Record Heat Increasingly Likely Next Weekend!

It was hot Sunday and today! My brother came down to Portland for the weekend, and yesterday, we went cliff diving at High Rocks Park on the Clackamas River. There were some pretty acrobatic divers jumping off the rocks, and there were several lifeguards and an ambulance van on standby in case there were any injuries and somebody needed to be transported to the hospital. It sounds like High Rocks Park is quite a dangerous place, but it’s such a perfect swimming and diving spot that prohibiting water sports would be an exercise in futility, so I was glad to see there was such a heavy lifeguard and safety presence there. Needless to say, we had a great time – I couldn’t think of anything better to do on such sunny, 93-degree day in Portland.

We drove back up to Washington last night, so I’m writing this blog from my folks’ place on South Whidbey Island and am enjoying the cooling influence of Puget Sound. As of 4pm, temperatures are on either side of 80 in our neighborhood but are in the mid 90s in the Portland metro area. Gotta love our “natural air conditioning” up here in Western Washington from Puget Sound & the Strait of Juan de Fuca!

But as hot as Portland is today, this weekend is shaping up to be much, much, MUCH hotter. Models are in increasingly good agreement that Portland and the Willamette Valley will experience their hottest June temperatures on record next weekend, and some models even show Portland approaching all-time record high temperatures. And all models – and most ensemble members of each model – show temperatures rising into the triple digits on Saturday, Sunday, and potentially even Monday. Before I delve too deeply into the details, here are a couple highlights:

• Expect warm (upper 80s) temperatures Tuesday-Thursday as a moderate/strong ridge builds in the NE Pacific but we see moderating onshore flow underneath this ridge. Unfortunately, this onshore flow will help fires east of the Cascades (such as the S-503 fire on Warm Springs Indian Reservation) grow eastward or southeastward.
• An upper-level low off the California coast spread will spread moisture northward into Central/Eastern Oregon, causing thunderstorms Monday – Wednesday afternoons/evenings with frequent lightning and relatively little precipitation.
• Temperatures will warm on Friday as the aforementioned ridge strengthens and approaches the coast.
• Triple digit heat is expected for Portland this weekend and potentially Monday as this ridge moves inland and flow turns offshore. Portland’s all-time June record of 102 stands a good chance of being broken, and there is a chance that Portland’s all-time record of 107 could be matched or exceeded. Fire danger will increase west of the Cascades due to the hot/dry conditions and potentially breezy offshore flow.
• The record heat should subside next week but things still look warmer and drier-than-average, and thunderstorms may ignite additional fires over southern and Central/Eastern Oregon.

Below is the current upper-level pattern over the Western US. We have a strong ridge over the Four Corners extending NW into the Pacific NW, and there is also a broad area of ridging in the NE Pacific. To the south of the ridge over the NE Pacific, there is an upper-level low centered several hundred miles off the Bay Area.

This upper-level low is directing some moisture into the Pacific NW, and the combination of hot, moist air at low levels and somewhat cooler (but still much warmer-than-average) air aloft has caused some instability and thunderstorms. The GOES-17 loop below shows how thunderstorms popped up over Central Oregon this afternoon.

These storms are visible on radar and are associated with a decent amount of rain – they aren’t dry thunderstorms by any means, but they aren’t particularly wet either.

More afternoon thunderstorms are expected Tuesday and Wednesday over Central/Southern Oregon and potentially some points to the north, and with relatively little rain expected from these storms and very dry fuel conditions, we’ll likely see additional lightning ignitions over the next several days. Nothing out of the ordinary for summer east of the Cascades and not a major dry lightning outbreak, but with fuels as dry as they are throughout the Western US and the drier/warmer-than-average long-range forecast, we want as few new ignitions as possible, since the environment is so favorable for any existing fires to grow.

Tuesday-Thursday should all be much cooler than this weekend but still sunny and warmer-than-average, as ridging brings clear skies and warm temperatures aloft while onshore flow helps cool temperatures at the surface. The images below show the upper-level heights at 5pm on Wednesday and the 10-meter (above-ground) wind gusts at 8pm Wednesday.

Unfortunately, the onshore flow will be gusty east of the Cascades and will allow the S-503 fire on the Warm Springs Indian Reservation to grow to the SE. As of this evening, the fire was 5% contained and was at 6,200 acres. For more updates on the S-503 fire, check out the Northwest Interagency Coordination Center’s current info on the incident and the Warm Springs Wildfire Management Facebook page.

The Heat Wave

Models have been showing a heat wave for the ages this weekend for the past several days, and although there’s still a significant amount of model disagreement in the ensemble solutions, I have very high confidence that this coming weekend will be by far the warmest period of 2021 thus far. In fact, there’s a high probability that highs this weekend across the Willamette Valley may break their all-time June records, and I think there’s a very good chance (30-40%) as of the writing of this blog that Portland could tie or break its all-time record of 107 degrees, which has been set three times in Portland’s history – 7/30/1965, 8/8/1981, and 8/10/1981. Portland’s hottest time of the year is typically the last week of July, so the prospect of breaking the record a month before that shows how perfect the pattern will be for extreme heat in the Willamette Valley.

By Friday, the ridge that was in the Northeast Pacific will have strengthened dramatically and approached the coast. The maximum heights are 596.8 decameters in this model (ie. the 500mb pressure level is measured at 5968 meters), which is near the value it was measured at during the historic heat wave the Desert SW/Intermountain West experienced last week.

Saturday – Monday should be the hottest days of the period as the ridge positions itself right over the Pacific Northwest.

The “operational” models – and the GFS in particular – have consistently shown outlandishly hot temperatures these three days, particularly Sunday and Monday. The 12Z and 18Z GFS, for example, both show Portland hitting 120 degrees on Monday, which would be the hottest temperature ever recorded in the entire state’s history! Given Portland’s elevation, ample vegetation, lack of strong downslope warming effects, and latitude, among other factors, I’d estimate Portland’s theoretical maximum temperature around 110-111.

Moreover, the operational models have consistently been among the warmest ensemble members, but the ensembles have been trending towards the extremely hot operational runs. The below chart shows the “spaghetti plot” for the 12Z 6/21/2021 GFS ensembles at Portland – the purple is the operational run (the one that showed 120F for Portland) and is the warmest run of all the ensemble members. However, given that the ensemble members have been trending warmer and the operational models have consistently shown record-breaking heat, I’m definitely leaning towards the more “reasonable” hotter solutions. I could see Portland getting to the low-mid 100s Saturday through Monday, with the potential for one of those days to match or break the all-time record of 107. We’ll have a little more clarity on the maximum possible high temps when the event comes closer, the ensembles show better agreement, and the operational runs (hopefully) start showing some more reasonable forecasts.

Why so hot? 

There are a number of factors coming together to create this record heat wave. The biggest and most unique is probably the strength of the ridge and the warmth of temperatures aloft. Temperatures at 850 mb (approximately 5,000 feet above sea level) are forecast to be as high as 30C, which would break the all-time record of 28.3C set back in September 3, 1988 (credit to Chris Callais for the tip). Air tends to sink under ridges of high pressure (hence the lack of clouds – the sinking motion inhibits cloud formation), and as this air sinks, it further warms via adiabatic compression at a rate of 9.8C/km.

Second, we’re expected to have offshore flow with this heatwave, will will prevent us from seeing any sort of cooling influence from the Pacific or Puget Sound and will instead give us toasty, continental air from the arid inland Northwest. Additionally, this offshore flow results in further adiabatic compression/warming as air sinks on the western slopes of the Cascades, resulting in additional warming for the Cascade foothills, Eastern Portland metro area, and Columbia River Gorge. Third, we’ll only be a few days past the summer solstice, so we’ll see our longest days and highest sun angles of the year to add to the intensity and duration of our warmth.

Finally, soils are drier-than-average due to the record-dry spring we’ve experienced, as last weekend’s deluge was just a drop in the bucket compared to what we’ll need to erase our drought, and much of the progress has already been undone by the hot/dry/sunny weather we’ve seen over the last week. The evaporation of soil moisture has a cooling effect since evaporation takes energy, so if we don’t have that extra cooling effect, temperatures can rise a degree or two above what they otherwise would if soil moisture was at above-average levels.

Conversely, there are a few things that could prevent temperatures from reaching the levels currently forecast. Offshore flow could be weaker than modeled or flow could just be northerly, both of which would likely cap us in the low/mid 100s. Some smoke from nearby fires could also drift over the Pacific NW and slightly filter sunshine – I remember August 2017, when forecasts had us reaching a high of 108 but high-altitude smoke turned the sun blood-red and brought our high temperature down to 105. The apocalyptic hues made it feel far hotter, though. Lastly, some models show some convection making its way northward, and if any clouds (let alone thunderstorms) were to pass over the Pacific NW, record temps would be off the table. Most models don’t show the potential for any convection until Sunday or Monday however, and most keep the convection over Southern Oregon or east of the Cascades.

Fire Threats

Unfortunately, any thunderstorms late next weekend or the following week would pose a significant threat for new, lightning-caused wildfires given how dry the environment will be after this heat wave. And even though the Willamette Valley isn’t expected to see thunderstorms, the extreme heat and dry, offshore flow will dramatically dry fuels west of the Cascades and increase fire danger. This is by no means a “September 2020” fire pattern – September 2020 was a dry, offshore windstorm that knocked out power to hundreds of thousands of people, caused widespread 40-55 mph gusts throughout the Willamette Valley, and caused 12 consecutive hours of 100 mph gusts on Mt. Hood, all with humidities in the teens or single digits. This pattern will just be extraordinarily hot with some light, non-destructive offshore winds, but the environment will be very dry and even a light breeze could allow fire to quickly spread out of control.

If you don’t have a portable air conditioner or fan, be sure to try and get one before the stores run out this week! And be sure to check up on your loved ones who may be sensitive to the heat – this type of heat is deadly and it’s critical that those sensitive to heat have a means to cool themselves during these events.

Have a great week and stay cool!
Charlie