An intense and expanding heat wave is baking the South and Southwest, pushing the mercury to record-challenging levels in areas notorious for their searing summer temperatures. The stifling heat is creating an additional public health challenge in a region where coronavirus cases are also spiking.

On Monday, heat warnings and advisories were in effect for at least 11 states from Southern California to the Florida Panhandle.

These include all of Louisiana, nearly the entire state of Texas and a portion of Arizona. Each of these states have seen dramatic surges in coronavirus cases in recent weeks, and the heat is complicating the effort to contain the illness by making drive-through testing centers less feasible and challenging cities’ heat wave response plans that rely in part on indoor cooling centers, where the virus could spread.

Perhaps more noteworthy than the heat is the expanse of real estate the anomalous temperatures cover, which makes this heat wave stand out from past events. Temperatures are already breaking records, and the National Weather Service’s Climate Prediction Center is expecting the excessive heat to continue for at least another two weeks, with above-average temperatures dominating much of the country into August.

In addition to prolonged elevated temperatures, the heat is likely to contribute to increased drying of vegetation in California and other fire-prone states.

Off to a sweltering start

The heat is roasting the South, where temperatures are exceptionally hot even for an area that’s used to summertime heat and humidity during July.

New Orleans Lakefront Airport saw a stretch of 40 hours over the weekend with heat indexes in the triple digits, with values topping 110 degrees for 10 hours. Afternoon heat indexes, which measure what the temperature feels like to the human body when the air temperature is combined with humidity levels, were expected to hit 118 around New Orleans on Monday.

New Orleans has managed five consecutive days with highs surpassing 95 degrees, hitting 99 on Saturday and 98 on Sunday. A high of 96 degrees is forecast Monday, with the mid-90s likely through at least the start of next week.

Miami tied or broke a record high every day from Tuesday through Friday in the past week, hitting 98 degrees Thursday. The city had also hit 98 degrees the week before. Miami has only seen temperatures of 98 degrees or greater a total of 15 days since 1895. Five have occurred in the past five summers.

Just a few weeks ago, Miami saw its hottest week recorded.

In general, heat waves are one of the clearest manifestations of long-term human-caused climate change, with numerous studies showing that such events are becoming more likely to occur and more severe. A study published in 2019, for example, found that extraordinary heat events that took place around the Northern Hemisphere that spring and summer “would not have occurred without human-induced climate change.”

When the heat becomes downright hazardous

In many places, the heat has been inescapable even in the dead of night, which makes this heat wave especially dangerous. Sunday morning was the first time in a week that the overnight temperature at Houston’s Hobby Airport had dipped below 80 degrees.

Heat is the No. 1 weather-related killer in the United States, according to some analyses, and heat illness is more likely to occur when people cannot cool off at night. Temperatures that fail to drop below 80 don’t provide much relief for those who lack air conditioning in particular, or those who work outdoors.

The health risks of the ongoing heat are underscored by the calendar. Monday is the 25th anniversary of the height of the 1995 Chicago heat wave, which killed about 750 and caused cities worldwide to rethink their extreme heat response plans to make sure vulnerable residents have access to air-conditioned areas.

In the Desert Southwest, the heat has been even more extreme — albeit without the sultry humidity present in Louisiana and Houston. In Phoenix, each day since July 2 has hit 110 degrees or greater, with overnight lows failing to drop below 90 since Wednesday. The city’s air temperature stood at 97 degrees as of 5 a.m. Monday morning, and Tuesday morning’s low isn’t forecast to drop below 94.

The low temperature in Tucson on Monday morning stood at 89 degrees, which if it holds through the end of the day, would tie for the city’s all-time warmest low temperature on record.

In Death Valley, Calif., one of the hottest places on Earth, the low temperature was a sizzling 100 degrees Saturday night. Sunday’s 128 degrees is the highest reliably measured temperature recorded anywhere on Earth this year.

Coronavirus complications

The exceptional heat is overlapping with soaring coronavirus cases in already hot states, including Arizona, Texas and Louisiana. This causes the heat wave to serve as a threat multiplier, since it amplifies heat risks for vulnerable populations, particularly older people, those with preexisting health conditions, and those who are living in urban areas and poorer neighborhoods that lack air conditioning.

There is an overlap between those who are the most vulnerable to heat illnesses and those who are most at risk for covid-19 complications.

Some locations operating outdoor, drive-through testing sites have had to alter their hours as temperatures skyrocket, while also making changes to their cooling centers to avoid spreading the virus while attempting to help people avoid heat-related illnesses. On Friday, heat indexes approaching 110 degrees resulted in the early shuttering of testing locations in Harris County, which includes Houston. Officials urged residents to take heat and coronavirus-related precautions simultaneously.

“Libraries, shopping malls and community centers can provide a cool place to take a break from the heat,” the Harris County Office of Emergency Management and Homeland Security tweeted. “Remember to wear your face covering and social distance from others when cooling off.”

Texas and Arizona are already dealing with long lines at testing sites and delays in getting test results. According to Washington Post data, as of Sunday, Texas had reported the second-largest number of new coronavirus cases nationwide in the past seven days, with 63,419 cases, behind only Florida. Arizona came in at No. 4 on that list, with 24,378 new cases during the same period.

“Exposure to extreme heat can cause a variety of health problems, including heat stroke and death,” wrote the Centers for Disease Control and Prevention in a guide to the novel coronavirus and cooling shelters on its website. “However, the use of cooling centers can result in congregating of groups of at-risk people, such as older adults or those with respiratory diseases, and potentially provide a route for … transmission.”

The CDC has urged local officials to house cooling shelters in buildings with “high ventilation capacity” and/or tall ceilings to maximize air flow and obviate the potential buildup of airborne contagions. Local officials have also been encouraged to provide face coverings to visitors to shelters and ensure a minimum social distancing of six feet be met.

A guide from Global Heat Health Information Network noted hot weather could complicate covid-19 responses by increasing patient load at care centers, but that fear of contracting the coronavirus might discourage people who have heat-related illnesses from obtaining care.

Heat forecast to expand east as jet stream retreats into Canada

The heat sprawled over the Southwest and the South is predicted to slowly expand north and eastward over the next week, evolving into a national event caused by a strong and expanding dome of high pressure.

That ridge of high pressure, also known as a heat dome, comes during what’s already the climatological peak of summertime temperatures. At this time of year, even an additional 5 to 10 degrees above average temperatures can be brutal.

Dallas and Oklahoma City will be near 100 degrees this week as the core of the hot air begins to spill north, with much of Kansas reaching 100 by late in the week. Kansas City could approach the century mark by this weekend — as could Lincoln, Neb. — as the epicenter of heat parks itself over the Plains.

That heat will pour over the Appalachians into the Mid-Atlantic and Southeast, with highs well into the 90s forecast through early next week in Washington.

Jason Samenow contributed to this report.