Heat wave continues to stifle large swath of U.S.

Updated at 3:09 p.m. ET

NEW YORK Cooler temperatures are within sight but probably not soon or cool enough for a large swath of the country hit with dangerously high temperatures for days, as the largest heat wave of the summer failed to budge from South Dakota to Massachusetts.

The relief is expected to begin arriving Thursday in some regions of the country as a cold front drops south from Canada.

But that's not soon enough for places such as New York City, which was bracing for another day of temperatures in the high 90s.

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Cooler temperatures are likely to sweep through the Midwest and into the Mid-Atlantic and Northeast regions by Saturday. They might be accompanied by severe thunderstorms.

The heat wave has stagnated over large regions, bringing sizzling temperatures and little hope of relief without rain -- a growing possibility for some hard-hit areas as the weekend approaches.

Most states in the U.S. had at least one area where the temperature hit 90 degrees Wednesday, according to the National Weather Service, though the worst heat was in the Midwest to Northeast. Humid air just made it all feel worse, with heat indexes in some places over 100.

The combination of heat and humidity made it feel like the temperature was in triple digits in many big cities Wednesday: 100 degrees in New York City; 102 in Washington, D.C.; 103 in Baltimore; and 105 in Detroit.

It was hot enough to buckle highway pavement in several states. Firefighters in Indianapolis evacuated 300 people from a senior living community after a power outage knocked out the air conditioning. The state of Illinois opened cooling centers. The Environmental Protection Agency said the heat was contributing to air pollution in New England.

Meanwhile Amtrak is slowing trains along the busy Northeast Corridor because the heat wave is driving up internal temperatures of the rails.

Passengers traveling between Boston and Washington on Amtrak and commuter trains can expect delays of up to 20 minutes Thursday. The speed restrictions also are in effect on portions of the Keystone Corridor between Harrisburg, Pa., and Philadelphia.

Heat and humidity are dangerous. They kill hundreds of Americans every year -- more than tornadoes, hurricanes, floods and lightning combined.

At St. Luke's-Roosevelt hospital and others, they're starting to see more people who have succumbed to the heat, reports CBS News correspondent Elaine Quijano. It's treating patients for dehydration and heat stroke.

Dr. Jeffrey Rabrich said they're averaging about a dozen a day. And he expects more.

"In this heat wave it's been staying hot at night -- and we've been seeing problems with that, particularly respiratory problems," he said. "The risk gets higher. Each day of added heat increases the risk we're going to see more people with both heat stroke and the respiratory problems."

In Philadelphia, where Wednesday was the hottest day of the year so far, the city activated its heat emergency hotline.

In Chicago, the windy city, there was barely a puff of wind to cool the air. CBS News correspondent Dean Reynolds reports that with temperatures stuck in the nineties for the next few days, air quality is becoming a concern.

Officials are blaming hot weather for at least one death. A 78-year-old Alzheimer's patient died of heat exhaustion after wandering away from his northern Kentucky home Tuesday in temperatures that rose to 93 degrees.

In New York City, where it was 96 degrees Wednesday, sidewalk food vendor Ahmad Qayumi said that by 11 a.m., the cramped space inside his steel-walled cart got so hot that he had to turn off his grill and coffee machine.

"It was just too hot. I couldn't breathe," he said, turning away a customer who asked for a hamburger. "Just cold drinks," he said.

CBS New York reports that power companies expect a surge in demand Thursday and extra crews are on hand to man the wires.

"Right now, it's just a bit crazy," said lineman C.J. Rodriguez. "A lot of transformers popping, a lot of wires melting because of the amount of load that's on the wires."

Con Edison has activated its 24/7 command center to keep the power flowing through the five boroughs and Westchester County. PSE&G and LIPA said they have extra workers on hand to handle any power outages that may occur.

All area utilities have asked customers to conserve energy where possible, like setting your thermostat above 78 degrees and turning up the temperature on big energy items like the refrigerator.

Con Edison told CBSNews.com that the electricity usage in its area was at 12,971 megawatts as of 3:00 p.m. The all-time peak electricity usage was 13,189 megawatts set on July 22, 2011 at 4 p.m.

Amid the heat, officials in Washington D.C.'s Maryland suburbs worked to keep a failing water main from cutting off hundreds of thousands of people, just when they needed it most. People in Prince George's County were asked not to run their faucets, water their lawns or flush toilets to keep the water system from emptying during emergency repairs.

Firefighters in southern California faced brutally hot -- but dangerously dry -- conditions as they battled a wildfire outside Palm Springs that had already consumed six homes.

New Mexico and parts of Texas turned out to be rare outposts of cool air Wednesday, but not without trouble of their own: Heavy rains prompted flood watches and warnings in some areas. More than five inches of rain fell in 24 hours in Plainview, north of Lubbock, according to the National Weather Service.

At the World Trade Center reconstruction site in New York City, workers building a rail hub dripped under their hardhats, thick gloves and heavy-duty boots. Some wore towels around their necks to wipe away the sweat.

"We're drinking a lot of water, down under by the tracks, in and out of the sun all day -- very hot," said carpenter Elizabeth Fontanez, of the Bronx, who labored with 20 pounds of tools and safety equipment strapped to her waist. Since the heat wave began, she said, she has been changing shirts several times during her shifts.