The big one?

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Forecast model ensemble paints heavy snow in northern Minnesota Sunday.

You can see them coming, but big winter storms keep many secrets until they arrive.

The numerical forecast models that handle these systems have biases, meaning the model errors in track and timing of storms can be frequent for some models.

This weekend's system promises to pack a punch for much of the upper Midwest.

The developing low is still out over the northern Pacific today, but it appears it will track through Iowa into Wisconsin. That's a perfect track for heavy snow somewhere in Minnesota.

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The big questions that remain are:

-Where will the exact surface and upper lows track?

-Where will the freezing line set up at around 5,000 feet?

-Will there be significant ice?

-Will a "dry slot" develop and limit snow amounts in some areas?

Snow, sleet freezing rain and wind look to affect much of our region Saturday night and Sunday. Right now it looks like the heaviest snow band could set up on a Sioux Falls-Redwood Falls-St. Cloud-Duluth line. Think Highway 23 and you get the picture. Everywhere north and west of that line could see heavy snow. If the system shifts east that may put the metro in play for heavy snow too.

A surge of warm air will push up ahead of this system, bringing a milder day Saturday and causing a mixed bag of precip over southern Minnesota. There may be some freezing rain and significant icing as well.

The rain snow line may set up right over the Twin Cities early Sunday. There could be a burst of heavy snow in the metro after the changeover to all snow as the system moves east Sunday afternoon.

Of course if the system tracks as little as 30 miles one way or another, the weather at your home can and will change.

This will be an awesome system to watch unfold as we head toward Sunday. It will also be a huge challenge for those of us who make our living forecasting winter storms!

Stay tuned.

PH