Major snow droughts in December in areas such as Iowa and Wisconsin will see some relief this weekend. Several inches of snow will blanket areas of brown, barren land by Sunday morning. An area of low pressure currently dumping heavy amounts of snow in the higher elevations of Arizona will makes its trek northeastward over the next few days into parts of the Midwest. As the surface low pressure deepens over northern Illinois, a narrow band of nearly a half of a foot of snow will set up on the northwest side of the track. Currently, models are hinting that the band will set up from central Iowa through northern parts of Lower Michigan, where lake enhancement should provide an amplification in snow totals. Furthermore, eastern Wisconsin could see an inch or two more than surroundings areas as winds will be blowing westerly off the relatively warmer waters of Lake Michigan for a period of time. This feature will create steep and unstable lapse rates, which may allow for some convection, or thunder in such areas, assuming the dynamics works out as modeled.
It appears there will be two rounds of precipitation that will affect the same areas, but are still associated with the same surface low pressure minimum. Vorticity (on the right) is basically a measure of spin in the atmosphere, and the more of it, the more upward motion seen in an area, which will then increase the rate at which precipitation falls and accumulates. The first part of the system will move through Saturday morning, and could create some quick hitting snows, but much of this energy will be in the warm, or rainy part of the storm. The more prolonged area of vorticity, "2" will pass through Iowa and Wisconsin by Saturday and will bring with it the bulk of the snow during this time. The exact location of highest values of vorticity will determine who sees the heaviest snow. We will also need to watch where the rain/snow line and thunderstorms set up. The pivoting thunderstorms into cold air can be a tell tale sign off highest snow rates. The NAM model on the left is the northern-most solution, but it brings rain into southern Wisconsin, which will certainly dampen snow totals. The transition line from rain to snow looks to be a quick one, meaning ice accumulation does not look to be much of an issue as vertical profiles through the storm have minimal inversions to create ideal icing conditions. Meanwhile, another weakening storm system will dump a quick hitting light fluffy snow near the Canada/US border. This system will pull the stronger cyclone to the north. However, with temperatures so cold, the snow will pile up quickly, as reflected by our snow map below:
