Sweltering heat to bake Southwest, California before South, Southeast

Although it has been a dry heat in places including Arizona, Nevada and Southern California, moisture from the Gulf of Mexico will be drawn northward in the central and eastern United States. The resulting combination of heat and humidity will produce conditions that feel well over 105 degrees for an extended period next week, contributing to dangerous heat stress for vulnerable groups, including older adults and the homeless.

Records have already fallen in a number of major cities, and more could be in jeopardy.

More than 30 million residents from the Desert Southwest through California’s Central Valley are under heat advisories and warnings into the weekend. Similar advisories were posted in south-central Texas, with a special bulletin issued for the Houston-Galveston metro area to emphasize the risk of heat-related illnesses.

Phoenix and Las Vegas are under excessive-heat warnings through Sunday, with temperatures expected to challenge records.

“In general, we typically see the first excessive-heat warning in early to mid-June, so that in and of itself isn’t incredibly abnormal, but temperatures will be approaching records,” said Jenn Varian, a meteorologist at the National Weather Service office in Las Vegas.

Record highs are forecast for Las Vegas — 108 is the number to beat Friday, and the forecast calls for a high around 110. That record has stood for 25 years. The average high in Las Vegas this time of year is about 100 degrees.

On Thursday, the city hit 109 degrees, just a touch below the 111-degree record observed in 1985.

“The excessive heat is here. No changes in forecast thinking,” the Weather Service office in Las Vegas wrote in its online forecast discussion. “Temperatures will remain elevated to dangerous levels.”

The heat that is affecting Las Vegas covers most of the Southwest, encompassing southern and western Arizona; most of Southern California, including the Inland Empire and deserts; and California’s heavily populated San Joaquin Valley.

Phoenix, a city of nearly 1.7 million, is forecast to hit 112 on Friday, 114 on Saturday and 113 degrees on Sunday. That should break records on Saturday. It hit 110 degrees for the first time this year on Wednesday, and made it to 109 on Thursday.

“High to Very High Heat Risk will be prevalent across the area through Sunday,” wrote the Weather Service in Phoenix, where an excessive-heat warning is in effect through Sunday.

The Arizona Department of Environmental Quality also issued an air-quality alert, which signifies that ground-level ozone might reach dangerous levels. Near-surface ozone production is catalyzed by excessive heat and can cause breathing difficulties. The agency is urging those who use gasoline-powered equipment to hold off until late in the day.

In Las Vegas and Phoenix, along with scores of other communities across the Southwest, the biggest concern isn’t daytime highs topping triple digits — it’s the exceptionally warm nighttime lows, which may not dip below the lower 80s in many locales.

“Overnight lows are the worst part of it in general,” Varian said. “If we just have highs approaching records and it cools off at night, we probably wouldn’t issue an excessive-heat warning. But for people who are homeless, maybe don’t have housing, or who are trying to save a dime with air conditioning, their bodies can’t cool down at night. That’s when the impacts start.”

Arizona’s Maricopa County has opened dozens of cooling shelters across the Phoenix metro area, although the vast majority are open only during the daytime. A website shared by the county allows residents to search for the nearest location using their address.

In Las Vegas, the Salvation Army received funds from Clark County to reopen the daytime cooling shelter that was closed in 2021 because of the coronavirus pandemic. A couple of other cooling shelters also were opening.

Excessive-heat warnings are in effect through this evening for northern parts of the Central Valley, where readings in the valleys and foothills should range between 100 and 107 degrees Friday. Alerts have been extended through Saturday, which should mark the final day of the excessive heat.

“In addition to hot temperatures during the daytime, there will be little overnight relief from the heat,” the Weather Service in Hanford, Calif., warned.

Sacramento hit 99 degrees Thursday afternoon but is expected to make it to 105 on Friday and 100 on Saturday before returning to the upper 80s by Sunday. That could tie a record on Friday that was set in 1985.

Heat advisories span the remainder of the Central Valley southward to where the excessive-heat warning begins in Southern California, but the advisories also reach west toward the Bay Area.

San Francisco’s downtown isn’t under any type of alert, but the high of 81 degrees expected on Friday is about nine degrees above average.

In Death Valley, the high should reach 122 degrees Friday and Saturday — establishing daily records, with overnight lows at the Furnace Creek Visitor Center in the lower 90s. Nearby Needles should be around 114 degrees Friday and Sunday and 117 on Saturday, just a hair shy of records.

Tens of millions in the Lone Star State are set to deal with toasty temperatures, too. Unlike in counterparts to the west, however, the heat that will grip Southeast Texas, including Houston and Galveston and the Interstate 10 corridor, isn’t a dry heat.

“The combination of near record high temperatures and high dew points will produce increasingly dangerous heat index values between 100 and 106 degrees today,” the Weather Service in Houston wrote in a special weather statement. The office is projecting heat indexes nearing 108 on Saturday and Sunday, which may lead it to issue a heat advisory. That is the threshold for a heat advisory.

“Heat-related illnesses and deaths are preventable, and all heat safety precautions should be taken even if there is no Heat Advisory in effect!” it wrote.

Houston could nick 100 degrees any day through Sunday before simmering back into the mid-90s.

There is a chance that the heat could be tempered a degree or two by a thin veil of Saharan dust expected to move overhead this weekend.

Farther north, Dallas should sit around 103 degrees on Saturday and Sunday before settling back closer to 100. With the heat dome lingering overhead, it is probable that the Metroplex will remain elevated over 100 into the middle of next week.

Austin will be between 100 and 105 each day, as will San Antonio, and no immediate relief is in sight. Austin hit 101 degrees Thursday, tying a record last hit in 2008.

The heat is all due to a dome of high pressure that is bringing clear skies and hot, dry, sinking air. The jet stream is diverting north of the high, carrying all inclement weather and storminess with it. That allows for copious sunshine, making it possible for sunlight to pour down and heat the ground unimpeded.

That heat dome will be centered over the Four Corners region on Saturday but should shift east over the Plains on Sunday and reach the eastern United States by Tuesday. Thereafter, it could linger, bringing widespread highs 10 to 15 degrees above normal.

It is likely to usher in what will be the first major heat wave of the season east of the Mississippi River. A slight dip in the jet stream over the Northeast may block the scorching temperatures from reaching the zone roughly north of D.C. However, this area — bridging the periphery of the heat and cooler to the north — may be vulnerable to strong-to-severe thunderstorms

Human-caused climate change is supercharging heat waves like this one, making them more intense and long-lived.