CNN
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Millions of people in the West are experiencing a dangerous and historic October heat wave, with temperatures so extreme they could be described as hot in midsummer.

The heat was so intense that on Tuesday the United States reached the highest temperature ever recorded in the month of October.

At least 125 locations from the West Coast to the Rocky Mountains have met or broken all-time October heat records since the start of the month. Many others have set daily high temperature records.

This is another reminder that extreme heat is no longer limited to summer as the world warms due to fossil fuel pollution.

“Climate change is causing heat waves to become longer and heat waves like this to occur more frequently in the fall,” said Kristina Dahl, a climate scientist at the Union of Concerned Scientists. “If we continue to warm the plant by burning fossil fuels, late-season heatwaves like this will become much more common.”

California was the epicenter of the extreme heat.

Palm Springs rose to 117 degrees on Tuesday, setting the highest October temperature ever recorded for the entire United States. It was the hottest reading in the country at this end of the year in more than 40 years.

In some places it was hotter this week than in summer.

San Jose, California was one of them. Thermometers in the city peaked at 102 degrees on July 2, but reached a staggering 106 degrees on Wednesday – 5 degrees higher than the city's warmest October day before.

The record-breaking heat began before the calendar changed to October. Last weekend was part of a hot stretch with record-breaking conditions in Las Vegas and Phoenix – two cities where the heat has been particularly brutal this year.

Record-breaking September heat in Las Vegas peaked for five consecutive days, with temperatures reaching 104 degrees on Saturday and Sunday. It was the hottest weather in the city this late in the year.

The heat was and is even more intense and extraordinary in Phoenix, where nine days in a row have seen record-breaking high temperatures. The city recorded its second warmest day in September when it reached 117 degrees on Saturday, a mark never reached this late in the month. Then on Tuesday it had its hottest October day on record, reaching highs of 113 degrees. Wednesday marked the 113th day this year that temperatures in Phoenix were at or above 105 degrees – another all-time record.

Phoenix is ​​in Maricopa County, Arizona, where the heat was responsible for more than 300 deaths this year, according to authorities. Hundreds more potential heat-related deaths remain under investigation, according to the county.

Heat is the deadliest form of extreme weather, and climate change is making heatwaves more severe, longer-lasting and possible at any time of year. It becomes particularly dangerous if it does not cool down sufficiently overnight – another symptom of climate change.

The heat wasn't limited to the Southwest and California. Dozens of Colorado cities, including Denver, have set or equaled new October temperature records since Tuesday. Denver set its all-time October record on Wednesday when the city reached 89 degrees, the hottest temperature recorded this late in the year.

The worst of the heat has just peaked across much of the Southwest and parts of the Rocky Mountains, but extremely hot conditions will continue into early next week.

More than 30 million people in the West are still under heat warnings – including 18 million people warned of excessive heat. Nearly 200 additional high-temperature records and record-warm low temperatures could be set this weekend from California to the Rocky Mountains.

“Dangerously hot temperatures will be commonplace in many valley, mountain and inland areas,” the National Weather Service in Los Angeles warned Thursday, underscoring the wide reach of the heat.

Highs ranging from over 90 degrees to near 100 degrees are forecast in Sacramento through at least Sunday. The California capital spent the first two days of the month with July-like triple-digit numbers.

Even coastal areas, which are often heat-resistant, will not escape it this time. Temperatures will gradually increase in Los Angeles over the weekend and into early next week. A high temperature of under 80 is possible through Monday, which could be one of the 10 warmest days of the year.

Phoenix could see record-breaking heat for two weeks in a row as it remains stuck in it until at least midweek.

And Denver will climb back into the 80s on Friday and highs will continue to exceed that threshold through at least the middle of next week.

The heat also increases the risk of dangerous forest fires – especially in California.

“Dry heat like this essentially strips moisture from plants and turns them into dry tinder, which can lead to a wildfire should a spark ignite,” Dahl explained.

About a month ago, California was hit by a heat wave, and devastating fires like the Line Fire and the Bridge Fire took advantage of it and flared to life shortly after. Santa Ana's wind season typically begins in October and could bring fire danger to a head once these strong winds meet tinder-dry conditions.