Children cool off in the fountain inside of Washington Square Park on a hot summer day in New York July 12, 2011. 
 
 CHICAGO (Reuters) - A massive heat wave is expected to develop over much of the central and eastern United States beginning on Friday with heat index values that could reach 115 degrees. 
 
 The big story for the coming weekend will be the building heat," said Jim Keeney, a National Weather Service meteorologist. "It looks like it's going to be a long-term heat wave." 
 
 In the thick of the heat wave is Oklahoma where Governor Mary Fallin asked Oklahomans to pray for rain this Sunday. 
 
 "The power of prayer is a wonderful thing, and I would ask every Oklahoman to look to a greater power this weekend and ask for rain," Fallin said in a news release on Thursday. 
 
 Fallin on Thursday also issued a ban on outdoor burning for the western half of the state because of the extreme drought conditions. 
 
 "The number of wildfires we have had over the last few months is extremely tough on our state firefighters," Fallin said. 
 
 "It's a drain on their resources as well as a physical drain. Anything that can be done to minimize fires will help to keep both our firefighters and the public safe. I'm asking all Oklahomans to be vigilant and to do their part in preventing fires." 
 
 Earlier this year Texas Governor Rick Perry asked Texans to pray for rain because of the drought there. 
 
 As of July 12, about 72 percent of Texas and 43 percent of Oklahoma are experiencing the worst possible drought conditions, according to the U.S. Drought Monitor. 
 
 On Thursday, 10 states were under heat advisories or watches, down from earlier in the week when 24 states were on the list. 
 
 "The worst states are Texas, Oklahoma and Louisiana," said Victor Murphy, a weather service meteorologist, who said Texas is seeing the warmest summer on record so far. 
 
 Operations at the Denver International Airport were returning to normal Thursday afternoon after hail and rain pelted the area Wednesday night during a severe thunderstorm, leaving 1,000 passengers stranded at the airport overnight. 
 
 Among those passengers was Wisconsin Governor Scott Walker, traveling to a National Governors Association meeting in Salt Lake City. 
 
 Walker tweeted, "Got stuck in Denver airport & slept on the floor (did get a pillow & blanket)." 
 
 Most airlines reported significant hail damage, according to Laura Coale, a spokeswoman for the airport, who said there were reports of damage to 40 planes. 
 
 The Denver area will be under a flash flood watch Thursday afternoon and through the evening. 
 
 "We are expecting some heavy rains, and there could be some hail with the storms," said Byron Louis, program manager for the weather service in Denver. 
 
 (Additional reporting by Keith Coffman and Steve Olafson; Editing by Jerry Norton) ||||| A man suns himself in McCarren Park on a hot summer day in New York July 12, 2011. 
 
 CHICAGO An oppressive and potentially deadly summertime mix of sizzling temperatures and high humidity baked a large swath of the United States on Sunday, pushing afternoon heat indexes in dozens of cities to dangerous levels. 
 
 Forecasters warned the heatwave would persist through much of the coming week and cautioned residents in more than three dozen states to take extra precautions. 
 
 The National Weather Service posted excessive heat warnings for much of the nation's midsection, including Illinois, Wisconsin, Minnesota, Iowa, Missouri, Kansas, Nebraska, Oklahoma, as well as South and North Dakota, where forecasters predicted heat indexes could hit 115 degrees Fahrenheit. 
 
 "This will likely be the most significant heat wave the region has experienced in at least the last five years," the weather service said. 
 
 Cities especially hard hit by the heat included Rapid City, South Dakota; Springfield, Illinois; and Minneapolis, Minnesota, where AccuWeather.com meteorologists were predicting long-standing high-temperature records would fall this week. 
 
 Kristina Pydynowski, a senior meteorologist at AccuWeather, said the heatwave would likely affect more than 40 states. 
 
 All the states will see temperatures of 90 degrees Fahrenheit or higher, she said, and "a large number of them will bake above 100 degrees for days on end." 
 
 The scorching weather is the latest in a series of meteorological problems to best the Midwest in recent months. 
 
 The list includes the devastating tornado that ripped through Joplin, Missouri, in late May, killing nearly 160 people and destroying more than 8,000 homes and other structures, as well as the ongoing flooding along the Missouri River, which has triggered weeks of evacuations and other emergency measures from Montana through Missouri. 
 
 While the heat wave is currently focused on the High Plains and Mississippi Valley, it is expected to press east by the middle of the week, the weather service said. 
 
 In Chicago, where high heat and humidity warnings were twinned with an alert for poor air quality, temperatures were expected to hit 95 degrees by afternoon, creating heat indexes as high as 105. 
 
 In Minnesota, the heat wave was expected to continue through Wednesday with possible thunderstorms in some parts. Highs in the Minneapolis-St. Paul area could reach 94 degrees F on Sunday, and 97 degrees from Monday through Wednesday. 
 
 The weather service is projecting possibly six consecutive days of temperatures at 90 degrees or higher in the Twin Cities, the longest stretch to far this year, but short of records, meteorologist Jim Richardson said. 
 
 "Basically, (Sunday) through Wednesday looks to be the warmest regime and humidity levels up there as well," Richardson said. 
 
 By mid week many locations on the East Coast will have heat index values approaching or exceeding 100 degrees, including Washington, D.C., the weather service said. 
 
 In St. Louis, temperatures are expected to reach 96 degrees on Sunday, 98 on Monday and touch the 100-degree mark on Tuesday. The heat index values are expected to range from 105 to 115 degrees. 
 
 In the Kansas City area, heat index values are expected to range from 105 to 110 degrees on Sunday and then average about 110 degrees through Friday with oppressive heat and humidity. (Reporting by James B. Kelleher and David Bailey; Editing by Maureen Bavdek)
– Another massive heat wave is poised to engulf much of the United States beginning today, threatening to send the heat index up to 115 degrees, reports Reuters. "It looks like it's going to be a long-term heat wave," says a meteorologist at the National Weather Service. A ridge of oppressive, triple-digit heat is expected to stretch from Mexico to the Canadian border, peaking on Sunday. Just how bad is it? Oklahoma Governor Mary Fallin banned outdoor burnings in the western half of the state—and called for people to pray for rain in church on Sunday, notes the Oklahoman. "The power of prayer is a wonderful thing, and I would ask every Oklahoman to look to a greater power this weekend and ask for rain," she said.