An unprecedented heat wave is scorching parts of the western United States at the start of October with record-breaking temperatures and lingering excessive heat alerts expected to last into the weekend.

Forecasters say the sweltering late-season heat wave will bring moderate to extreme heat risks across portions of the Southwest through the week and into the weekend. On Wednesday, about 39 million people in the region were under heat alerts, according to the National Weather Service.

The weather service extended excessive heat alerts for areas near major cities, including Las Vegas, Phoenix, San Francisco, and Los Angeles. High temperatures in parts of Arizona, California, and Nevada are expected to be 10 to 25 degrees above normal, forecasters said.

"Similar areas will experience major to extreme heat impacts over the next few days as 90-110F temperatures develop each afternoon, threatening record high temperatures," the weather service warned Wednesday.

The hot conditions are caused by a heat dome — a strong high-pressure system that traps heat over a region and prevents cool air from getting in. AccuWeather meteorologist Isaac Longley told USA TODAY that the ridge of high pressure has been persistent across the Southwest.

"This, certainly, is a very impressive stretch of heat, especially for some locations and cities across the West and Southwest," Longley said. "It's not really moving. We're not getting any dip in the jet stream to really bring a cooler, or at least more comfortable weather to the areas of the Southwest and even along the West Coast."

Longley also noted that the heat dome is "expansive," impacting other areas such as Denver with unseasonably warm weather. Forecasters said dozens of heat records will likely be set through the weekend across the Southwest and West Coast.

The early-autumn heat wave follows a summer of extreme heat that hit both the western and eastern United States. The National Oceanic and Atmospheric Administration reported last month that the summer of 2024 was the fourth-hottest summer on record for the country.

Several states, including Arizona and California, had their warmest summer on record, according to the NOAA. "As a result of global warming, summers are getting hotter across much of the U.S. West, and both the number of heat waves and the length of the heat wave season are increasing," the NOAA's Climate Prediction Center said.

Fall is here:Weather forecasters say brace for more heat in autumn

Phoenix has no relief with weeklong high temperatures

After facing its warmest summer on record this year, Phoenix saw little relief in September with persistent high temperatures over 100 and even 110 throughout the month. 

Temperatures continued to set records into October when both Phoenix and Yuma reached 113 degrees on Tuesday — making it the hottest October day in the city’s history. Prior to this week, Phoenix had never reached temperatures at or above 110 degrees in the month of October, according to a weather service forecast.

Tuesday marked the eighth day in a row of record high, tied, or broken temperatures in Phoenix, according to the Arizona Republic, part of the USA TODAY Network. Records continued to be challenged on Wednesday when the weather service in Phoenix reported that temperatures reached 108 degrees, which breaks the previous record high of 107 degrees set in 1980.

An excessive heat warning issued by the weather service was extended through the weekend as "record-breaking daily high temperatures are expected to continue," the agency said Wednesday. Temperatures were expected to be between 105 and 112 for the lower desert areas.

Record temperatures in California

AccuWeather said temperatures reached season-high levels along portions of the California coast. In San Francisco, temperatures climbed to 94 degrees on Tuesday — which was the city's hottest day since 2022, according to the weather service.

"94 degrees in San Francisco yesterday with similar expected today," the weather service in San Francisco said Wednesday. "For October, we've only reached this benchmark 15 times over the last 150 years."

The weather service extended its excessive heat warning for inland portions of the Bay Area, Central Coast, and the city of San Francisco through late Thursday. Several daily record temperatures were set or tied in the region on Wednesday when San Jose hit 106 degrees, breaking its records of 96 degrees set in 1980 and 2012, according to the weather service.

Los Angeles will also continue to feel high temperatures through Thursday with the hottest areas likely reaching 100 to 106 degrees, the weather service said. Several records were already reached on Tuesday across the Los Angeles County as some cities saw triple digit temperatures.

Temperatures through the week will be 5 to 10 degrees above the historical average, according to AccuWeather.

Contributing: Skylar Heisey and Hayleigh Evans, Arizona Republic