The heat is on for much of U.S.

People in northeastern Kansas are bracing for a heatwave forecasters say will hit triple digits and expand over a large swath of the nation through the week. The Topeka Capital-Journal reports the National Weather Service issued a heat advisory for Sunday and Monday for much of the state\'s northeast. Temperatures of about 101 with high humidity levels are expected to push the heat index to 107 degrees, the newspaper said. Temperatures are to be in the 90s Tuesday through Thursday, then returning above the century mark Friday, the weather service said. Thunderstorms were expected across the northern Plains states Sunday. Accuweather.com said expanding high pressure over the middle of the country will be to blame for the coming sweltering heat that will make it unpleasant for millions and even life-threatening for some. Accuweather.com said two high pressure areas -- one off the Atlantic Coast and other over the southern Plains -- are merging into one system over the middle Mississippi Valley. With the jet stream bulging northward, it means tens of thousands of square miles of the United States will experience temperatures in the 90s for days. Temperatures could hit 100 degrees from Kansas City to Cincinnati, Accuweather says, adding urban areas will bear the brunt of it since pavement and concrete-and-brick structures give off the heat they absorb. Meanwhile, the National Hurricane Center in Miami said Saturday night Tropical Storm Calvin had dissipated into a post-tropical remnant low about 400 miles south-southwest of the southern tip of Mexico\'s Baja California. It was moving to the west-northwest at 10 mph with maximum sustained winds of 35 mph. Severe thunderstorms in the Washington, D.C., area Friday caused street flooding and may have contributed to a fatal car crash in Prince George\'s County, Md. Thousands of homes lost power for a time in the region.