Publication Date July 18, 2013 | Los Angeles Times

'Heat dome' grips two-thirds of the U.S.

United States
Nine-year-old William Williams, right, and his brother Xavier Robertson, 10, wait for the Big Splash machine at the Como Town water park to dump water on them in St. Paul, Minn. Temperatures hit 92 degrees in St. Paul on Wednesday, with a heat index of 97 degrees. Photo: John Autey / Associated Press
Nine-year-old William Williams, right, and his brother Xavier Robertson, 10, wait for the Big Splash machine at the Como Town water park to dump water on them in St. Paul, Minn. Temperatures hit 92 degrees in St. Paul on Wednesday, with a heat index of 97 degrees. Photo: John Autey / Associated Press

Cooler temperatures are approaching, but the year’s worst heat wave continues to punish areas from South Dakota to coastal Massachusetts on Thursday, prolonging withering conditions that have already claimed one man’s life in Kentucky.

Officials warn that persistent overnight heat, high humidity and intense sunshine threaten areas of the country, especially the Midwest, that aren’t accustomed to near-tropical conditions. Excessive heat still blankets parts of 21 states and the District of Columbia, with temperatures reaching into the 100s when accounting for humidity.

“It’s oppressive and dangerous for people exposed to this condition,” warned Chris Vaccaro, spokesman for the National Weather Service headquarters in Silver Spring, Md. He said only a Miami resident vacationing in Minneapolis would be accustomed to the muggy conditions in the Midwest.

“This is the most significant heat wave of the year,” Vaccaro said.

One man, a 78-year-old Alzheimer’s patient, died of heat exhaustion Tuesday after he wandered away from his northern Kentucky home, Mason County Deputy Coroner David Lawrence told the Los Angeles Times.

In Indianapolis, where temperatures reached 95 degrees Thursday, 300 people living in a senior community were evacuated after an electrical outage knocked out their air conditioning.

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The heat wave comes on the heels of June temperatures that a National Oceanic and Atmospheric Administration report released Thursday shows were the month’s fifth highest on record.

Dr. Jessica Blundon, one of the report’s authors, said the June temperatures are “consistent with what we expect to see in a warming world.”

She also said the near-record June heat created dry conditions in the Western U.S. that set the stage for devastating forest fires