PORTLAND, Ore. (KOIN) — Record-breaking, triple-digit temperatures are in the forecast as the National Weather Service issues an excessive heat warning, with the spike expected to start the Fourth of July — but just how long will this pattern of hot weather last?

Unfortunately, the summer heat isn't expected to cool down any time soon.

KOIN 6 Meteorologist Josh Cozart says an "omega blocking pattern" is the reason behind the extended hot forecast, which is taking shape over much of the western half of the U.S.

"This type of setup blocks any cooler air from moving into the Pacific Northwest," he says. "This blocking pattern will last through much of the next week."

Furthermore, even though temperatures are expected to cool by the middle of next week, it will still be warmer than average for this time of year by about 10 to 15 degrees, Cozart says.

Overnight hours aren't expected to see much relief from this heat wave, with early morning lows forecasted to be just slightly below 70 along much of the Willamette Valley.

The National Weather Service's excessive heat warning for much of the Willamette Valley starts at noon on Thursday, July 4 and is expected to end at 11 p.m. on Sunday, July 7, but Cozart says that doesn't mean the hot temperatures will drop at that point.

"Even though the warning ends with the weekend, temperatures are still expected to remain dangerously high through at least Monday," he said. "The heat will continue to linger through much of next week."

Multnomah County announced on Wednesday that a state of emergency will begin on Friday, July 5 for it to open cooling shelters in Oregon's most-populated county. Oregon Gov. Tina Kotek's office also announced on Wednesday that state agencies are mobilizing resources ahead of the heat wave, and reminded residents to check in on neighbors, especially those who may not have air conditioning.

"High heat can also be a catalyst for wildfires," Kotek said in a statement. "I strongly urge all Oregonians to practice extreme caution this weekend, stay cool, and take preventative measures to reduce the risk of human-caused wildfires."

Stay with KOIN 6 News as this story develops.