These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulations are usually accumulations of 0.25 inches or greater. A combination of severe winter weather types occurring over a wide area is usually called a winter storm . Winter-storm formation requires below freezing temperatures, moisture, and lift to raise the moist air to form the clouds and cause precipitation. Lift is commonly provided by warm air colliding with cold air along a weather front.
Various causes exist for winter storms in the United States.
Winter storms in Midwestern and Plains States typically develop over southeast Colorado on the lee side of the Rockies. These storms move east or northeast and use both the southward plunge of cold air from Canada and the northward flow of moisture from the Gulf of Mexico to produce ice, snow, and sometimes blizzard conditions. These fronts may push deep into the interior regions, sometimes as far south as Florida. The occurrence of severe winter weather has a substantial impact on communities, utilities, transportation systems, and agriculture, and often results in loss of life due to accidents or hypothermia. Between 1988 and 1991, a total of 372 deaths, an average of 93 each year, were attributed to severe winter storms nationally.