(Reuters) – A grueling heatwave that has gripped the U.S. Southwest for three days spread eastward to Iowa and Missouri on Thursday, while punishing the hardest-hit areas with record high temperatures that are challenging local power systems.

The National Weather Service issued excessive heat warnings for much of the Southwest, including Arizona, southern Nevada, much of California and parts of Utah. Heat advisories were issued parts of the Central Plains, including Kansas, Nebraska, Iowa and Missouri.

“It’s a pretty big impact with respect to where the record heat is,” said National Weather Service meteorologist Bob Oravec at the Weather Prediction Center in College Park, Maryland.

A high-pressure air mass parked across the Southwest has been bearing down for three days on the area where temperatures of around 100 degrees Fahrenheit (38 degrees Celsius) are not uncommon between now and September, Oravec said.

“But now the temperatures in the last several days, especially today, are going anywhere from 10 to 20 degrees above average,” he added.

The wave is set to topple numerous records on Thursday, including Phoenix with a forecast of 117 degrees Fahrenheit (47 degrees Celsius), Sacramento, California with 108 degrees (42 degrees Celsius) and Las Vegas which is expected to tie its daily record of 113 degrees (45 degrees Celsius).

California’s Death Valley National Park, typically one of the hottest places in the country, recorded 129 degrees Fahrenheit (54°C) on Wednesday.

“Up to a certain temperature it’s OK, like maybe 120, but once it gets above that is when it really gets hard,” said Willo Alford, who runs a general store in Death Valley Village and has lived there most of her life.

With air conditioners cranked up in homes and businesses, Texas and California urged consumers to conserve energy this week to reduce stress on the grid and avoid outages.

“The public’s help is essential when extreme weather or other factors beyond our control put undue stress on the electric grid,” said Elliot Mainzer, chief executive of the California ISO, which operates the grid in most of California.

The Electric Reliability Council of Texas (ERCOT), the state’s grid operator, expects Thursday’s demand to break the June record set on Monday.

Both states have previously imposed rotating or controlled outages to prevent more widespread collapses of their power systems – California during a heatwave in August 2020 and Texas during a brutal freeze in February 2021.

The heatwave and several wildfires scattered across the region, most of which were at least 50% contained, set social media abuzz with complaints.

“Mother Nature, please bring on a productive monsoon. Be gone: #heatwave #wildfires,” Monica Surfaro of Tuscan, Arizona, wrote on Twitter.

A cold front is expected to bring relief to the midsection of the country on Sunday as temperatures in the Southwest slowly moderate as well.

“The heatwave, at least the record portions of the heatwave, looks like it will be coming to them this weekend,” Oravec said.