Extreme heat is scorching much of the eastern United States, and it's not expected to let up anytime soon.

Experts at the National Oceanic and Atmospheric Administration's Climate Prediction Center say much of the southern half of the country will be hotter than normal in August, with the worst conditions in Texas, Louisiana and parts of Mississippi and Arkansas.

"We have quite a few records being set," said Deke Arndt, chief of the climate modeling branch of the National Oceanic and Atmospheric Administration's National Climatic Data Center. "This is a very large heat wave."

Most damaging, federal forecasters said yesterday, are rising night temperatures.

During the first three weeks of July, 12 weather stations have recorded all-time daytime highs. But 93 weather stations have seen their all-time warmest nighttime temperatures, Arndt said.

In other words, the coolest parts of the day are getting warmer, making it harder for people -- especially those without air conditioning -- to recover from daytime heat exposure.

Studies suggest that kind of prolonged exposure to high temperatures increases the risk of death for vulnerable groups like the elderly, said Rebecca Noe, an epidemiologist at the Centers for Disease Control and Prevention. The agency's study of a 2005 heat wave in Maricopa County, Ariz., found that elderly people were more likely to die indoors, while those who died outside were more likely to be younger, working in outdoor occupations or homeless.

"Heat is a 'silent killer,'" Noe said. "The danger is not well-recognized, and people can be caught unprepared for heat waves."

The shift to hotter nights has been developing for some time, Arndt said yesterday.

"This trend to have even more dramatic numbers of overnight lower temperatures being exceedingly warm is consistent with what we have seen in recent decades," he said.

Hot days, high humidity, little relief at night

That was one conclusion of a 2009 study by researchers at the National Center for Atmospheric Research and NOAA, who found that nights are getting warmer. Their study, published in the journal Geophysical Research Letters, also found evidence that climate change is skewing the proportion of record high temperatures to record low temperatures in the continental United States, with extremely hot days now outnumbering extremely cold days by 2-to-1.

Meanwhile, the current heat wave's scorching temperatures are exacerbated by high humidity, which makes it harder for the body to cool down by sweating.

The National Weather Service is predicting highs of 103 degrees Fahrenheit in Washington, D.C., and Baltimore, with muggy conditions making it feel even hotter -- the equivalent of 115 degrees.

Conditions will be slightly cooler in New York City, where the mercury is expected to hit 99 degrees, with a heat index of 108 degrees.

The agency has placed large swaths of the Midwestern and mid-Atlantic regions, as well as the Ohio Valley, under "excessive heat warnings," which it issues when it expects a heat index of at least 105 degrees Fahrenheit for three hours a day, two days in a row, or a heat index of 115 degrees for any length of time.

The current heat wave comes after record or near-record temperatures last month in many parts of the country.

This year saw the seventh-warmest year on record, globally, while the first six months of 2011 were the 11th-warmest year-to-date on record, Arndt said.

In the United States, last month was the 26th-warmest June in 117 years of record-keeping.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500