Record Pacific Northwest Heat Wave Finally Ends This Weekend

Jon Erdman
Published: August 12, 2017


A record-shattering two-week heat wave in the Pacific Northwest is finally coming to an end this weekend, thanks to an abrupt change in the jet-stream pattern.

Since late July, an expansive dome of high pressure aloft has taken a vacation from its usual mid-summer home either in the Southwest or Plains; instead, it has camped out near the Pacific Northwest or far western Canada.

(MORE: Strange Start to August 2017 Weather)

This map shows a pronounced area of much stronger-than-normal high pressure aloft (orange, red contours) near the Pacific Northwest from July 22 to Aug. 7, 2017, responsible for the excessive heat and dry weather.

Near this feature, air sinks and dries, inhibiting clouds, trapping pollutants and, most notably, allowing temperatures to soar well above average – even more in the middle of summer.

Finally, that is about to change.

This weekend, the jet stream will once again plunge into the Pacific Northwest, kicking the stubborn heat dome of high pressure into central Canada.

(MORE: Pacific Northwest the Most Extreme Weather Region in 2017)

The southward dip in the jet stream, illustrated by the color contours, responsible for the much cooler weather in the Pacific Northwest this weekend.

In the heat-fatigued Interstate 5 corridor of western Washington and western Oregon, 90s will be replaced by 70s in most areas by Sunday. Seattle only reached a high of 77 degrees on Friday, which is the first time a high in the 70s was recorded since July 28.

Even the interior Northwest, where heat is typical in the summer, will see highs in the 90s and 100s gradually replaced by 80s early next week.


Forecast Highs Next Five Days

Along with this pattern change is something residents west of the Cascades are familiar, but haven't seen much this summer: rain.

Instead of the hit-or-miss thunderstorms, some of which have either started new wildfires or worsened existing ones, a steady light rain should accompany the weak cold front this weekend in western Washington and western Oregon.

(MAPS: 7-Day Rainfall Forecast)

A Record Stretch of Heat, Dry Weather and Smoke

This torrid heat wave has set several notable records for longevity.

Salem, Oregon, topped its previous record streak of 90-degree-plus highs of 10 days set in 1967 and 1938 by reaching 13 days in a row. Oregon's capital city averages just 17 such days in a year, but have already recorded 23 this year.

Spokane, Washington, has broken its record 90-degree-plus high streak of 14 days that stood since 1894, when Grover Cleveland was president. The current count is 15 days through Friday.

(MORE: 6 Things You Should Know About Heat Waves)

The first nine days of August were the hottest such period on record in Seattle, Portland, Salem, Eugene, Oregon, and Yakima, Washington, according to data compiled by the Southeast Regional Climate Center.

Then, there's the smoke.

Given the rash of wildfires in the Northwest, northern Rockies and British Columbia, Canada, this high-pressure dome aloft trapped smoke in valleys and also, at times, channeled some smoke south and west into cities such as Seattle and Portland.

(MORE: How Wildfires Can Create Their Own Weather)

High-resolution satellite imagery of wildfire smoke (gray, milky texture) over the Pacific Northwest, northern Rockies, and southwest Canada Aug. 1-9, 2017.

Spokane International Airport reported "haze or smoke" for 11 consecutive days in observations Aug. 2-12.

Ironically, the areas of smoke may have provided just enough reflection of incoming sunlight to keep Salem, Oregon (107 degrees), from reaching its all-time high on Aug. 2, and also held Seattle's highs in the low to mid-90s, as opposed to forecasted mid- to upper 90s in that same time frame.

(MORE: Smoke Creates Spectacular Sunrises/Sunsets)

Then there are the dry streaks.

As of Aug. 11, Seattle-Tacoma International Airport still hadn't recorded measurable rainfall since before summer officially arrived, June 18, breaking a new record dry streak.

It has still been the Emerald City's third-wettest year-to-date, through Aug. 11, according to the Southeast Regional Climate Center.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.


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