Low pressure in northern Illinois has only crawled to the northeast from its position 24 hours ago near St. Louis. That is one reason why the snow has taken so long to get here. Another reason is the big high-pressure center in Ontario sending dry air at us on strong easterly winds.
The upper-air system supporting the low will charge to the northeast into the U.P. during the night tonight into tomorrow. That means that the dry air will be overcome with snow spreading across the U.P. as the night wears on. Snow will come down heavily, especially over northern into eastern Upper Michigan. That??s where a Winter Storm Warning is in effect later tonight into Friday. The heaviest snow over the entire U.P. is expected during the night and then into tomorrow morning over the northeaster half of Upper Michigan. Afterwards, light to moderate snow will fall through Friday night, especially over higher elevations of the northern U.P.
As for amounts, it looks like 4 to 8 inches will fall from Iron River to Iron Mountain southeastward to Menominee where a Winter Weather Advisory is in effect. The least is expected in the far south around Menominee. Six to ten inches of snow is likely over the Gogebic Range through the Porkies into the Copper Country and much of the northern and eastern Upper Peninsula. Over the higher elevations of the north-central U.P. west of Marquette, amounts in excess of a foot are likely.
The low will drift slowly through southern Wisconsin into the northern Lower Peninsula and then off to the northeast on Saturday. We will be caught in its circulation until then and that means at least light snow will fall into Saturday, particularly over northern portions. There will be a break in the action later Saturday into early Sunday, but then a new system just beginning to roll through the Gulf of Alaska will threaten to bring more precipitation starting later Sunday over western portions. At this time, it appears that while the precipitation may start as snow, enough warm air will work in ahead of the low induce a change to rain during Sunday night.