The National Weather Service forecast excessive heat from Friday morning through Monday night, with temperatures topping 100 degrees across inland and mountain areas, the Los Angeles Times reported Wednesday
NWS issued excessive heat warnings or watches from San Luis Obispo to San Diego counties.
The weather service urged people to schedule strenuous activities for the early morning or evening hours and, to prepare shady areas for animals and people working outdoors.
The potentially historic heat wave also could affect parts of Nevada and Arizona, Climate Control said. Several cities in California, Nevada and Arizona already known for their high heat could see all-time record highs in danger.
Weather officials said the blast-furnace heat is courtesy of a "stuck" weather pattern setting up across the United States and Canada.
Meanwhile, parts of the Midwest and Northeast will experience rain, floods and high winds, through Thursday, meteorologists said.
Up to 100 million people in cities from Chicago to New York could be affected by flash floods, wind gusts of up to 70 mph, giant hail and the possibility of tornadoes, Accuweather.com reported.
Cities at greatest risk include New York, Boston, Philadelphia, Pittsburgh, Baltimore, Chicago, Nashville, Indianapolis and St. Louis.
In Chicago, heavy rains forced flight cancellations at O'Hare Airport, halted trains and flooded streets, WLS-TV in Chicago reported.
Weather.com said Madison, Wis. picked up nearly 3 inches of rain in three hours Wednesday, while slightly more than 4 inches of rain was recorded in three hours in Wheeling, Ill.
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