One of the most favored areas for this re-development is in Colorado and the surrounding region. Water Survey climatologists discovered that this area was where most of our severe winter storms in Illinois come from. There are also two other areas that favor development of severe winter storms, although to a lesser extent. The source of the storm - and The track the storm follows Not all low pressure systems that develop in the favorable areas during the winter become severe storms. Thankfully, only a relatively small percentage of all lows that form, regenerate, or intensify in these three source areas actually lead to severe winter storms in Illinois.
Storms tend to form where there are great contrasts in temperature.
During the winter months the lee side of the Rockies and the western Gulf coast provide these battlegrounds for the air masses. Once the storms have developed, the winds in the upper atmosphere determine where and how fast they move, and ultimately if Illinois will be their target. The severe winter storms that affect Illinois tend to follow certain paths. Cold air north of the storm center in the Great Lakes or upper Midwest can then feed into the storm - the necessary ingredient for snow or ice. If the air is cold enough in the winter, the precipitation will be snow, sleet, or freezing rain.