Freezing rain was falling around the Baltimore region into Sunday afternoon, with as much as a few tenths of an inch of ice accumulation possible in most areas and a quarter of an inch or more northwest of the city.
An ice storm warning is in effect through 9 p.m. for Carroll and Frederick counties, where the most significant accumulations could occur. A freezing-rain advisory for Howard, northern Baltimore and Harford counties was lifted by about 3:30 p.m.
The region could soon be in for its coldest temperatures in 20 years Monday and Tuesday, after having its coldest morning in four years early Saturday. Temperatures fell to 6 degrees at BWI Marshall International Airport early Saturday, just above the record low for Saturday's date of 4 degrees, set in 1918.
Temperatures close to or below zero with wind chills as much as 20 degrees below zero are expected Monday into Tuesday as a low-pressure system bringing Arctic air much farther south than usual moves across the country, said Jared Klein, a meteorologist with the National Weather Service's Baltimore-Washington forecast office.
A system known as a polar vortex, an area of low pressure tens of thousands of feet up in the atmosphere, is anticipated to shift southward from its usual position around the North Pole, Klein said. The deep freeze across the northern U.S. plains, including North and South Dakota, and through the Great Lakes region and Ohio Valley, could bring the region's coldest weather in two decades, with blizzard conditions expected in the Central Plains and Great Lakes, National Weather Service forecasters said.
"Occasionally we get pieces of energy that move south into the country and we get our colder outbreaks," Klein said. "This time we're getting almost the whole polar vortex moving south toward the Great Lakes, which is pretty rare. That's going to allow the really cold Arctic air to come down into a large part of the United States, including the Baltimore area."
Record low temperatures were expected from Montana to Michigan, including 20 degrees below zero in Chicago, before reaching the Northeast by early Tuesday, forecasters said.
"Incredibly, it may feel as cold as negative-50 to negative-60 on Sunday night over sections of the north-central states," including Minnesota and Wisconsin, the National Weather Service said in a statement.
In Green Bay, Wis., temperatures around 4 degrees below zero and average wind chills of 19 degrees below zero are forecast for the Packers-49ers game, according to the weather service's forecast office there. Those figures could rank it among the five coldest NFL games in history, according to the weather service.
Meanwhile, a respite of seasonable temperatures is forecast Sunday in Baltimore after the chance for freezing rain passes. Normal highs this time of year are in the lower 40s.
But by Monday morning, the cold is expected to arrive quickly. The day's high is expected in the early morning hours, dropping throughout the day to the single digits in the city and zero or below in the suburbs by early Tuesday morning, Klein said. The record low for Jan. 6 is 5 degrees, set in 1904; the record low for Jan. 7 is 8 degrees, set in 1988. If the temperature at BWI drops below zero, it would be for the first time since Feb. 5, 1996, when the airport reached 1 degree below zero.
Reuters contributed to this article.