From the Deep South to the Northern Plains, much of the eastern half of the nation sweltered through another day of scorching temperatures today as a heat wave that has killed at least 71 people lingered with little relief expected until the weekend.

The hot weather hit Middle America the hardest. It has been blamed for 27 deaths since July 19 in Missouri alone; Illinois reported a total of 19 fatalities, 11 of them in Chicago. Ohio had 10 deaths attributed to heat, while Arkansas, Oklahoma, Georgia, North Carolina, Pennsylvania and Maryland reported heat-related fatalities ranging from one to three each.

In St. Louis, which has counted 14 heat victims, two men in their eighties were found dead in their homes with their air conditioners turned off, authorities said. In Columbia, Mo., a woman who police believe had been sunbathing was found dead in her yard by a neighbor who noticed that the victim's radio was on.

In the Washington area, from the beginning of the year through Wednesday, there have been 28 days of 90 degree or higher temperatures recorded at Reagan National Airport. Meteorologists predict more of the same, with temperatures in the mid- to upper-90s through Tuesday. However, the National Weather Service says temperatures in Washington area have been only slightly above normal for this time of year. In the 56 days between June 1 and July 26, the average temperature at Reagan National Airport has been 78.4 degrees; the normal for that period is 77.6.

The heat wave is the result of a high pressure system that has anchored itself from the Plains states through the lower Mississippi Valley, preventing cooler weather systems in the northwest from moving across the nation.

"It's summertime in the United States, and you get systems like this," said Pat Slattery, of the National Weather Service's central region headquarters in Kansas City, Mo. "It looks like for the next day or so it's staying put."

Slattery said excessive heat advisories were issued today through Friday from Colorado and Wyoming through Illinois and Indiana, with temperature-humidity heat indices of up to 110 degrees expected for those states and parts of Missouri and western Kentucky, as well.

The nation's high temperature was 111 in Pierre, S.D., while Little Rock had the highest heat index at 118 degrees. In Bismark, N.D., temperatures reached 100 degrees for the first time in eight years.

Although temperatures fell short of reaching records in most areas, the heat wave's severity could be measured by its duration and geographic span, Slattery said. "This system covers a big area and just keeps hanging around, which makes it seem a lot worse," he said.

Joseph Schaefer, director of the National Severe Storms Forecast Center in Norman, Okla., said unstable weather systems make severe thunderstorm activity possible from Knoxville, Tenn., to Raleigh, N.C., and from lower Michigan to northeastern Ohio. Flash flood warnings were issued in parts of Arizona and in southern Utah.

However, forecasters predicted that the weekend could bring a break in the heat as a cool front from the northwest moves into the nation's midsection.

After moderating somewhat Wednesday, dangerously high temperatures returned to Chicago today, prompting the city to reopen its heat emergency command center. Authorities activated a response plan devised after as many as 700 people -- most of them elderly or poor -- died during the region's worst heat wave four years ago.

Mayor Richard M. Daley, citing the heat's "devastating impact" on the health of low-income and elderly residents, has asked President Clinton to release part of $100 million in federal funds that are available to 17 states to subsidize people who are reluctant to use air conditioning or fans because of the cost of electricity.

"No one in Chicago should be forced to choose not to use their air conditioning or fans because of their inability to pay electric bills," Daley said.

With temperatures today hovering in the high 90s and the heat index nearing 110 degrees, city officials said they hoped the lessons they learned from the 1995 heat wave would continue to keep the death toll from rising as fast as it did before the emergency plan was created. Chicago recorded temperatures of over 100 degrees for three straight days in 1995.

Scores of air-conditioned public buildings were designated today as "cooling centers" and Human Services Department workers were providing transportation to residents who could not make it to the centers on their own. Also, police officers were checking on the well-being of at-risk residents -- by telephone and in person -- and urging apartment dwellers to look in on elderly neighbors.

Kevin MacGregor, spokesman for the Chicago Fire Department, said that the major lesson learned from the 1995 heat emergency is providing temporary relief for at-risk residents.

"If the body core temperature can be cooled down, even if it's only for a couple of hours, it can make a world of difference," MacGregor said. "We learned we need to get that word to the elderly people, and also make sure that neighbors and family members check up on them frequently."

Staff writer Fern Shen in Washington contributed to this report.