A relatively mild January will come to an abrupt end Friday as a frigid air mass moves into Minnesota, kicking off a cold snap that will last at least two weeks.

Temperatures at the Minneapolis-St. Paul International Airport, the official reporting station for the Twin Cities, have yet to fall below zero this month. But the mercury will drop into negative territory by Saturday morning and low temperatures will be at or below zero through the first full week of February, the National Weather Service said.

Though hardly record-setting, Monday's forecasted low of 11 below will feel bone-chilling at the tail end of a month that has seen temperatures 7.5 degrees above average and has featured just three days with average daily temperatures below normal. The average high temperature for the week is 25 degrees, and average low temperatures are in the mid teens, the Minnesota Climatology Office said.

"It will be a shock to the system because it has been so balmy," said Pete Boulay, assistant state climatologist with the Minnesota State Climatology Office. "Winter is returning."

The Twin Cities normally sees 10 nights with readings at or below zero in January, but the month "has been a lot warmer than usual," Boulay said. However, December was much colder than normal, including the season's lowest temperature of -12 degrees recorded on Dec. 23.

The warmest reading this month was 37 degrees on Jan. 15 and 16; the lowest was 0 on Jan. 7 and 8, according to the climatology office. Some readings in the suburbs have dropped below zero a couple of times this month.

A deep snowpack — 10 inches on the ground as of Tuesday — with another inch or two possible Wednesday and Thursday is helping set the stage for frigid weather that promises to hang around through the run of the St. Paul Winter Carnival, which starts Thursday and continues through Feb. 5.

"The deeper the snow, the deeper the cold," Boulay said

Arctic air often brings sunny skies, but not this time. Cloudy conditions are forecast at least through Tuesday, the National Weather Service said, with highs over the next week in the single digits.

"This is not unusual," Boulay said. "You just expect it."