A dangerous heat wave is impacting a large swath of the United States, with the Southern Plains, Midwest, and Mid/Lower Mississippi Valley experiencing particularly oppressive conditions. Heat Advisories are in effect for much of the region as heat index values are expected to soar into the 105-110 degree Fahrenheit range. Adding to the discomfort, overnight low temperatures will only dip into the mid-to-upper 70s, offering minimal relief from the daytime heat.