The Southwest is baking. High temperatures at least 15 degrees above average have scorched the zone from Southern California to parts of Texas for days. In the desert, temperatures soared past the century mark and flirted with April records last week, and are predicted to challenge more records in the coming days.

The heat, which has affected millions, doesn’t look likely to relent any time soon, probably persisting until a pattern change in the middle of the month.

In Phoenix, the last five days of April all climbed above 100 degrees — setting a record April 26 and coming in second place for each of the following four days. The first day of May also reached the triple digits, while the past three days have reached the upper 90s.

Tucson has been on fire, too. The heat wave has brought the city four 100-degree readings, which have set three daily high-temperature records.

Las Vegas is bracing for hazardous heat by the weekend, with temperatures anticipated to nick 100 on Saturday. Last week the city hit 99 degrees, tying the hottest temperature ever recorded during the month of April. Its low of 79 degrees April 30 was the warmest low ever recorded during the month.

Temperatures in Death Valley could spike past 110 on Friday and Saturday. This desert hot spot, known for attaining some of the most extreme temperatures on the planet, posted highs of 110 and 112 degrees on April 28 and 29, respectively. Its low temperature of 90 degrees April 30 was the warmest ever recorded during April.

The heat has even managed to make it to the coast in Southern California. Heat advisories blanket the map south and east of Los Angeles, while the city proper will flirt with 90 degrees Wednesday and Thursday. Last week, the hot weather sent Californians flocking to local beaches amid the coronavirus crisis, much to the dismay of Gov. Gavin Newsom (D), who ordered them closed in response. However, some of the beaches are now being reopened.

The heat wave stretched east to Texas, where Lubbock has seen its hottest start to May on record, with highs of 101, 100, 96 and 99, in its first four days. Midland set record highs on each of the first four days of the month.

Looking ahead, the National Weather Service is predicting more than a dozen record highs Thursday across the Southwest, including in Tucson and Phoenix:

A long-duration heat event

It’s not just the intensity of the heat that’s unusual — the duration has also been remarkable. The average high in Phoenix for May 1 is 90 degrees, a figure that has been eclipsed by nearly 10 or more degrees for the past week and a half. Another excessive-heat warning is in effect through Thursday, when temperatures are expected to exceed 105 degrees.

“This is an exceptionally hot airmass for this time of year, and will very likely lead to many new daily records,” wrote the National Weather Service in Phoenix in their online forecast discussion Tuesday. “Forecast high temperatures in Phoenix on Wednesday and Thursday are currently 108 and 107, respectively. Statistically the chances of reaching 107 or greater this time of year in Phoenix is less than 1 percent.”

The city’s average first 100-degree day tends to be around May 4, but it’s well ahead of schedule to be getting into the hundreds routinely.

“I think probably the most remarkable thing is the duration of this heat,” said Marvin Perche, a meteorologist at the National Weather Service in Phoenix. “We’re actually expecting temperatures to be over 100 all the way through at least next Monday. You’re looking at two weeks’ worth of temperatures at 100 or greater. That’s quite unusual for Phoenix this time of year.”

A distant end in sight

The hot weather is the result of a stubborn zone of high pressure that has remained sprawled over the Southwest for more than a week. The high-pressure zone has formed as the jet stream has lifted north toward Alaska but taken a big dive in the eastern United States, where temperatures have been abnormally cool.

The jet stream pattern is predicted to begin reversing by mid-May, allowing the stifling heat to ease.

However, any brief lull in the heat will eventually succumb to the calendar as Phoenix heads into summer. By that point, its average high temperature exceeds 100 degree for more than three months.

Climate connection

Because of the long-term warming trend in the Southwest, which scientists attribute to the buildup of greenhouse gases in the atmosphere, Perche says the odds of this kind of long, intense heat wave have increased.

“As temperatures do warm, you kind of load the dice to make it more likely for these events to occur,” he said. “It’s kind of like having a six-sided dice, and putting two ones on there. You’re more likely to roll a one, but you’re not necessarily going to roll it every time.”

That trend has already manifested itself in an expanding season of 100-degree days. In 1950, it was typical for Phoenix to see its first 100-degree reading around mid-May. Now, the average first date is in late April.

The same thing is happening on the back end of the season, where the average last 100-degree date falls in early October, about 10 days later than it did in 1950.

That has led to an increase in the number of 100-degree days during meteorological spring — from March through May. The average has jumped from six days to 11 since 1950. Average spring temperatures have warmed a little over four degrees in the interim.

Phoenix now sees an average of 15 more days that hit 100 degrees per year than it did 70 years ago.