Folks in the western U.S. are sweltering under an unusually intense June heat wave, with temperatures soaring to near-record highs from Oregon to Arizona.

Heat warnings and/or advisories were in effect Tuesday for a number of major metro areas in the West, including Las Vegas, Phoenix, Portland, Sacramento and San Francisco.

In Palm Springs, California, where highs could hit 114 degrees Tuesday and Wednesday, officials warned the high temperatures could cause heat exhaustion, heat cramps or heat stroke.

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In Las Vegas, shelters and temporary cooling stations were open during daytime hours because of that city's excessive heat warning. Highs there could hit 105 each of the next three days.

On Monday, normally mild San Francisco soared to a brutal high of 100 degrees, the first time that city has ever hit the century mark in June. The heat warped tracks on the city's transit lines, the Weather Channel said, and led to tens of thousands of power outages.

Monday was also only the seventh time on record the city reached 100 degrees, AccuWeather said. More record heat was forecast for Tuesday, where the average high is in the upper 60s.

An excessive-heat warning was even in effect in typically hellish Death Valley, California, where a high of 120 degrees was forecast for Wednesday.

In the Northwest, the National Weather Service has issued a heat advisory from noon Tuesday through 9 p.m. Wednesday for the Portland, Oregon, and Vancouver, Washington, metro areas. Near-record highs in the upper 80s and 90s are expected Tuesday and Wednesday.

Portland, Oregon, was forecast to hit 95 degrees on Tuesday, which is hotter than either Miami or Tampa in Florida were forecast to be.

In Oregon, the unusual hot weather brings up many concerns, but one of the most acute is people swimming in rivers and creeks still cold from snowmelt, experts warned.

“When you have temperatures this hot this early, what happens is people jump in rivers and creeks that are probably still around 50 degrees," said meteorologist Jon Bonk of the Portland office of the National Weather Service. "The drop from 90 degrees air temperature to 50 degrees in the water causes a physical response – sometimes shock or a big intake of water or air, and that can be dangerous."

Fires are also a concern in the West: "The hot and mostly dry weather will increase the risk for wildfires," AccuWeather meteorologist Brett Rathbun said.

Fortunately, by Thursday, temperatures will begin to moderate as a Pacific front pushes into the Pacific Northwest, the Weather Service said.

While any single weather event can't be exclusively linked to climate change, unusual, extreme heat like what's hitting the western U.S. and also India on the other side of the world likely have a global warming influence, scientists say. One expert, UCLA climate scientist Daniel Swain tweeted Monday that "our recent work suggests that we're reached the point where a majority (perhaps even a vast majority) of unprecedented extreme heat events globally have a detectable human fingerprint."

Contributing: The Associated Press; Zach Urness, Salem Statesman-Journal; Nicole Hayden, Palm Springs Desert Sun