Tens of millions of people across the desert Southwest and along the California coast are being urged to curtail outdoor activities this week, as high temperatures rarely seen this early in the year are expected to topple records across those regions.

The heat will last through the week. “It is going to be a marathon,” the National Weather Service’s office in San Francisco warned. Heat advisories and even extreme heat watches, unusual for March, have been issued in many areas.

Meteorologists said they were expecting several types of records to fall: daily records; overall highs for the month of March; and earliest 100-degree days of the year. Official record-keeping in many of these locations, including Phoenix, Los Angeles and San Francisco, goes back more than 100 years.

Because the heat is hitting so early in the year, forecasters said they expected it to affect people in ways they might not be used to. “We are not acclimated to this level of heat this early,” said Gabriel Lojero, a meteorologist with the weather service’s office in Phoenix.

San Francisco Bay Area and Central California

The Bay Area is under a heat advisory from Monday morning through Friday night, and forecasters in the San Francisco office of the weather service may extend the advisory into the weekend. It is the first time since that office began issuing heat advisories in 2006 that it has issued one in March.

A heat advisory means temperatures are expected to be dangerously high, but it stops short of extreme heat warnings and watches. In a heat advisory, people are urged to avoid outdoor activities and to take frequent breaks in the shade.

Temperatures along the coast may reach into the 80s, and highs in the 90s and even approaching 100 are expected farther inland.

It’s unusual for the temperature to hit 80 degrees in San Francisco at any time of year, and temperatures that high or higher are expected every day this week.

Air conditioning is not widespread in homes in the region, but the San Francisco Department of Emergency Management warned people seeking relief at the area’s beaches to be careful: “Be aware that ocean water is extremely cold and strong currents are common.”

Los Angeles and Southern California

Parts of Southern California got a taste of the heat last week, and temperatures around the Los Angeles area set daily records in the upper 80s and low 90s. After a brief weekend dip, temperatures are expected to climb as high or higher, and many heat advisories, warnings and watches were in place.

“It’s going to feel like summer, for sure,” said Chandler Price, a meteorologist for the weather service’s office in San Diego.

Forecasters in Los Angeles warned that the week would bring a “historic, long-duration heat event.” Temperatures in the city are expected to be at their highest Tuesday and Wednesday and will continue to be well above normal through the rest of the week.

The highest March temperature for downtown Los Angeles — 99 degrees — was recorded in 1879, and the highs forecast for this week will creep close to that number. Away from the coast, highs in the upper 90s and 100s are expected.

Mayor Karen Bass of Los Angeles directed people to “hundreds of locations open for relief from the heat,” including libraries and recreation facilities.

Phoenix and the Desert Southwest

Phoenix, no stranger to heat, is expected to record highs that are unusual — and unusually persistent — for this time of year. The weather service’s office in the city warned that summerlike temperatures were expected by Tuesday, followed by “widespread triple digits” beginning Wednesday. The service’s forecasters said that daily records might be shattered by as much as 10 degrees.

“The all-time March records for Phoenix, Yuma and El Centro are 100, 102 and 101 degrees, respectively,” the Phoenix forecasters said. “All of those values could be tied or eclipsed as early as Wednesday.”

Another record that is likely to fall is the date for Phoenix’s first 100-degree day of the year. The record, March 26, was set in 1988; it is the only time in Phoenix’s history that the weather service recorded a 100-degree day there in March.

“It appears that a new earliest instance of 100 degree will be set as it is a matter of when, not if, it will happen,” the forecasters said.

An extreme heat watch will be in effect in Phoenix from Thursday to Sunday.

What Else to Know About the Heat

Meteorologists are using the term “heat dome” to describe the cause of the hot weather.

The term describes an area of high pressure that’s high in the atmosphere and building over California and the Southwest. Air descends to the ground and heats up, and the sinking air acts like a lid, trapping warmer air close to the ground.

While tying a single heat wave to climate change requires analysis, scientists have no doubt that heat waves around the world are becoming hotter, more frequent and longer-lasting. As humans continue adding greenhouse gases to the atmosphere by burning fossil fuels, record-breaking heat will become even more common, scientists say, as will extreme weather events like droughts, wildfires and floods.

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This article originally appeared in The New York Times.

By Erin McCann
c. 2026 The New York Times Company