Excessive heat warning issued as heat dome expected to hit Portland metro

PORTLAND, Ore. (KOIN) — An excessive heat warning has been issued for the region starting on Thursday as a heat dome hanging over the Western U.S. will bring three days of record temperatures to the Portland area for the Fourth of July weekend.

KOIN 6 Meteorologist Kelley Bayern forecasts daily highs above 100 degrees from Friday to Sunday. Portland’s current records for July 5, 6 and 7 are 98, 97 and 95 degrees, respectively. The National Weather Service has issued an excessive heat warning in response to the "dangerously hot" temperatures. The excessive heat warning will last from noon Thursday to 11 p.m. Sunday.

“Some model forecasts range maximum temps around 104 to 108 degrees,” Bayern said.

Bayern said that the record temperatures are caused by a large ridge of high pressure that will lock in over the Western U.S. for several days, trapping heat at the surface.

"The heatwave coming this weekend is due to a heat dome that will anchor-in over the Western U.S over the weekend,” Bayern said. “In meteorological terms, we have an omega blocking ridge pattern developing that will amplify all the way into northern British Columbia, bringing excessive heat far north.”

The National Oceanic and Atmospheric Administration states that a heat dome occurs when a persistent region of high-pressure traps heat over a particular area for days or even weeks.

“Under high pressure, the air sinks toward the surface,” NOAA states. “This sinking air acts as a dome capping the atmosphere. This cap helps to trap heat instead of allowing it to lift. Without lift, there is little or no convection and, therefore, little or no convective clouds with minimal chances for rain. The end result is a continual build-up of heat at the surface that we experience as a heatwave.”

While temperatures could rise as high as 108 degrees, Bayern said that it is very unlikely the region will experience conditions as severe as the deadly heat wave that set an all-time high of 116 degrees in Portland on June 28, 2021.

“It is important to note that the chance of this heatwave being like the 2021 heat is very low, at about 10%,” Bayern said.

Multiple government agencies have issued fire safety warnings ahead of the heat wave, which will continue to dry out forestlands across the Pacific Northwest during a holiday weekend that’s synonymous with fireworks.