Shown above is a forecast for 500 millibar heights just 5 days from today. We see a sharp dip in the height contours and a cooling of temperatures over the Northern Plains, and it is fron this that we gather where the storm system is. A couple days ago, I was telling of how models would most likely shift back south and give snow to the heart of the Midwest. Right now, model guidance is in good agreement that the storm track will give the heaviest snow to the Upper Midwest and North Plains.
The reason for this track change is the persistent high pressure in the Southeast. If you look closely, you may be able to see how the height contours (in solid black) begin to spread out when you get into the Northeast and Mid-Atlantic. This is symbolic of high pressure in the area, and we know how high pressure deflects low pressure. The problem with this storm system is that it will not be able to dive deep enough south to move in a track favorable for the heart of the Midwest. If high pressure in the West Coast was stronger, the storm would be persuaded to dive further south into the central Plains. If the high pressure in the East was weaker and not as spread across the Eastern Seaboard as it is projected to be, the storm would have an easier time moving more to the east than northeast. The northeast movement is preferred if there is dominant high pressure spread across the East US, as we will see with this storm system.