Apologies to anyone trying to fly out of Chicago.

The bitter polar vortex caused hordes of flights to be canceled across the country on Wednesday, with almost 2,000 flights canceled at Chicago's O'Hare and Midway airports alone.

In total, with a mass of extremely cold air freezing out the Midwest and Northeast, 3,576 flights were canceled.

As Amtrak suspended service into and out of Chicago, fuel lines at O’Hare Airport froze, forcing some planes to refuel elsewhere before continuing to their destination, an airport spokeswoman told the Associated Press.

American Airlines confirmed to USA TODAY they experienced issues refueling in the extreme temperatures.

Extreme weather: Polar vortex in Midwest, record heat in Australia, odd warmth in Alaska. What's going on?

"In (an) attempt to operate as many flights as possible and accommodate as many customers as possible, some flights are being sent in with an overage of fuel to get to their next destination without needing to take on fuel at ORD, and others are departing ORD and making a fuel stop before they reach their final destination," American Airlines spokesman Ross Feinstein said in a statement.

The temperature reached minus 23 at O’Hare International Airport, with a wind chill of 52 degrees below zero, according to the National Weather Service.

Another 2,159 flights nationwide scheduled for Thursday have also been called off, according to flight tracker FlightAware.

This article originally appeared on USA TODAY: Polar vortex causes another lousy travel day for fliers</s>
• The polar vortex that brought deadly cold to the US is coming to an end, and temperatures could rapidly rise by as much as 80 degrees Fahrenheit this weekend.
• The warmer conditions will be welcomed after record-low temperatures disrupted towns and cities, and killed at least 21 people.
• But the rapid thaw could create some new problems like flooding, potholes, and burst pipes.
• Wintry conditions could also return to some parts of the US Midwest and Northeast next week, though it likely won't be as cold as during the polar vortex.

The deadly polar vortex that brought record-low temperatures to the US and killed at least 21 people is coming to an end — and temperatures could climb by as much as 80 degrees Fahrenheit this weekend.

The polar vortex brought temperatures lower than minus 50 degrees Fahrenheit (minus 46 degrees Celsius), and forecasters predict that some places will become up to 80 degrees Fahrenheit higher than their lowest temperatures during the week.

The rebound in temperature could set its own records. Jeff Masters, meteorology director of the Weather Underground firm, said: "I don't think there's ever been a case where we've seen [such a big] shift in temperatures," according to The Associated Press.

"Past record-cold waves have not dissipated this quickly," he added. "Here we are going right into spring-like temperatures."

Read more: The polar vortex ravaging the US with extreme cold has killed at least 21 people

Ice builds up along the shore of Lake Michigan, Chicago, on Thursday. Scott Olson/Getty Images

The more than 20 million Americans that experienced the life-threateningly low temperatures will likely welcome the milder weather after thousands of flights were canceled, schools were closed, and the US Postal Service suspended operations due to the severe weather.

But the speed at which much of the country is expected to get warmer brings its own problems.

Extreme cold followed by a quick thaw can damage roads, leaving them cracked and filled with potholes.

The heating and cooling of the pavement cause it to expand, contract, and then crack, The Weather Channel warned. Melted water from ice can get into those cracks and freeze overnight, expanding and making the cracks even bigger.

Cars and trucks driving over those cracks can break the road surface further, creating potholes.

People walk through the snow in lower Manhattan on Wednesday. Spencer Platt/Getty Images

Meteorologist Brian Donegan wrote on The Weather Channel: "Since the temperature was so bitterly cold and is now expected to warm up significantly in the days ahead, road surfaces will become stressed due to that quick transition from freeze to thaw. This is why numerous potholes will likely form in the Midwest and Northeast."

Another problem that the rapid thaw can produce is the phenomenon of "ice jams" — the large chunks of ice created when a frozen river begins to melt, and which can get stuck or pile up in rivers. These can cause water to overflow and flood the surrounding area.

Melting icewater from surrounding areas can also flow into rivers and lakes, causing flooding.

Pipes can also freeze and burst.

Anthony Tornetta, a spokesman for the American Red Cross, told USA Today: "Unfortunately, this polar vortex is gripping a large portion of the country and frozen pipes is an unfortunate reality in some cases."

Read more:A polar vortex is engulfing the US. Here's what that really means, and why these events might be getting more common.

Photographers take pictures of the sunrise as temperatures hover around minus 20 degrees Fahrenheit (minus 29 degrees Celsius) and wind chills nearing minus 50 degrees Fahrenheit (minus 46 degrees Celsius) in Chicago on Thursday. Scott Olson/Getty Images

Some parts of the country may have to embrace the cold again

Some parts of the US may have to brace themselves for the return of winter weather after the warm spell, though it won't be as cold as the polar vortex.

Donegan, the meteorologist at The Weather Channel, predicted that the "mild pattern is not expected to be prolonged" for some parts of the country.

"Below-average temperatures might return to the Midwest and Northeast next week," he warned, though "the good news is that it likely won't be as harsh as this week's arctic blast."

Some branches of the National Weather Service (NWS) are warning people to prepare for cold weather next week after a warmer weekend.

The NWS in Sioux Falls, South Dakota, warned that an "Arctic chill" may return next week after a "great" weekend.

The NWS in Chicago predicts a significant increase in temperatures next week, with a high of 56 degrees Fahrenheit (13 degrees Celsius) on Monday.

However, it warned of rain and ice over the weekend and into next week.
• Read more coverage of the polar vortex:
• Here's what it's like in one of the coldest cities in the US, where the windchill is in the -50s
• The frigid 'polar vortex' cold snap engulfing the US is so bad it can give people frostbite in 5 minutes
• From frozen lakes to indoor icicles, here are the most dramatic polar vortex photos from around the US that will make you want to stay inside
• Parts of the Midwest are colder than Antarctica due to a polar vortex. Can you guess which photo was taken where?
• A good Samaritan paid for hotel rooms in Chicago for 70 homeless people who were sleeping in tents with no source of heat during the polar vortex
• This is what Midwesterners actually do to stay warm in below-freezing temperatures</s>As frigid temperatures and life-threatening winds batter parts of the US, cities in the Midwest are shuttering to keep citizens inside and safe from the dangerous cold.

Currently, below-zero temperatures have been reported throughout the area, with wind-chill temperatures predicted to reach a record-low of -31C (-25F) in Chicago by Thursday with wind-chill temperatures dipping to below 48C (-55F).

The sudden extreme temperatures are the result of a polar vortex, a weather phenomenon that occurs in the winter and impacts parts of the northern hemisphere.

The polar vortex is a low-pressure area comprised of cold air that sit above the Earth’s north and south poles.

The area of cold air is present throughout the year but is typically kept in place in the arctic regions by winds moving in a counter-clockwise direction at the north pole and clockwise at the south pole.

“It ALWAYS exists near the poles,” the National Weather Service states on its website.

However, during the winter months, the polar vortex above the north pole can expand into the northern hemisphere - bringing freezing cold temperatures with it.

The polar vortex, which is naturally occurring, can split and stray if the winds are weakened - as is the case when temperatures warm in the stratosphere.

When this occurs, the polar vortex sends cold air south, which can affect the United States, Europe, and Asia.

According to the NWS, this occurs frequently: “Many times during winter in the Northern Hemisphere, the polar vortex will expand, sending cold air southward with the jet stream.”

How do you prepare for a polar vortex?

With millions of Americans currently facing below-freezing temperatures as a result of the polar vortex, it is important to take extreme precautions to protect against the cold.

If you must go outside, dress appropriately in several layers and ensure that there are no exposed areas of skin.

In these temperatures, it takes as little as five minutes for frostbite to begin.

You should also cover your mouth with a scarf and avoid talking or taking deep breaths, according to the NWS, to avoid damaging your lungs with cold air.

Children, elderly, and those with weakened immune systems should avoid going outside.

Additionally, ensure that you have adequate heating or seek out alternative housing until the temperatures rise.

Many cities have opened warming centres where people can stay if they do not have proper heating available.

In parts of the US where the polar vortex has led cities to declare states of emergencies, it is integral that people take the proper precautions and do not spend significant time outside.

When was the last polar vortex?

As previously stated, cold temperatures caused by a polar vortex are not uncommon.

The last polar vortex occurred in January 2014, according to the NWS, and before that in 1977, 1982, 1985 and 1989.

Where is the polar vortex now?

On Wednesday, parts of the US were still in the midst of the bitter cold.

According to meteorologists, temperatures will continue to plunge in the Midwest until Thursday.

Chicago, Illinois is facing the brunt of the cold, but the impact can be felt in cities in Iowa, Minnesota, North Dakota, South Dakota, Wisconsin, and parts of Kansas, Missouri, Montana and Nebraska.</s>Frozen winds have brought record-low temperatures to the US midwest as a blast of Arctic air, known as the polar vortex, unnerves residents accustomed to brutal winters... Wet clothes freeze after 10 minutes, ice arrives after a hot shower and ships break their way through the Chicago River
• Polar vortex: eight dead as Arctic air spreads across midwest
• 'Cold as ºF': Chicago residents make best of life in 'Chiberia' during polar vortex</s>SALT LAKE CITY — The United States has suffered a bit of a cold spell this week.

As headlines put it, a polar vortex descended upon the country. Areas in the Midwest experienced “once-in-a-generation” temperatures — 23 below zero in Chicago, 77 degrees below zero in Minnesota and 50 to 60 degrees below zero in Iowa.

So: Will Utah ever experience a polar vortex?

No, according to KSL meteorologist Kevin Eubank.

That’s because “polar vortex” is a made-up term, one that meteorologists laugh about, Eubank said.

Eubank said the term “polar vortex” is a headline-grabbing name for what's really just a cold storm. The low pressure, cold air floods down from the North Pole and descends upon Montana and sweeps eastward.
• Context: The National Weather Service defines a "polar vortex" as a counter-clockwise flow of cold air from the North Pole, adding that it's "only recently been popularized, bringing attention to a weather feature that has always been present."

So, no, Utah won’t experience that specific weather pattern. But it doesn’t mean Utah won’t experience temperatures similar to the Midwest’s chilly temperatures.
• “If you’re asking if a cold storm can come to Utah, if it has origins of the cold air at the pole, or, in essence, the Arctic — yes. We get polar air or arctic air to Utah all the time,” he said.

Still: Utah will experience temperatures as cold as Chicago, Minnesota and the Midwest had this week.

By the numbers: A quick look at the record books show Utah temperatures aren’t that uncommon from those we’ve seen in the Midwest.
• Peter Sinks, Utah, holds the record of the second-lowest temperatures in the contiguous 48 United States at minus 69 degrees Fahrenheit, which happened on Feb. 1, 1985.
• Roger’s Pass, Montana, has the coldest temperatureamong the 48 states with minus 70.0 degrees.
• Prospect Creek, Alaska, holds the overall record with minus 80 degrees, which occurred on Jan. 23, 1971.

This year: Peter Sinks experienced minus 40 temperatures this year, too, showing it’s possible for Utah to experience temperatures similar to what we saw in the Midwest, Eubank said.
• “You hear about this polar vortex and how crazy it is,” Eubank said. “Well it’s not that unusual, No. 1. And No. 2, Utah’s had temperatures that equal to or are colder than what they‘ve experienced over there.”</s>A record-breaking cold wave has swept through the US Midwest, with 22 states hitting sub-zero temperatures. Among cities, Chicago dropped to a low of -30°C, slightly above the city’s lowest-ever reading of -32°C from January 1985. Minneapolis recorded 32°C and Sioux Falls (South Dakota) -31°C.

The extreme cold has been caused by a blast of Arctic air, which in turn is a result of what is known as a “polar vortex” event.

Essentially a low-pressure area, it is a wide expanse of swirling cold air surrounding both polar regions. The counter-clockwise flow of air helps keep the colder air near the poles. “Polar vortexes are not something new. The term ‘polar vortex’ has only recently been popularised, bringing attention to a weather feature that has always been present… However, when we feel extremely cold air from the Arctic regions at Earth’s surface, it is sometimes associated with the polar vortex,” the US National Oceanic and Atmospheric Administration (NOAA) explains on its website.

So, when does the polar vortex cause extreme cold?

In winter, the polar vortex sometimes becomes less stable and expands. “Many times during winter in the northern hemisphere, the [north] polar vortex will expand, sending cold air southward with the jet stream,” NOAA explains. This is called a polar vortex event, defined by NOAA’s SciJinks as the “breaking off” of a part of the vortex. “Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south,” NOAA says. “But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.”

Is all cold weather the result of a polar vortex event?

No. Though the polar vortex is always “hanging out” up North, it takes pretty “unusual conditions” for it to “weaken” for it to migrate far south, NOAA explains. It is not confined to the US either. “Portions of Europe and Asia also experience cold surges connected to the polar vortex. By itself, the only danger to humans is the magnitude of how cold temperatures will get when the polar vortex expands, sending Arctic air southward into areas that are not typically that cold,” it states.</s>Image copyright Getty Images Image caption Chicago's frozen shoreline

At least 21 people have died in one of the worst cold snaps to hit the US Midwest in decades.

Ninety million people - a third of the US - have seen temperatures of -17C (0F) or below. Some 250 million Americans overall have experienced the "polar vortex" conditions.

Hospitals have been treating patients reporting frostbite as parts of the country ground to a halt.

Temperatures are expected to swing to above average over the weekend.

Who are the victims?

Homeless people have been particularly at risk, with warming shelters set up across cities.

But some still braved the freezing conditions and one woman, aged 60, was found dead in an abandoned house in Lorain, Ohio.

A hospital in Chicago has already treated 50 patients for frostbite, and some may end up losing a limb, CNN reports. Half of those patients were homeless individuals , while others had jobs that required them to be outdoors.

Some people were found dead a short walk from their homes:

A Michigan man who froze to death in his neighbourhood had been "inadequately dressed for the weather", officials said

In a wind chill of -46C (-51F) an 18-year-old student was found unresponsive a short walk from his dorm on Wednesday and later died in hospital

On Tuesday, a man froze to death in a garage in Milwaukee, Wisconsin, having "apparently collapsed after shovelling snow", according to a medical examiner

Dangerous roads have also been a factor in the deaths. A man was fatally struck by a snow plough near Chicago on Monday and in northern Indiana, a 22-year-old police officer and his wife died after a collision on icy roads.

Media playback is unsupported on your device Media caption So what actually is a polar vortex...?

What's the forecast?

The icy cold is expected to loosen its grip on Friday.

By the end of the weekend, Chicago could see temperatures as high as 10C (50F).

"It's going to be at least a 60-degree swing for Chicago," David Hamrick, a National Weather Service forecaster, told Reuters news agency.

The sudden weather change coming this weekend may be the fastest warm-up on record, meteorologists say.

But as the temperatures abruptly turn warmer, US emergency officials warn of flooding and utility risks.

Pipes can burst with such temperature fluctuations, and rapidly melting snow and ice could cause flooding, the Federal Emergency Management Agency cautioned.

How cold did it get?

More than 30 record lows were broken across the Midwest.

Cotton, Minnesota, was the coldest place in the US on Thursday with a low of -48C (-56F) based on preliminary data.

Chicago passed the record low for 31 January, while Mount Carroll has probably beaten the Illinois record with a morning temperature of -39C (-38F).

Media playback is unsupported on your device Media caption Chicago is using fire to melt snow on the railway and keep the trains running

Cities across Iowa have also broken temperature records.

The chill drifted eastward on Thursday, bringing sub-zero temperatures to north-eastern cities such as Boston.

Media playback is unsupported on your device Media caption Much of Chicago River has frozen over

With wind chill factored in, temperatures of -40C (-40F) in the Midwest and Great Lakes have felt closer to -53C (-63F), which is enough to cause frostbite in less than five minutes.

Image copyright Reuters Image caption Snow plough in Buffalo, New York

How is the cold snap affecting daily life?

The Arctic weather could cost the US billions of dollars. In 2014, a similar polar freeze cost the country an estimated $5bn (£3.8bn), CBS News reports.

In Minnesota and Michigan, residents were asked by gas companies to turn down their home thermostats to help handle heating demands.

Consumers Energy, a natural gas provider in Michigan, had a fire on Wednesday morning that damaged equipment and temporarily affected how much gas could be sent out to customers.

Native American tribes in the northern Midwest states helped their members obtain heating supplies as many live in poor-quality housing, the Associated Press reports.

Andrea Cusack, a pharmacist in Michigan, began using her snowmobile to deliver essential prescriptions to snowed-in residents, according to the Lansing State Journal.

More than 2,300 flights have been cancelled and another 3,500 delayed due to the polar vortex.

Social media has been full of photos and memes showcasing just how shockingly cold the Midwest became.

Image copyright AFP Image caption Niagara falls covered with snow

What about Canada?

Areas across the provinces of Ontario, Quebec and Saskatchewan, and in the north, remain under extreme cold warnings.

But many parts of the country are moving towards more seasonable temperatures on Friday and over the weekend.

In Toronto, wind chills near -30C (-22F) were expected to continue early Friday before beginning to warm.

There were also winter storm and blizzards warnings active across the country from the east to the west coast.

Environment Canada was urging residents to limit their exposure to cold and keep pets indoors.

Canada did not experience a spate of deaths linked to the polar vortex like the US.

Stephen Hwang, an associate professor with the University of Toronto's department of medicine, suggested that Canadian cities and public health authorities probably had more experience dealing with the deep cold.

Most homeless shelters also already had protocols in place for when the extreme cold hits.

But he said it was still "fortunate" that cities like Toronto, where homeless shelters have been stretched for resources in recent months, did not see any cold-related deaths among its most vulnerable citizens.</s>At least eight people have died in the US Midwest as the region shivers in the grip of its worst cold snap in decades.

An Iowa student found dead outside a college building is among victims of the deadly freeze.

Hospitals have been treating patients reporting frostbite as life across a swathe of the nation grinds to a halt.

The iciest blasts may still come on Thursday. Ninety million people - a third of the US - have seen temperatures of -17C (0F) or below.

Some 250 million Americans overall have experienced the "polar vortex" conditions, but southern states such as Florida have escaped the brutal chill.
• How cold is it where you are?

How did the fatalities occur?
• University of Iowa student Gerald Belz, 18, was found unresponsive behind a campus building before dawn on Wednesday and later died in hospital. Officials said weather was a factor
• A 70-year-old Michigan man was found dead near his home in Detroit, Michigan, on Wednesday. Officials said he was "inadequately dressed for the weather"
• Another Michigan man was also found frozen to death in his neighbourhood
• On Tuesday, a 55-year-old man froze to death in a garage in Milwaukee, Wisconsin, having "apparently collapsed after shovelling snow", according to a medical examiner
• An 82-year-old in Pekin, Illinois, died from hypothermia just outside his home on Tuesday
• A 75-year-old man was fatally struck by a snow plough near Chicago on Monday. The driver has since been placed on paid leave pending an investigation, according to WGN9 News
• In northern Indiana, a young couple died after a collision on icy roads

Thursday could see America's third largest city of Chicago breaking its 1985 record low of -32C (-25F), according to meteorologists.

The chill is drifting eastward on Thursday, bringing sub-zero temperatures to Northeastern cities such as Boston.

Areas downwind of the Great Lakes are expected to be buried by intense lake-effect snow lakes into Thursday night.

The region near Buffalo, New York, should see the heaviest snowfall. Snow could fall at rates of 3in-5in (7cm-12cm) per hour.

The icy cold is expected to loosen its grip on Friday.

With wind chill factored in, the Midwest and Great Lakes have felt temperatures closer to -40C (-40F) and -53C (-63F), which is enough to cause frostbite in under five minutes.

But by the end of the weekend, Chicago could see temperatures as high as 10C (50F).

"It's going to be at least a 60-degree swing for Chicago," David Hamrick, a National Weather Service forecaster, told Reuters news agency.

How is the cold snap affecting daily life?

The Arctic weather could cost the US billions of dollars. In 2014, a similar polar freeze cost the country an estimated $5bn (£3.8bn), CBS News reported.

In Minnesota, residents have been asked by natural gas company Xcel Energy to reduce their home thermostats to 17C (63F) in order to help the company handle heating demands.

Michigan residents have had similar requests from their utility companies as providers struggle to keep the states warm.

Detroit, Michigan, has had over two dozen water mains freezing over this week. A city spokesman told the Associated Press the pipes were installed up to 1.8m below the frost line, but with such drastically low temperatures, the ground has still frozen through.

The US Postal Service has suspended all mail deliveries for the second day to parts of six states.

Over 2,300 flights have been cancelled and another 3,500 delayed due to the polar vortex.

As ice and snow continue to build up, roads have become increasingly dangerous across the northern US.

At least two people were critically injured in a 27-car pile-up on icy roads in Wyomissing, Pennsylvania.

In Minnesota, prison visits were cancelled over the cold, according to the New York Times.

The Illinois city became a frozen ghost town after temperatures fell to -30C (-23F), colder than parts of Antarctica.

A Good Samaritan paid for 70 homeless Chicagoans to stay in a hotel as temperatures dropped.

Salvation Army spokeswoman Jacqueline Rachev told the Washington Post: "It's a deadly situation for anyone. We're thrilled that someone was in a position to be able to do this."

Most of the thousands of cancelled flights this week were coming out of Chicago's airports - O'Hare International is ranked as one of the top 10 busiest airports in the world.

Amtrak also cancelled all trains into Chicago on Wednesday, affecting 55 trains, and said most would be cancelled on Thursday as well.

As the Midwestern city is one of the company's hubs, train service nationwide could also be impacted.

The chill was even too much for Chicago's Disney on Ice show, which cancelled its Wednesday performance.

Most of Canada was under some sort of weather warning - from extreme cold in the Prairies, Quebec and Ontario to heavy snows in Alberta and Nova Scotia.

In Toronto, where winters tend to be milder compared to cities such as Montreal and Ottawa, temperatures had plummeted to -18C (0F). Icy roads and several transit delays made for a hellish commute for millions of the city's residents.

Environment Canada issued extreme cold warnings for most parts of Ontario, Quebec and Manitoba, urging residents to limit their exposure to cold and keep pets indoors.

"If it's too cold for you to stay outside, it's too cold for your pet to stay outside," the weather agency warned.

Advocates expressed concern for homeless people living in cities hit by the extreme temperatures.

In Winnipeg, the capital of Manitoba, temperatures were a frigid -40C (-40F), with wind chill factors making it feel like -50C (-58F).

"If you don't get it in December, you will get it in January or February or March. What do you expect? It's 'Winterpeg'," Caroline Rodriguez told the CBC, in reference to the city's nickname.

In Calgary, Alberta, temperatures were a relatively balmy -3C (27F), but were expected to plummet overnight and through the weekends.

In parts of rural northern Alberta, 15in of snow were expected to fall.</s>US Weather: Polar Vortex takes hold of US

Parts of the US are facing temperatures colder than Antartica as a polar vortex sweeps over the country. The Arctic winds caused temperatures to plummet to life-threatening temperatures across the US midwest yesterday. Cities in the region have been forced to shut down as US officials have urged the public to stay indoors.

US snow MAP: Chicago as cold as NORTH POLE

US snow MAPPED: Where in the US has snow fallen today?

Temperatures have fallen to -30C in Chicago and -37C in North Dakota with wind chills as low as -52C in Minnesota on Tuesday.

Chicago Mayor, Rahm Emanuel said: “These conditions are actually a public health risk and you need to treat it appropriately.

"They are life-threatening conditions and temperatures."

Mr Emanuel also added that the city is making sure homeless people are in shelters while the Governor of Illinois, JB Pritzkler declared a state of emergency earlier this week - which has also been called in Michigan and Wisconsin.

READ MORE: Polar vortex: What is a polar vortex - latest alerts as SNOW buries US</s>One social media user filmed a video of a soap bubble freezing solid and then breaking in the dangerously cold weather that continued to shatter records across the midwest on Thursday, due to the polar vortex.

Chicago broke all-time low temperatures on Thursday morning, earning it the nickname Chiberia. There, the mercury dropped to -29C (-21F).

Polar vortex: midwest to creep out of icy grip after record lows
The death toll from a North American cold wave caused by a polar vortex rises to at least 21 people.