Visualization of heat and humidity Friday. (Earth.nullschool.net) A stifling heat wave has begun to take shape across large portions of the United States, with millions likely to see temperatures creep toward the century mark, along with even higher heat indexes by this weekend. The heat wave is already generating excessive heat watches in the central United States, and by Wednesday the national weather map is likely to feature a blanket of heat advisories from the National Weather Service. The combination of sultry dew points and scorching air temperatures approaching will help make this event a dangerous one from a public health perspective. Cities including Chicago, St. Louis, Washington, Philadelphia, New York, Nashville and Kansas City, Mo., are likely to see at least three days with temperatures between 95 degrees and 100 degrees, along with dew points — a measure of the amount of moisture in the air — above […]