That's the sound of the temperature dropping early this week as a batch of cold air moves from the Tennessee Valley and lower Midwest over to the East Coast.
There will be 3 to 9 inches of new snow to go along with that from the Virginias to Massachusetts, the National Weather Service said. Much of it is expected to fall in the daylight hours Tuesday.
Add the areas that are frigid already, and the eastern half of the nation will shiver through temperatures 10 to 25 degrees below average.
The vast area is expected to stay in the icebox until the weekend. That can mean lows in the teens in Memphis, Tennessee, and in the single digits from Louisville, Kentucky, to Boston.
A second frigid weather system is moving down from Canada and will plunge low temperatures into the double-digit minus degrees all along the border. Whipping winds will knock the wind chill down to 20 to 40 below zero from the Upper Plains states to northern Missouri.
A small but intense snowstorm is dumping up to 10 inches of snow in metropolitan Chicago as moisture over Lake Michigan mixes with frigid air.
Though it covers only a small area, Chicago's storm appears to be enough to make air traffic at least more annoying. O'Hare International had canceled about 5% of flights in and out of the airport before dawn Tuesday, according flightaware.com.
But much more frightful nightmares appear to be taking hold in the Northeast. From New York City to Washington, major airports had canceled a quarter to a third of scheduled flights by early Tuesday.
At Philadelphia International Airport, Southwest Airlines plans to suspend operations at 3 p.m., CNN affiliate WPVI reported.
The flakes should be falling at full tilt about then, the weather service said.
The federal government is telling D.C. employees to dispense with the drive to work and is closing its offices. Only emergency employees are required to appear on the job, the Office of Personnel Management said.
Even Boston's Logan International Airport had reduced air traffic by at least 20% early Tuesday, according to flightaware.com.
The cancellations at snow-doused airports may cause ripple effects throughout the nation's air traffic system.
By Tuesday morning, more than 2,500 flights were canceled nationwide, flightaware reported.
State of emergency in Ohio
The icy weather has collided with a shortage of propane and heating oil in Ohio, CNN affiliate WKYC reported.
More than 10% of residents have tanks of one of these fuels attached to their homes to warm them against the brutal cold.
Joe Butano is one of them, and on Monday, with more cold on its way, his propane tank was less than half full.
Customers are clamoring for what is left, and Butano is afraid of coming up short. He has added portable electric heaters and firewood to the mix.
"It makes sense where there's a shortage, it just seems to make sense in preparation for what's the worst that could come," he told WKYC.
The shortage may seem like an inconvenience for customers like Butano, but Ohio's government is taking the it, paired with the intense cold, more seriously.
Gov. John Kasich has declared a state of emergency that allows trucks carrying hazardous materials, like propane or heating oil, to be on the road longer.
Fuel suppliers also warn that the dearth is acute. Propane company The Gas House says it hasn't seen one like this in 23 years.
"I'm concerned because I don't know when we're getting it or if we're getting it, and all the propane companies are in the same boat," spokesman Scott Zura said.
Some sidewalk cafes got crowded in the nation's capital Monday, as people enjoyed that last temperate day before the cold snap's arrival, CNN affiliate WJLA reported.
"I'm loving this today; this is true winter weather for me," Bill Dilodovico said.
Some Washingtonians were soaking up the weather in shorts and T-shirts.
"It is fantastic yet ironic at the same time," said Dan Tarvin, who was out jogging ahead of the icy weather. On Tuesday, he'll be running indoors.
Others opted to use the day to brace for what may come -- Larry Luebbers of Oakton, for example.
"I picked up my snowblower, I got gas ... four-wheel drive, so I'm ready to go," he told WJLA.
Pam Rao and her son Josh of Vienna, Virginia, were looking forward to the snow. "It's a lot of fun, and you know, you only get it once a year or twice a year in this area, so, it's good," Rao said.
Dozens of schools in and around the District of Colombia will shut their doors Tuesday, WJLA reported.