Cities across the West recorded their hottest March days ever on Wednesday as a heat wave that has been baking the region since last week intensified, and forecasters warned that some of the hottest days were yet to come.
Phoenix hit 101 degrees just before 3 p.m., setting a record for the city’s hottest day in March and also the earliest point in the year that it reached 100 degrees. Palm Springs hit 105. At 94, Las Vegas, too, set a record for March.
And it’s only going to get hotter before things cool off, particularly across the southern portions of California, Nevada and Arizona.
“Through Friday, we’re going to pile on records that have already been broken this week,” said Frank Pereira, a meteorologist with the Weather Prediction Center.
Mr. Pereira and other forecasters said on Wednesday that some relief from the worst of the heat would arrive over the weekend and into early next week, but the weather was expected to stay dry and warmer than normal in most locations.
Temperatures are expected to peak on Friday.
The heat is expected to reach its highest intensity on Friday, with Palm Springs, Calif., forecast to top 108 that day and Phoenix 106.
As temperatures have climbed in recent days, California and Arizona have been getting most of the attention because they are poised to see the hottest weather. However, temperatures are well above average across the entire Southwest, including in Utah, Colorado and New Mexico.
The extreme heat is likely to ease slightly on Saturday, particularly across Northern and Central California, with San Francisco expected to be 10 degrees cooler on Saturday than on Friday. Temperatures drop a little more on Sunday.
It’s not until Monday and Tuesday when more widespread cooling is expected across Southern California and southern Arizona.
“If you’re in San Francisco, it’s going to be a lot better by Sunday,” said Brian Hurley, a meteorologist with the Weather Prediction Center. “If you’re in Phoenix, it won’t get better until Monday.”
The oppressive heat is the result of what meteorologists call a heat dome. The term describes the area of high pressure that’s trapping heat close to the ground across California and the Southwest.
The dome is expected to weaken over the weekend, allowing temperatures to drop.
Mr. Hurley said the high pressure could start to strengthen again as early as next Wednesday, but “not to the extent or the vigor as we saw this week.”
How hot has it gotten so far?
Dozens of places have shattered same-day records this week, measuring their highest temperature for a specific day. It’s not uncommon for this type of record to be set as temperatures rise and fall across the country.
More significant to meteorologists, over a dozen major weather stations have broken monthly records during this heat wave. On Tuesday, Long Beach, Calif., soared to 99 degrees, and Anaheim to 100, the highest temperatures ever recorded in March in these locations.
The duration of the heat is also notable. Downtown Los Angeles hit 90 degrees on Monday, 98 on Tuesday and was forecast to stay in the 90s through at least Saturday.
“Getting that hot for five or six days in a row in March is outrageous,” said Robbie Munroe, a meteorologist with the National Weather Service office in Oxnard, Calif. The city typically doesn’t record its first 90-degree day of the year until early May.
San Francisco was predicted to record temperatures in the 80s Monday through Friday. This may not seem hot to people living in other parts of the state or country that annually experience triple-digit heat. However, these temperatures are unusual on the San Francisco Peninsula, which is surrounded by the cool waters of the Pacific Ocean and San Francisco Bay on three sides.
San Francisco hits 80 degrees or above on average only 17 days a year, according to its climate record over the last 30 years. The city has never recorded five consecutive days with temperatures in the 80s in March.
While the record-breaking high temperatures are getting a lot of attention, Brian Garcia, a meteorologist with the Weather Service, said that the minimum temperatures were especially concerning.
“If these don’t drop below 60 degrees, it’s difficult for people to recover and makes the days ‘feel’ hotter because they will start hotter,” Mr. Garcia said.
Weather Service offices across California and the Southwest began raising the alarm about these temperatures last week, calling some of the predicted highs “unprecedented.”
Heat is generally easier to accurately forecast than other types of extreme weather, such as strong storm systems, Mr. Pereira said. Heat waves are driven by calm, often large and slow-moving areas of high pressure in the atmosphere, rather than chaotic, usually smaller and faster-moving air masses.
“Heat waves are more predictable,” Mr. Pereira said. “There are less variables.”
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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