Fog across east-central Minnesota and the Duluth area and snow across the northern part of the state are leading to tough driving and school delays Monday morning.

Conditions were improving across the Twin Cities metro area where fog was prevalent early Monday. But a dense fog advisory where visibility was less than a mile in some places remained in effect until 9 a.m. for Aitkin, Pine, Itasca, Carlton and southern St. Louis counties, the National Weather Service said.

Cities affected include Pine City, Duluth, Grand Rapids, Hill City and Aitkin.

To the north, a winter weather advisory remained in effect until noon for Koochiching, northern St. Louis, and northern Cook and Lake counties, along with Voyageurs National Park and the entire Boundary Waters also included. As much as 4 inches of snow could fall in the area, the National Weather Service in Duluth said.

Lesser totals with gusty winds are expected across northwestern Minnesota, where an advisory also is in effect until noon, the Weather Service said.

"Plan on slippery road conditions," the Weather Service warned.

Several school districts delayed the classes Monday morning to allow weather conditions to improve. Among districts starting late are Aitkin, Bemidji, Brainerd, Cook County, Hill City, Nevis, Park Rapids, Pequot Lakes, Staples-Motley and Walker-Hackensack-Akeley.

Mild conditions will give way to arctic air by Wednesday sending the mercury into the teens below zero from Brainerd to International Falls to Ely, and windchill readings even lower, perhaps minus 40 degrees, the Weather Service said.

"Frostbite occurs in as little as 15 minutes with these windchills," the Weather Service said.

Even in the metro, thermometers could register below zero readings for the first time this season Wednesday night into Thursday and highs both days only in the single digits above zero, the Weather Service said.