Another Scorcher To Begin The Week

I don’t know about you, but I thoroughly enjoyed our sudden switch to cooler weather on Saturday. I was in Seattle for a weekend of boat chores and band practice, but even more fun than pressure-washing our boat was having all my windows open Friday night and feeling a strong, westerly, marine breeze suck all the heat out of my room and expel all the smoke from our skies. And I love pressure-washing.

Saturday even featured some rain showers across the region as this upper-level trough moved through, and the unstable atmosphere associated with this trough meant that some of these showers became quite intense. One was nearly heavy enough to do a bit of pressure-washing on its own, making us question whether it was worth it for us to have spent 2-hours what Mother Nature could have done in 30 seconds.

While it was nice to get our of the haze on Saturday, smoke returned with a vengeance today as NNW upper-level winds in the wake of Saturday’s departing upper-level low, now located over Central Alberta, sent smoke from BC fires into the region. With weak winds aloft over our region (or, in the words of the Portland NWS forecaster’s discussion, an “axis of dilation created by the departing upper low and a pair of high pressure regions over the central CONUS and also over the NE Pacific), this smoke isn’t expected to go anywhere and should hang around through at least midweek. 🙁

Take a look at a 24-hour comparison from NASA’s polar orbiting AQUA satellite. That’s a pretty stark (and sad) difference.

Here’s an image of the 500 mb pattern over our area at 5 pm tonight as modeled by this morning’s 12Z GFS. You can see the trough exiting Alberta sandwiched between two ridges – one over the NE Pacific and another over the eastern CONUS. The “axis of dilation” is the SW-NE “U-shape” in the height contours over our area extending SW into the Eastern Pacific.

By Monday, this ridge will move east into British Columbia, resulting in further warming.

Still, this ridge isn’t nearly strong enough to bring us the low-mid 90s forecast for the Portland metro area on Monday, especially with all this smoke overhead. If not the warm air associated with this ridge, than what else?

Thermal Troughs and Offshore Flow

Monday will also feature something that we haven’t seen too much of this year – thermal troughs and offshore flow. Thermal troughs form when a ridge of high pressure forms over our area and directs a “tongue” of warm air through the Central Valley of California into Western Washington/Oregon, and because warm air is less dense than cold air, this tongue of warm air has locally lower pressure than its surroundings. The image below shows a textbook thermal trough affecting the area Monday afternoon/evening, with a lobe of warm air and locally lower pressure pushing all the way into British Columbia.

By creating locally lower pressure over Western Washington and Oregon, thermal troughs create offshore flow. Offshore flow warms us up for two reasons – the origins of the air itself (Eastern Washington/Oregon is much warmer than the Pacific Ocean) and “adiabatic warming,” which is the process by which an air parcel warms without exchanging any heat energy with its surroundings. Adiabatic warming occurs when an air parcel is compressed, as an increase in temperature must balance out a reduction in volume to maintain the same quantity of heat energy. Even though the Columbia River gorge has relatively little decrease in elevation as you travel westward. air flowing through the gorge in these scenarios can originate from higher elevations, resulting in Portland being blasted with adiabatically-warmed air. As you might expect, adiabatic effects from offshore flow can strengthen the thermal trough, which then strengthens offshore flow in return.

Offshore flow separates our run-of-the-mill heat waves from our true record-breaking heat waves. The relatively low mid and upper-level temperatures over our region (due to the aforementioned “axis of dilation” and the ridge not being notably strong) will prevent temperatures from rising into the triple digits around the Portland metro area, but we should still rise low 90s on Monday and the mid 90s on Tuesday as hot, dry winds rush through the Columbia River Gorge. I call this kind of weather “hair dryer weather” because the stiff, hot, dry breeze brings to mind a massive hair dryer.

With hot, breezy, and extremely dry conditions, fire danger will be extremely high and I wouldn’t be surprised to see some of the weather forecast offices issue some “Red Flag Warnings” (high fire danger warnings, essentially) for parts of the region. Don’t even think about throwing a lit cigarette butt out the window – you could very well start a forest fire.

Tuesday will be our hottest day as this thermal trough continues to strengthen, and temperatures will finally “moderate” to either side of 90 degrees on Wednesday as this thermal trough heads east and winds turn lightly onshore.

Things get interesting Wednesday evening. The GFS model develops an upper-level-low off our coast and takes it far enough east to direct moist, unstable, southerly flow into the region. This could spark a few thunderstorms over the Cascades and Eastern Oregon, and with tinder-dry fuels, any lightning strikes from these storms could easily ignite some new wildfires. To make matters worse, these storms likely wouldn’t be accompanied by a ton of precipitation to help wet these fuels.

However, the generally-more-accurate ECMWF model keeps this low far enough offshore to avoid any threat of convection over the Pacific Northwest. It’s notoriously hard to predict how these upper-level lows will evolve and I believe the most likely outcome at this point is that we won’t see any thunderstorms over our region. Even so, I thought it was worth mentioning with fire danger already being so high throughout the Pacific Northwest.

Regardless of whether we see thunderstorms during the latter half of this week, we are expected to rise into the 90s once more over the weekend as yet another big ridge of higher pressure settles over the region. No thermal trough is expected over the weekend, so low-level onshore flow will help keep temperatures closer to 90 degrees.

Unfortunately for those hoping for a surprise onshore push instead of yet another ridge, ensembles are in pretty darn good agreement on this weekend ridge. We may see a temporary return to more seasonable weather towards the latter half of next week, but something tells me that our hot and dry weather isn’t going away anytime soon.

Tl;dr:

A thermal trough will move into the area on Monday and stick around through Tuesday, sending temperatures into the low 90s on Monday and mid-upper 90s on Tuesday with dry, offshore flow and smoky skies overhead. Fire danger will be extremely high, so please heed all burn bans throughout the state as it won’t take much to start an out-of-control blaze. Models differ with the strength and location of an upper-level low that may form over the Pacific Tuesday night/Wednesday and spark convection over our area Wednesday night/Thursday as it approaches the region and directs moist, unstable southerly flow into the area, but the ECMWF model shows this trough weaker and further offshore and does not show any risk of convection over area. Regardless of what happens from the Wednesday night/Friday period, models are in good agreement that yet another ridge of high pressure will move over us for the weekend, but low-level onshore flow will help moderate temperatures somewhat and prevent us from rising into the mid 90s like we’re expected to on Tuesday. We’ll moderate during the first half of next week after this ridge moves through, but warm weather may return once again later in the week.

Thanks for reading, and have a fantastic Monday!
Charlie