US Heat Dome: Sizzling

A heat dome is searing down across the central US, focusing on the Great Plains, Midwest and Mississippi River Valley, including Memphis and St. Louis, driving up demand across regional power grids and leaving more than 100 million people sweltering.

Separately, in the Pacific, Hurricane Fausto is forecast to lose strength, passing north of the Hawaiian Islands as a tropical storm on Tuesday and Wednesday, while Hurricane Genevieve is intensifying off Mexico's Baja California coast.

Extreme heat warnings and advisories cover the central US from the North Dakota-Canada border south to Dallas. At least 20 daily high temperature records are set to be challenged or broken across the region on Sunday, with 33 warm overnight lows threatened as well, the US Weather Prediction Center said.

The heat and humidity will make temperatures feel as scorching as 112F (44C) as the heat index rises across large parts of the central US, the National Weather Service said. In St. Louis, where Sunday's high is forecast to hit 97F, the moisture in the atmosphere could create a feels-like temperature closer to 116F, and in Dallas a high of 102F is expected, with a heat index as high as 109F.

“A big heat dome is centered over the Rocky Mountains and Plains that's going to bring record breaking temperatures,” said Alex DaSilva, a meteorologist with the commercial forecaster AccuWeather Inc. “Triple-digit heat is stretching from the Rockies to the Plains. There's some dangerous heat there.”

At least 105 million people across the US are under heat advisories or extreme heat warnings for Sunday, including in Southern California, the Weather Prediction Center said. As the weather sends people fleeing to air conditioners for relief, the Southwest Power Pool and the Midcontinent Independent System Operator, which together cover at least parts of 20 states, have said that loads may rise. Stagnant air under the large high-pressure system may also crimp wind-generated power at peak times.

High temperatures can hurt some crops but benefit big retailers as sales of air conditioners and fans jump.

On top of the sweltering temperatures there's a chance for severe thunderstorms, wind and hail at the edges, DaSilva said. Saturday brought 89 filtered reports of high winds and two of large hailstorms, mainly in Montana and South Dakota, the US Storm Prediction Center said.

There's a level two risk of dangerous wind and thunderstorms across the upper Midwest again for Sunday, focused mainly on Minnesota and Wisconsin, the agency said.

“Any time you get these areas of high pressure, you have to watch the periphery as well,” DaSilva said. “There will be multiple chances to see severe weather.”

The heat dome will continue to raise temperatures across the western and central US this week before the focus shifts to the west by next weekend, said Bob Oravec, a senior branch forecaster at the Weather Prediction Center.

Heat across the Southwest and California is likely to last into the beginning of August, he said.

In the Pacific, the main impact from Fausto is likely to be rough surf and rip currents to the northern facing sides of the Hawaiian islands, said AccuWeather's DaSilva. “Overall, I don't think this will be a big deal for them.”

As Fausto fades, Hurricane Genevieve is intensifying off Mexico's southwest coast, potentially on its way to becoming a Category 4 major storm, bringing rough seas to the southern parts of the Baja California peninsula, the National Hurricane Center said.

Genevieve is the eastern Pacific's seventh storm, a mark typically reached by Aug. 11, showing the season that began on May 15 is slightly ahead of its normal pace.