After wilting heat 10 of the last 11 days — where Portland temperatures reached or exceeded 90 degrees — forecasters say relief in the form of cooler ocean air will arrive Thursday night.

That still means Portland and most of northwest Oregon and southwest Washington will reach or exceed 90 degrees on Monday, Tuesday and Wednesday.

Thursday’s forecast high is 89 degrees, but that drops to 84 on Friday, 81 on Saturday and 83 on Sunday.

Forecasters say the extended heat wave is the result of a so-called Rex blocking high-pressure system

According to the National Weather Service, a Rex block is “characterized by a high-pressure system located immediately north of a low pressure system. Because the flow of the air is basically north-south, there is very little eastward progression of the system.”

The phenomena was recognized and named for Dr. Daniel F. Rex, who discovered the pattern in 1950. The West Coast of the U.S. and Western Europe are two of the main places where Rex blocks occur.

Washington and Oregon are just to the right of the Rex blocking high indicated by the blue H in this weather chart from the National Weather Service. 

“Finally, an end is in sight to this historic heat wave,” Shawn Weagle, a meteorologist for the National Weather Service in Portland, said Monday in his forecast discussion.

Weagle predicts that by Thursday night he expects the marine layer to become deeper and for cooler air to start flowing east into the interior of the Willamette Valley through the gaps in the coast range and up the Columbia River.

Although Portland has not seen any high temperature records fall during the  heat wave, a record for consecutive days of 90 degrees or higher could fall if the mercury tips that level each day through Wednesday. A heat advisory for Portland remains in effect through late Wednesday night.

The record for consecutive days of 90 degree or higher came July 25 through Aug. 3, 2009.

Portland would have tied that 10-day record Sunday if not for the high of 85 degrees on June 28. If the highs reach 90 or higher Monday through Wednesday this week, Portland would tie the 2009 record.

Monday’s forecast high of 93 degrees will make it the 11th day out of the last 12 to reach or exceed 90 degrees, and give Portland its 15th day at 90 or above, three more than the annual average.


Portland extended forecast:

Monday: Sunny; high near 95. 

Tuesday: Sunny; high near 95.

Wednesday: Sunny; high near 94. 

Thursday: Sunny; high near 89.

Friday: Mostly sunny; high near 83.

Saturday: Mostly sunny; high near 81.

Sunday: Mostly sunny; high near 83.

— Stuart Tomlinson

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