(CNN) -- Just when you thought the heat couldn't get any worse, it looks like it will.
On Friday afternoon, temperatures topped 100 degrees Fahrenheit from Chicago to Kansas City, Missouri, and Columbus, Ohio, and many places in between. Heat watches, warnings and advisories touched 25 states -- an area spanning roughly one-quarter of the nation and encompassing more than 115 million people, or more than one in three Americans.
Around 4 p.m., the thermometer in St. Louis read 105 degrees, the ninth straight day it has topped the century mark in that Missouri city. That may sound bad, but it's relatively cool compared with the 110 degrees the National Weather Service is forecasting for Saturday.
St. Louis won't be alone in going from unbelievably hot to unbearably hot. New York should hit 100 degrees Saturday, as should Lexington, Kentucky. And in Washington, D.C., the thermometer is expected to reach 103 degrees, and the heat index will make it feel like 113.
"The record-breaking heat wave currently affecting parts of the central and eastern U.S. will continue into the weekend," the weather service said dryly.
About 350,000 customers across 12 states and the District of Columbia will be suffering in this heat without the benefits of electricity, including power for their air conditioning and for refrigeration to keep their food edible, according to a CNN count from Friday night. Because utilities typically define each residential and business account as a customer, the actual number of people affected was not clear.
Many of these people have gone without power for a full week, thanks to strong storms fueled by the heat that barreled east from Indiana to New Jersey. Others have watched more recent but similar storms leave them in the dark.
The hardest-hit state continues to be West Virginia, where about 167,000 customers had no power Friday night.
"It's been a very, very serious and critical situation," said John David, a West Virginia University Institute of Technology professor and volunteer with the South Appalachian Labor School who has been working in Fayette County. "But people here are used to sacrifice."
Nationwide, more than 4,500 record-high daily temperatures have been set in the past 30 days, according to the National Climatic Data Center.
And since June 23, scores of cities have been the hottest they've ever been, on any day ever recorded. That includes 107 in Colorado Springs, 109 in Nashville and 106 in Atlanta. In Washington, the thermometer has gone past 95 degrees for nine straight days -- the longest such streak since modern record-keeping began.
The high temperatures have been linked to a number of deaths nationwide.
That includes five deaths -- of men ages 48, 58 and 59 and two women ages 81 and 91 -- because of "heat stress" in Chicago, city public health spokesman Efrat Stein said Friday. Another heat-related death was reported in nearby Cook County, according to Stein.
Another three heat-related deaths were reported in Rock County, Wisconsin, according to Chief Deputy Coroner Louis Smit. Heat was ruled an "exacerbating factor" in two of those cases, and it was a "major factor" in the death of an 83-year-old woman found dead "from a cardiac-related condition" in a home that was 95 degrees inside and had "no cold water or other fluids."
Powerful storms tied to the weather system have left dozens dead. Most recently, two people died and eight were injured when a storm struck Tennessee's Great Smoky Mountains National Park on Thursday night, park officials said.
Thankfully, there is some relief on the horizon. A cold front should move across the Upper Midwest and Great Lakes on Saturday, and then into Ohio and the Northeast by the following day, bringing significantly cooler air with it.
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CNN's Greg Botelho, Dave Hennen, Brandon Miller, Amanda Watts and Jake Carpenter contributed to this report.