little too risque for scientific journals.
In any event, this thermal trough will build throughout the day as high pressure offshore nudges its way inland, and with clear skies and light offshore flow, temperatures will rise into the upper 80s and will likely hit 90 in a few spots tomorrow. Thankfully for those sick and tired of the heat (i.e. just about everybody), nighttime temperatures will cool all the way to the mid-upper 50s Wednesday morning due to clear skies and excellent radiational cooling, and Wednesday will see a return to stronger onshore flow and temperatures close to average as a weak disturbance approaches the area from the northwest.
Image valid 11 AM Wednesday, 8/29/2018
We’ll chill to the lower 70s Thursday and Friday as cool, northwesterly flow continues to pour in behind this trough. We could see a few showers along and just behind this trough, but most rainfall should stay north of Portland. Those in North Seattle may be graced with a fairly strong (for this time of the year… everything is relative!) convergence zone on Thursday as this northwesterly flow splits around the Olympics and meets again over Northern King/Southern Snohomish County.
As we move into Saturday, another upper-level trough approaches the area, but the different models are all over the place with respect to the details. The American GFS model is stronger and brings it in on Saturday, while the European model shows a much weaker trough arriving Sunday. The Canadian model is somewhere between the two. Regardless, temperatures should stay near or below-normal through the weekend and into next week as onshore, northwesterly flow continues.
Drought Status
While I’m talking about precipitation, I might as well touch upon the complete lack of it for the summer. Since May 1st, Portland has only had 1.24 inches of rain, and Seattle has been even worse, with only 1 inch since then. Just skimming through some climate data, this is the driest May-August period at both sites in the climatological record, and by a significant margin. Couple our dearth of summertime precipitation with a drier-than-average February-April for many places and well-above-average temperatures, and most of Oregon is now in a “severe drought,” despite the 2015-2016 and 2016-2017 winters being the wettest on record for many regions around the Pacific Northwest.
Credit: NOAA/NCEI (map made by Jessica Blunden)
One of the most well-documented effects of global warming will be an increase in the frequency and severity of heat waves and droughts, and it seems like we are already seeing some of that here in the Pacific Northwest. I haven’t done any in-depth climatological analysis, but every summer since 2014 (with the exception of 2016) has been way warmer than average, and every June, July, AND August from 2013 onward has been warmer-than-average at Portland AND Seattle. The fact that we are just beginning to feel the effects of anthropogenic global warming here in the midlatitudes is unnerving – our droughts and heat waves over the West will become more severe throughout the coming decades.
Yesterday’s 18Z GFS
Now that I’ve sobered you all up, I’ll leave you with a couple images from yesterday afternoon’s 18Z GFS, which was one of the most unrealistic model runs in recent memory. The 18Z GFS is known in the weather geek community as the “Drunk Uncle” because it spits out all sorts of weird weather in the extended, and yesterday’s did not disappoint. In fact, it showed a polar vortex dropping down from the Arctic into the United States, bringing widespread snow to Montana with sub-20 degree temperatures for much of the northern tier of the state.
The images below are at hour 384 (11 am PDT 9/11/2018), which is the farthest the GFS goes out and the point at which the Drunk Uncle is the most inebriated.
OK folks, it’s time for me to head to bed. To summarize: pretty darn warm Tuesday, but moderating Wednesday with perhaps some light precip to our north Wednesday night/Thursday. Thursday and Friday will be cooler-than-average with strong NW flow, and another upper-level trough arrives for the weekend.
Thanks for reading, and have a great week!
Charlie
