European model simulation of heat dome over Pacific Northwest Thursday. (WeatherBell.com)

(This post, originally published Wednesday afternoon, was updated Thursday morning and Friday afternoon based on the latest temperature reports.)

Friday midday update: While it will remain considerably hotter than normal in the Pacific Northwest for the next several days, the worst of the heat is over. Many daily record highs were set in the region Thursday. But all-time records, which would’ve otherwise been threatened, did not materialize due to thick haze over the region. The haze was a byproduct of raging wildfires in British Columbia.

In Portland Thursday, the mercury reached 105 degrees, shattering the daily record of 99 set in 1952 and ranking among its top ten hottest days ever. But it fell two degrees short of the all-time record of 107.

To the north, Seattle and Olympia both hit daily record highs of 94 and 96 degrees – but fell well shy of the triple digit heat which would’ve been in play but not for the smoke.

While the smoke lessened temperatures, it intensified sunrises and sunsets in the region. Check out this view Friday morning:

Original post from Wednesday and Thursday

An overpowering dome of hot air has entrenched itself over the Pacific Northwest and is primed to cook population centers like Seattle and Portland in record-crushing heat through Friday.

Excessive heat warnings blanket the western third of Northern California, Oregon and Washington state. High temperatures just inland from coastal locations are forecast to soar to between 100 and 110 degrees.

“We are talking about one of the major sustained heat waves in a long time around here,” writes Cliff Mass, professor of meteorology at the University of Washington. He said that “there is a lot of confidence” temperatures will at least reach the mid-90s in Seattle and notes the GFS model projects a high of 100 on Thursday — a reading he can’t “remember ever seeing”.

Seattle has posted only three days in the triple digits in the last 123 years. The hottest temperature ever recorded in Seattle was set July 29, 2009, when the mercury reached 103 degrees.

“If Seattle hits triple digits on Thursday or Friday, it’ll be the third time in the past 23 years — after reaching it only once in the first 100 years of weather observations,” notes The Seattle Weather blog. It has never before hit 100 in August.

The National Weather Service office serving Seattle said high temperatures are likely to be within five degrees of all-time highs.

Already, Seattle logged its second warmest low temperature ever recorded Wednesday, only dropping to 69 degrees. Its high temperature was 91, which broke the Aug. 2 daily record of 89 set in 2009.

Mass said in Seattle the heat wave is “major” but that in Portland it would be “historic.”

In Portland, the National Weather Service office serving the region says the temperature could surge to 107 degrees or higher Thursday, which would match or exceed its hottest temperature on record (at any time of year) of 107, set in 1942, 1965 and 1981. Only smoke flowing into the region from British Columbia could put a lid on temperatures, preventing all-time highs. “With that said, several daily high temperature records will certainly be shattered in the interior,” the Weather Service said.

On Wednesday, despite smoke wafting through the city, Portland climbed to 103 degrees, shattering the Aug. 2 daily record of 96 set in 1986.

Statistics geek will find fascinating that the GFS model predicts temperatures near the surface in Portland to be seven standard deviations (7 sigma) from the mean Thursday. In plain English, this means that this is an incredibly rare and extreme heat situation.


Temperature difference from normal forecast Wednesday late afternoon by GFS model. (WeatherBell.com)

The hottest temperatures in the region are expected in Oregon’s southern interior where highs could approach 120 degrees. Medford’s all-time record high of 114 degrees, set July 20, 1946, could be threatened.

On Wednesday, Medford hit 112 degrees, its third highest reading on record.

These sweltering temperatures, 20 to 30 degrees above normal, are expected in a part of the country not accustomed to such heat. The New York Times reported Tuesday that only a third of Seattle has air-conditioning. Seattle is opening cooling centers to help its citizens find relief.

The heat is the result of an unusually intense high pressure system aloft, sometimes referred to as a heat dome. The core of the heat dome is forecast to drift northward allowing more moderate temperatures to replace the punishing heat this weekend.