Heat wave intensifies from Phoenix to LA, north to Seattle

• Extreme heat watches, warnings and advisories are in effect for 31 million people as of this morning, in Southern California and the Pacific Northwest, where the National Weather Service is warning of "major" to "extreme" levels of heat risk.
• The heat wave is also affecting metro LA and San Diego, with areas away from the immediate shoreline seeing skyrocketing temperatures into the triple digits again through the weekend after a scorching Thursday and Friday.

By the numbers: Downtown LA reached 112°F on Friday, just 1°F shy of its all-time record high, and only the third time temperatures have climbed to such heights.

• Burbank, Calif. tied a high temperature record on Thursday and again Friday, with a high of 114°F. And in LA, one of the city's longest stretches with highs of 100°F or above is expected through the weekend.

Stunning stat: Phoenix has now gone a record-setting 103 days straight with highs of 100°F or above. The next 10 days are forecast to stay above that threshold. This crushes the previous record of 76 straight days, set in 1993.

• It also broke the city's record of 55 days in a year with temperatures already exceeding that today, with additional 110-degree-plus days to come.
• The high temperature of 116°F in Phoenix on Thursday tied for the hottest day on record there in September, and was the latest occurrence of 116°F.
• The low temperature Friday morning was only 93°F, setting a new record warm overnight minimum temperature for the month of September.

Between the lines: Extreme heat contributed to 645 deaths in metro Phoenix last year, which is likely an undercount given how such deaths are counted and classified.

• The Maricopa County Office of the Medical Examiner has already confirmed 177 heat-related deaths this year and is investigating another 436, per reporting from Axios Phoenix.
• Poor air quality will exacerbate public health impacts in the big cities of southern California.

Zoom in: This heat wave, courtesy of a strong dome of high pressure parked above the Southwest, comes on the heels of the hottest summer on record for tens of millions of people.

• Las Vegas, Phoenix and Sacramento are among dozens of cities and towns setting seasonal records.

• The hot, dry summer leaves the region primed to burn. Multiple large wildfires are in progress in California, Idaho, Oregon and other states, and more are likely to come. Air quality warnings for wildfire-related smoke are in effect today across portions of Oregon and Idaho.
• These fires follow the Park Fire this season, which stands as California's fourth-largest wildfire on record.
• Red flag warnings for dangerous fire weather conditions are in effect in northeastern California, parts of the Cascades in Oregon, and several other states in the West as gusty, dry winds combine with the extreme heat.

What they're saying: "An upper level high pressure system is expected to continue aiding well above average and potentially dangerous temperatures throughout the West into the first full weekend of September," the NWS stated in an online forecast discussion on Friday.

Context: Climate change is causing extreme heat events to be more frequent, longer-lasting and intense, studies show.

• According to Climate Central's Climate Shift Index, human-caused global warming is causing the forecast high temperature in Fresno, Calif. today to be at least four times as likely as it would have been in the preindustrial era.

What's next: While the most intense period of this upcoming heat wave is likely to wane by early next week, longer-range outlooks show the likelihood for more heat events in the Southwest through September.

Editor's note: This story was updated with the latest forecast information.