As December snowstorms go, the one that dumped several inches of snow on the metro and much of the southern half of Minnesota on Thursday was fairly average. But the first significant snow of the season was still enough to snarl morning traffic, give some students an unplanned day off school and greatly increase the odds of a white Christmas.

Minneapolis and St. Paul declared snow emergencies with instructions for vehicle owners to revisit their parking plans. The emergencies in both cities go into effect at 9 p.m. Thursday.

By midmorning, 4 to 6 inches of snow had fallen in many areas, including the south Twin Cities metro and southeastern Minnesota. A second wave of snow pushing into the metro area as lunchtime approached was poised to bring more.

What had already fallen was enough to create headaches on for drivers. By 9 a.m., the State Patrol had responded to nearly 100 crashes across the state and 28 vehicles that went into the ditch. Troopers also reported a handful of spinouts and five jackknifed semitrailer trucks.

Ice- and snow-covered roads kept commuters crawling even as Taylor Drift, Clearopathra and the rest of the Minnesota Department of Transportation’s named and unnamed plows tried to clear them. Snow falling at about a half inch to an inch an hour made the job tough, said agency spokeswoman Anne Meyer.

“This will be a long day with long impact,” Meyer said. ”The longer it snows, the longer it will take to clear off the roads.”

Main roads in the metro were mainly wet as of noon. But wind coming behind the snow is expected to blow around what fell, making for dangerous driving conditions that should persist until about midnight, when the winter storm warning expires, the National Weather Service said.