Between now and early spring, Chicago would have to see almost 43 inches of snow to have the same amount it had last winter. But that’s unlikely, said Richard Castro, a meteorologist for the National Weather Service’s Chicago office.
After getting more than 50 inches of snowfall for four consecutive years, why was Chicago’s first real snow day halfway into the season?
The mildness comes from two weather systems: La Niña and the Arctic Oscillation.
At its very basic, La Niña is a polar jet stream from the Pacific Ocean that causes chilly winters. La Niña allows cold air to drop south into the Midwest, Castro said. “But there’s been a very persistent feature above Alaska that’s keeping cold air from dropping down here.”That feature: Arctic Oscillation, which refers to pressure patterns over the Arctic and has two phases, positive and negative.
Castro said it’s in a positive phase this year. This means less pressure at the polar region, which moves storms toward the north.
Since 2000, Chicago had comparatively cold winters as well as mild ones, but it’s not that unusual.
“It’s all over the map,” Castro said. “It fits the theme as how variable the weather patterns can be in this part of the country.”
To get more details, read Castro’s article about this year’s mild winter.