A lot has happened in the weather department since I wrote my last blog two weeks ago. Washington, Oregon, and California all declared a State of Emergency due to widespread wildfires, and the entire Western US saw one of the hottest August heat waves on record. This heat wave gave California its worst energy crisis since the 2000 California Energy Crisis, as existing generation resources were inadequate to cover the increased electric demand (primarily from air conditioning) from consumers.
Last week, our eyes shifted from the Western U.S. to the Gulf of Mexico. Two tropical systems entered the Gulf of Mexico, strengthened, and made landfall along the Gulf Coast. The first – Hurricane Marco – was a relatively small Category 1 hurricane that weakened into a low-end tropical storm by the time it made landfall near the mouth of the Mississippi River on 8/24, bringing minor impacts to the US. However, the second storm – Hurricane Laura – was a moderate-to-large, extremely intense storm that made landfall as a high-end Category 4 tempest that made landfall at Cameron Parish on the extreme western Louisiana Coast wind 150 mph sustained winds.
Wow! Uncanny – we got the “before” of Cameron LA while I assume @bclemms is where this still (that was sent to me via email by a follower of mine) nailed the “after” of Laura’s storm surge. pic.twitter.com/KLdfKcCeXc
— Mark Sudduth (@hurricanetrack) August 28, 2020
Hurricane Laura was the 4th time in three years that a major hurricane made landfall on US soil (Harvey & Maria in 2017 and Matthew in 2019 were others) and the third in the Gulf of Mexico. Before Harvey, the US had a notable “hurricane drought,” with only nine Category 1/2 and no Category 3+ storms making landfall since 2005.
Harvey, Michael, Laura.
The Gulf of Mexico has seen three jaw-dropping eyes over the last four years.
All peaking at landfall. pic.twitter.com/OnNaGmO55z
— Dakota Smith (@weatherdak) August 28, 2020
Unfortunately, Western fires and Eastern hurricanes will continue to be a threat this week and beyond. A large ridge of high pressure will develop along the West this week and spread hot/dry conditions through the Central Valley into the Pacific Northwest, and we are tracking several more tropical waves that may develop into named storms over the next week or two.
Califonia Fires:
Northern California is currently experiencing a historic wildfire season. A series of dry lightning outbreaks near the San Francisco Bay Area during the mid-August heat wave caused numerous fires, with the most prolific outbreak occurring from the early morning hours of August 16th through August 17th as deep moisture from the remnants of Tropical Storm Fausto combined with an upper-level disturbance tracking northward along the West Coast and sparked unseasonably intense thunderstorms. Several of the large fires near the Bay Area combined into larger fire “complexes,” and two of these complexes – the SCU Lightning Complex and LNU Lightning Complex – have burnt a total of over 750,000 acres and are the 2nd and 3rd largest wildfires in California history.
Credit: NASA Worldview
With 1,660,332 acres burnt as of 8/30, 2020 is California’s second most prolific fire season on record. This is especially sobering considering that California’s main fire season is from September through November, which is the time of year that extremely strong and dry Santa Ana and Diablo winds affect the state. With tinder-dry fuels from the extreme heat this summer and aging infrastructure that cannot withstand these winds, I fear that there will be significant public safety power shutoffs this year, which could be absolutely catastrophic if we get another wave of the virus and health facilities become overwhelmed with COVID patients. Hospitals have backup generators for power outages, but other health facilities that do not have generators will need to prepare for power shutoffs if they haven’t already.
Current Conditions:
We currently have a big ridge of high pressure in the Northeast Pacific with troughing and NW flow aloft over the Pacific Northwest. A quick-moving system embedded in this NW flow is currently spreading precipitation over Washington, but Northwest Oregon and the Portland metro area should stay dry from this system.
Credit: University of Washington
Yesterday, a drier system passed through the Pacific NW in this northwesterly flow and triggered Red Flag warnings, allowing existing blazes to spread east. Counterintuitively, strong, cool, dry summertime onshore flow is an extremely dangerous fire pattern for Eastern Oregon and Washington provided that fuels are dry enough. This is due to the cool sea-surface temperatures by our coast, as these help keep dewpoints in the 50s for much of the summer west of the Cascades and the 40s east of the Cascades. Because Eastern Washington/Oregon are typically hotter than Western Washington and Oregon during the summer, relative humidities can drop as low as 10-20% east of the crest in the afternoon during periods of onshore flow. These low relative humidities, coupled with extremely dry fuels and gusty winds, make for high fire danger.
Forecast:
Onshore flow will continue Monday for one last mild day. But by Tuesday, our mild weather will be a distant memory as yet another strong ridge builds over the Western US. This ridge will be strong but “flat” – in other words, it won’t perturb the jet stream very far northward.
In the 500mb height anomaly chart above, note how the strong zonal flow aloft over British Columbia while the Central Valley is directly under the influence of the ridge. This will cause Western Washington to see moderating onshore flow while the Central Valley sees another heat wave. And with very weak winds aloft, smoke will collect over much of California
Credit: Tropical Tidbits
Credit: NASA
This flat ridge will amplify over the week, but flow is still expected to remain weakly onshore. This type of pattern generally gives Portland temperatures in the low 90s with much hotter weather east of the Cascades where the ridge is stronger.
Our main shot for a bonafide, early September heat wave with offshore flow occurs next weekend. A “rex block” – a blocking pattern with a ridge poleward of a cutoff low – is predicted to form in the Northeast Pacific, and the preexisting ridge over the Western US is expected to direct a “thermal trough” into the I-5 Corridor.
Credit: University of Washington
Portland hasn’t yet seen any serious smoky days this summer. Sure – some smoke has drifted north from California at times, but we haven’t seen anything like the smoke we saw during the 2017 and 2018 summers. Unfortunately, our luck may come to an end next weekend if this thermal trough forms and we end up seeing easterly flow, as this would transport smoke from the many fires over the Northern Oregon Cascades into the metro area. A late-season heatwave with offshore flow would also dramatically increase fire danger due to dry, hot, gusty air flowing through the Columbia Gorge and well-below-average fuel moisture for nearly all of the Pacific Northwest. The Eagle Creek fire occurred during an early September offshore flow heat wave, so our upcoming event needs to be monitored closely for fire danger.
To summarize, the Pacific Northwest will be much warmer than average beginning Tuesday, but onshore flow should prevent Portland from rising much above 90 degrees. However, a thermal trough could arrive by next weekend, turning flow offshore and allowing temperatures to rise into the mid 90s. If this offshore flow does materialize, Portland could also see a bit of smoke this coming weekend and the following week, but it’s still too far out to try and trace the exact trajectory of smoke plumes from fires east of the Cascades.
Have a great week!
Charlie
