Heat wave to expand across much of the U.S.

A stifling heat wave has begun to take shape across large portions of the U.S., with millions likely to see temperatures creep toward the century mark, along with even higher heat indices by this weekend.

The heat wave is already generating excessive heat watches in the central U.S., and by today 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 100 degrees mark will help make this event a dangerous one from a public health perspective.

Cities including Chicago, St. Louis, Kansas City, Washington, Philadelphia, New York, and Nashville are likely to see at least three days with temperatures between 95 and 100 degrees, along with dew points — a measure of the amount of moisture in the air — above 70 degrees.

This is likely to create ideal conditions for setting hot overnight low temperature records, with the possibility that some all-time overnight low temperature records might be broken.

For example, the GFS model, which is generally bullish on the hot weather, is projecting a low temperature in Washington of 86 degrees on Sunday morning. The all-time warmest low temperature for D.C. is 84 degrees.

Faster warming of overnight lows compared to the warming rate of daytime highs are consistent with model projections and observations of how global warming is reshaping our weather. In cities, urban heat-island effects also help keep overnight temperatures higher than in surrounding areas.

When overnight lows fail to drop below a particular threshold, it’s harder for the human body to cool down and rest, elevating the health threat posed by a particular heat wave.

The sauna-like conditions forecast are the result of a sprawling and intense heat dome or zone of high pressure that will swell over the eastern-two thirds of the nation beginning Wednesday. Temperatures will soar as many as 10 to 20 degrees above normal. That may not sound like a lot, but considering it is hottest time of the year, the resulting heat will challenge records in some areas.

Excessive heat watches are already in effect for the entire state of Missouri and portions of Kansas, Illinois and Iowa as heat indices are forecast to range from 100 to 110 degrees during an extended period of time, from today through Saturday.