http://www.auseco.com.au/?action=page.view&page=133

Ice Albedo Feedback
The reflectivity of the earth’s surface is called its albedo. Heat and light from the sun will either be absorbed by the earth or reflected back into space. If the albedo is high, more radiation is reflected and the earth cools. If the albedo is low the earth’s surface absorbs radiation and the world will warm. As the earth cools during one of earth’s orbital cycles, snowfields and ice sheets expand increasing the earth’s albedo. The increase in albedo increases surface reflection of heat and light resulting in further cooling of the earth. This in turn increases the albedo and the earth quickly begins to enter and ice age.

Basicly, if snow is covered the Entire United states, it can reach a critical point, where it's reflecting too much heat to be warmed up, so the rains in March and April, won't be rain, but snow, massive amounts of spring snow, which in turn, reflects more heat. A huge area of snow cover, such as they have all across the northern hemisphere right now, could precipitate an ice age.