12 dead in U.S. storms and scorching heat wave

• Storms cause about 3 million power outages from Indiana to West Virginia
• "If you don't have a good pair of boots, it'll burn clear through to your feet," roofer says
• Authorities around Kansas City are probing three deaths that may be heat-related
• Scorching temperatures are expected to stretch into next week in some areas

(CNN) -- More than 3 million homes lost power early Saturday across the Midwest as a fierce line of thunderstorms and winds pounded the region after record-setting temperatures.

The storms moved east from Indiana through Ohio and into West Virginia, according to utilities. Virginia was the most hit with power outages to about 860,000 homes.

The outages come as tens of millions in the central and eastern United States are battling a sweltering summer and stormy fury.

Temperatures Friday soared past 100 degrees Fahrenheit from Kansas to Washington, with scorching conditions expected to continue through the weekend and beyond.

"If you don't have a good pair of boots, it'll burn clear through to your feet," said roofer Zach Bruner in Evansville, Indiana, where he said the 103-degree temperatures were spiking to 130 on the job site.

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The severe thunderstorms moving across the Midwest are fueled by the record-high temperatures, bringing with them lightning and wind gusts as strong as 80 mph, according to the National Weather Service.

As storms moved east from Indiana through Ohio and into West Virginia, they left a trail of damage that included power outages.

"The storms may reach as far as the Atlantic coast by late tonight or early Saturday morning," the weather service said. "People planning outdoor activities this evening in the path of the squall line are urged to pay attention to local weather warnings and take shelter as storms approach."

The massive systems left one in three Americans baking in scorching heat and has threatened lives and doled out misery for those not fortunate enough to find a splash of cool water or air conditioning.

Heat warnings, watches or advisories Friday spanned 730,000 square miles of the central and eastern United States, an area roughly the size of Mexico, CNN meteorologist Brandon Miller said. About 100 million people were affected, he said.

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By Friday afternoon, temperatures had climbed to 100 degrees in Indianapolis; 101 in Richmond, Virginia; and 102 degrees in St. Louis, where highs were forecast to stay above 100 through Thursday, the National Weather Service said.

And it felt even hotter in some places, with the heat index topping out around 115 degrees.

The Jackson County medical examiner in western Missouri is investigating three deaths that may be related to the heat, according to the Kansas City Health Department.

"Heat-related illness such as heat exhaustion or heat stroke are a real threat," the National Weather Service said in issuing an excessive heat warning through Sunday for eastern Missouri, including St. Louis, where temperatures should soar up to 106 this weekend.

"This is especially true because of the longevity of this heat wave and the effects of extreme heat are cumulative."

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In Memphis, where highs hit 105 degrees Friday, firefighters went door to door, checking on residents. Churches and faith-based institutions were also urged to reach out to people and to ask people to check on their neighbors and relatives.

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"Please, if you know of someone who doesn't have air conditioning or who might be struggling with the heat, just stop by and see how they are doing," Mayor A.C. Wharton Jr. urged residents.

The weather service posted excessive heat warnings for 12 states, from Nebraska to New Jersey, with watches and advisories posted for at least six other states. Arizona was also under an excessive heat warning, with a predicted high Friday in the Phoenix area of 112 degrees.

Baltimore Mayor Stephanie Rawlings-Blake activated the city's emergency operations to coordinate storm recovery operations. In West Virginia, Gov. Earl Ray Tomblin declared a state of emergency for the entire state after the powerful storms.

CNN's Nick Valencia, Melissa Abbey and Jake Carpenter contributed to this report.