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100 million in U.S. under alerts as heat expands coast to coast

Washington, D.C., in the midst of its first heat wave of the year, could enter rare territory this weekend with a forecast of 100 degrees.

Dangerous heat will continue to grip much of the country Wednesday as cities report record-breaking temperatures from coast to coast.

Dallas, Houston, San Antonio and Austin, Texas, have experienced more days of 100 degrees Fahrenheit this year than they typically do over an entire summer. The number of 100-degree days to date is ranked in the top five for these locations with more expected into next week.

Oklahoma City hit 110 degrees for the first time in a decade Tuesday, while the entire state registered temperatures of 103 or higher by the afternoon.

Farther south, Dallas tallied its 24th 100-degree day of the year, with six of them soaring above the 105-degree mark.

The next city to watch is Washington, D.C., where 100 degrees is forecast for Sunday. The last time the nation's capital hit the mark was in 2016.

A total of 100 million people in the United States were under heat alerts Wednesday, touching parts of 28 states from California to the Southwest deserts, up to the Plains and into New England. The U.S. population that will experience highs above 90 degrees is expected to reach 219 million.

Areas across the Mississippi and Tennessee valleys, including Memphis, are expected to reach highs near 100 degrees, which combined with high humidity will lead to heat index values of 112 to 115 degrees.

Across the Northeast and New England, highs in the 90s combined with high humidity will lead to heat index values of 95 to 105 degrees Wednesday and Thursday.

Both Washington, D.C., and New York City could experience heat indexes as high as 102 degrees, particularly on Thursday.

Image: Large Swathes Of U.S. Under Excessive Heat Warnings
Latisha Allen, 29, holds Kario, her son, at the Emancipation Swimming Pool on Tuesday in Houston.Brandon Bell / Getty Images

The human body is not equipped to handle such high temperatures, especially over a prolonged period, without mechanisms to stay cool.

Meteorologists warned people to look out for signs of heat illness, including headaches, dizziness, nausea, vomiting and fatigue.

Prolonged exposure at 80 degrees can cause fatigue and dehydration. At 90 degrees, heat cramps and heat exhaustion are possible. Above 100 degrees, heat exhaustion becomes increasingly likely with heat stroke possible. Finally, with prolonged exposure to 115 to 125 degrees, heat stroke, the most life-threatening level of heat-related illness, becomes highly possible.

The climate connection

The warming planet is creating more intense, frequent and longer-duration heat waves, raising the bar for baseline temperatures. The higher starting points result in higher temperatures compared to 50 years ago.

According to Climate Central, 74% of the country has experienced an increase in extreme heat days each year since 1970. Extreme heat days are defined as the number of days above 85, 90, 95, 100 or 110 degrees based on location.

The average number of 100-degree days has also increased since 1970. Dallas, for example, experiences an average of 22 100-degree days, compared to 15 in 1970. Houston sees an average of eight such days, compared to just two in 1970.

Summer nighttime temperatures are warming nearly twice as fast as daytime highs. Since 1970, average summer nights have warmed 2.5 degrees across the continental U.S.