(CNN)The brutally cold weather that's killed 12 people and trapped millions in a historic deep freeze will finally let up by week's end.
The extreme cold was still evident Thursday as frigid air and dangerous wind chills stretched from the Upper Midwest all the way to Maine, CNN meteorologist Dave Hennen said.
At the cold's peak Thursday morning, about 7 a.m. ET, more than 216 million people saw temperatures below freezing, including 84 million who dealt with subzero temperatures, Hennen said.
By the afternoon, about 30 million people were still under wind chill warnings and advisories -- down from a peak of 140 million in the morning.
But the misery will slowly melt away Friday, with a warming trend that could give many Americans thermal whiplash.
"Today is the last of the extreme cold air," Hennen said Thursday. "Temperatures will rebound quickly over much of the area that saw the extreme cold, creating a yo-yo effect of extreme temperature difference."
Chicago, for example, will see a temperature rise of almost 75 degrees -- from extreme cold of 20-25 below zero to temps in the low 50s on Monday.
And Atlanta, which has shivered in the 20s this week, will enjoy temperatures in the 60s when it hosts the Super Bowl on Sunday.
Chicago might be hit with 'frost quakes'
Across the country, the bone-chilling weather has shattered dozens of records.
Chicago came close to breaking its record of 27 below zero when temperatures plunged to 21 below, Hennen said.
But the Windy City now has something other than a negative-41 wind chill to worry about: frost quakes.
A train travels in Chicago as a gas-fired switch heater on the rails keeps the ice and snow off switches.
Some Chicagoans were startled awake Wednesday by a series of large booms, CNN affiliate WGN reported.
"I thought I was crazy! I was up all night because I kept hearing it," Chastity Clark Baker said on Facebook, according to WGN. "I was scared and thought it was the furnace. I kept walking through the house. I had everyone's jackets on the table in case we had to run out of here."
That boom was probably a weather phenomenon known as cryoseism -- and dubbed a "frost quake." It happens when water underground freezes and expands, causing soil and rock to crack.
12 deaths are now linked to brutal weather
As millions grapple with frigid temperatures, at least 12 deaths have been linked to this week's extreme weather.
Ice lines the shore of Lake Michigan before sunrise on Thursday in Chicago.
The latest reported death was in Marquette Heights, Illinois, where an 82-year-old man died from cold exposure after he slipped on an ice patch outside his home, according to police Chief Bradd Elliott. The man had just returned home and was about to go inside when he slipped.
A neighbor noticed him a few hours later and called 911, Elliott said. He died at a hospital.
In Michigan, the body of a 70-year-old man was found frozen near his Detroit home, police said. It was not clear why he was outdoors.
Officials in Iowa said there have been four deaths there this week, including the discovery Wednesday of a University of Iowa student.
The student, a sophomore, was found unresponsive about 4 a.m. ET behind a campus recreational facility. The temperature in Iowa City at that time was about 21 below zero, and it had been below zero all day, the National Weather Service reported.
Storm-related deaths were also reported in Illinois, Minnesota, Indiana and Wisconsin, authorities said.
Mail service is still suspended
The massive cold snap has also frozen some mail and blood donation services.
The US Postal Service said due to arctic temperatures, Thursday deliveries are suspended in parts of Illinois, Indiana, Michigan, Ohio, Pennsylvania and Wisconsin.
Mike Calen, left, and Steve Goyette, right, prepare to tie off as the ice-covered fishing boat Buzzards Bay arrives in the harbor in New Bedford, Massachusetts on Thursday.
The American Red Cross said 370 blood drives across the country were canceled as temperatures dropped.
"The Red Cross currently has an emergency need for blood and platelet donors of all types to help ensure lifesaving medical treatments and emergency care are not delayed or canceled this winter," spokeswoman Stephanie Rendon said in an email.
State government offices in Michigan were closed for a second day on Thursday due to "emergency weather conditions."
Michigan wants residents to weather storm on less heat
Despite the frigid temperatures, Michigan's governor is asking some residents to turn their heat down.
A fire at Consumers Energy's Ray Natural Gas Compressor Station in Macomb County -- which is responsible for about one-fifth of the state's natural gas storage supply -- shut down all gas flow from the facility.
With gas delivery inhibited, Gov. Gretchen Whitmer made a late-night appeal Wednesday to residents of the lower peninsula, asking them to turn their thermostats down to 65 degrees or lower until noon Friday.
It's important for Michiganders to follow through, Whitmer said, "so that we can get through this storm with minimal harm."