Yes, you read that right. Unless something goes horribly awry (say, a massive brush fire right next to the weather station at Portland International Airport), next week will feature the hottest weather for Portland until at least mid-April 2019. After January 1, Portland’s record high doesn’t rise above 80 degrees until April 7, and at this point, Tuesday-Friday are all looking like they’ll reach the low-mid 80s, with Thursday being our hottest day. Do yourselves a favor and GET OUTSIDE this week because you won’t see weather like this for a long, long time.
Why so warm? Well, a thermal trough will spread northward along the Oregon Coast while a big ol’ ridge of high pressure sits just offshore, giving us clear skies and offshore flow. The thermal trough is too weak to give us record-breaking heat, but we’ll get plenty warm nonetheless, and after our extended period of cooler-than-average temperatures, I’m sure things will feel downright toasty across the area.
Current Conditions:
Let’s start by looking at one of my favorite charts: the infrared satellite with overlaid 500mb height lines and temperatures.
As you can see, there’s a big ridge of high pressure offshore centered near 145W with a tiny little cutoff underneath it near 25N. Over our neck of the woods, we have a trough moving to our east and moderate onshore flow and a weakly unstable airmass. This combo led to a pretty sweet MODIS image from NASA’s polar-orbiting TERRA satellite when it made its daily passage over the Pacific Northwest earlier this afternoon. Here, Western Washington/Oregon and the Cascades see cumuliform clouds/weak convection as the sun warms the ground and lower atmosphere, while the air over the ocean doesn’t warm enough (actually, it hardly warms at all!) to create convection. The Columbia Basin and Central/Eastern Oregon are “cloud-shadowed” and had beautiful sunny skies today.
Credit: NASA
Finally, take a look at how our weather of late compares to the weather we experienced earlier this summer. You’ll have to forgive me… the below plots are for Kelso/Longview… the University of Washington didn’t have any Portland plots. Anyway, the 5 consecutive days of below-normal lows from the 17th-21st this week (the 21st is not shown yet) is the longest stretch of cooler-than-average low temperatures this summer, and Kelso/Longview hasn’t had an above-average high since September 7. And while most of the Pacific Northwest is drier-than-normal, we made some serious progress erasing our summer deficit over the past two weeks.
Credit: University of Washington
Credit: University of Washington
This Week’s Heat Wave
On Monday, the upper-level trough currently over our area will push through the Northern Rockies and displace the dome of hot, thermally-induced low pressure that typically sits over the Intermountain West westward to the Pacific Coast. Meanwhile, both the ridge offshore and the weak cutoff low just barely visible in the current 500mb analysis above strengthen and move slightly to the east. The images below show the 500mb heights/anomalies and 925 mb temperatures at 5 am Tuesday morning.
Valid 5 am Tuesday morning
Look at how the thermal trough evolves Wednesday and Thursday.
If we look back again at our 500mb chart at 5 am Thursday, you’ll notice that although the ridge is beginning to retrograde NW, there is still that very distinctive upper-level pattern with the upper-level low directly to the south of the ridge. This is a type of block called a Rex Block, and though I have no idea who Rex is, he sure is responsible for some pretty persistent blocks. A Rex Block is a very difficult pattern to break out of, and they can be absolutely infuriating for winter weather junkies like me when they form off our coast during the winter and give us a week or two of inversions and smog. I don’t mind them in September though!
Temperatures will cool slightly on Friday to either side of 80 at the surface, but we’ll see a much more dramatic cooldown Friday night/Saturday morning as the sun temporarily disappears (just kidding, it’s because the thermal trough will move east of the Cascades and we’ll see a strong onshore push). We may see some precipitation Saturday due to both an upper-level trough approaching from British Columbia and/or the cutoff low moving inland, but, to borrow my all-time favorite weatherman excuse (and there are plenty), it’s too far in advance to know the details.
However, models generally show us being cooler and wetter-than-average to begin October week as the Pacific storm track resumes and a series of weak systems plow through the area.
In other words, the warm weather we’ll see this week doesn’t look like it will be returning anytime soon, so be sure to get out and enjoy it while you can!
Thanks for reading, and have a great week!
Charlie
