Winter storm to our south today, Thanksgiving rain and snow possible

Now that temperatures finally feel like a real November, it's time for a major snowstorm. And that's what is going on just to our south. Winter storm warnings have been posted for most of Iowa and east to Milwaukee and Chicago.

Heavy snow has been falling at rates of one to two inches per hour as the storm crosses Iowa this Friday afternoon. Visibilities are down to around a quarter of a mile in moderate to heavy snow with gusty winds across northern Iowa late this afternoon. Most locations in the warned area will pick up five to 10 inches of snow, with some places likely picking up 12 inches or more.

Snow has been falling on the fringe of this fast-moving storm in Minnesota's southern counties. Accumulations of one to four inches are likely by tonight, especially near the Iowa border.

Radar was tracking the snow eastward this afternoon, mainly along the I-90 corridor.

The chillier-than-normal weather will hang around for the weekend. Lows Saturday morning are likely to be in the teens in much of the state with low 20s from the Twin Cities to the southeast corner, followed by highs only from the low 20s to the low 30s. The temperature at kickoff for the Golden Gophers football game at 11 a.m. Saturday probably will be near 25.

Sunday will be milder. Expect highs in the 30s. The temperature should be about 35 at kickoff for the Vikings-Packers game and then fall slowly as the afternoon becomes evening.

A weak low pressure system from Canada will slide southeastward and bring a little snow to mainly northeastern Minnesota Sunday afternoon into Sunday night.

Uneventful weather Monday, Tuesday and Wednesday will be followed by a storm from the southwest beginning Wednesday and continuing through Thanksgiving and possibly beyond.

Forecast tracks and storm arrival times are highly uncertain so far ahead of time, but Thanksgiving week involves travel for many of us so an early look can set the stage for future updates. The long-range GFS model is bringing the heaviest precipitation from Iowa across extreme southeastern Minnesota toward Wisconsin.

Of course, precipitation type -- rain versus snow or some mix -- is extremely important and can be difficult to forecast very far in advance.

One tool meteorologists use is called "critical thickness." We look at the thickness between various levels in the atmosphere, such as the 1000 millibar (near the surface) up to 500 millibar thickness to try to determine where the atmosphere will be cold enough to cause snow to form and remain frozen all the way to the ground instead of melting into rain.

There are several of these critical thickness parameters to consider. When plotted on top of forecast precipitation, they give us an idea of where rain is likely to fall on the warm (usually southern) side of the plotted lines and where snow is likely to fall on the cold (usually northern) side of the lines. Near the lines, the precipitation type can be highly uncertain and can even fall as a mix.

Below is one of the forecast critical thickness maps for Thanksgiving Day. Also shown in shades of purple, for the weather geeks, is 850 mb average relative humidity.

This model would bring rain to southeastern Minnesota and a little snow where precipitation falls northwest of the Twin Cities. The metro is in between where we could get rain changing to snow or a rain-snow mix.  The uncertainty can be addressed in future model runs.