The high temperature could easily tie or break the all-time record temperature of 100 degrees Fahrenheit set July 20, 1994, at Seattle's Sea-Tac Airport, the weather service's Jay Albrecht said.
"There's not going to be a whole lot of places to get away from the heat tomorrow," he said Tuesday.
Another day of high heat for the normally temperate region follows a Tuesday that saw the thermometer hit 106 F at Portland International Airport, just short of the 107 F all-time mark for the area set in 1981.
"The thing about a place like Portland is there are some buildings and residences that don't have air conditioning," said Andy Bryant, a hydrologist with the National Weather Service. "You go to Phoenix or Dallas, yes it would be very hot there, too, but they have more of a system in place to deal with it."
A Home Depot store in Portland had to order up air conditioners from one of its stores on the cooler Oregon coast to restock for the week. The units arrived Tuesday morning only to sell out three hours later, Home Depot employee Cliff Baker said.
"Fans are even getting hard to find," he said.
Shaved ice was also in high demand, said Matthew Ho, owner of Portland's Ohana Hawaiian Cafe. Normally the cafe sells about 20 a day, but lately it's at least double that, he said.
"We actually have a friend from Hawaii who pretty much just flew in to run the shaved ice machine," he said.
Temperatures pushed up to 108 F Tuesday in Medford in southern Oregon, and the 93 degrees F in Hoquiam on Grays Harbor in Washington state crushed the old record of 81 degrees F that dated back to 1965.
Cooling centers for the elderly were open late in Portland, and the city of Seattle extended hours for the International Fountain, where hundreds of people soaked in sprays that were timed to move along with recorded music.
In Washougal, Washington, the fire department opened up an air-conditioned training room to help cool folks off, and set up a shower in a city park during the afternoon.
Relief was in sight, however, as temperatures were expected to begin dropping into the 90s F by Thursday in areas where the high has been over 100 F, said Miles Higa, a weather service meteorologist in Portland.
In eastern Washington, temperatures around Spokane are forecast in the 90s F for the rest of the week, which is a little above normal. More worrisome is a forecast for thunderstorms that could produce lightning that ignites wildfires.
Heat advisories were issued throughout the region.