Officially, the National Weather Service defines a blizzard as a storm which contains large amounts of snow OR blowing snow, with winds in excess of 35 mph and visibilities of less than 1/4 mile for an extended period of time (at least 3 hours). When these conditions are expected, the National Weather Service will issue a "Blizzard Warning". When these conditions are not expected to occur simultaneously, but one or two of these conditions are expected, a "Winter Storm Warning" or "Heavy Snow Warning" may be issued.
Blizzard conditions often develop on the northwest side of an intense storm system. The difference between the lower pressure in the storm and the higher pressure to the west creates a tight pressure gradient, or difference in pressure between two locations, which in turn results in very strong winds. These strong winds pick up available snow from the ground, or blow any snow which is falling, creating very low visibilities and the potential for significant drifting of snow.
The Upper Midwest and Great Plains of the United States tends to be the region that experiences blizzards most often. With few trees or other obstructions to reduce wind and blowing snow, this part of the country is particularly vulnerable to blizzards. However, blizzards can occur in any location that has a climate that experiences snowfall. Northern Arizona can experience blizzard conditions when a strong low pressure system moves across southern Arizona and high pressure builds strongly into the Great Basin. However, these conditions are rarely met due to the infrequency of strong low pressure systems moving through the state.
The strong winds and cold temperatures accompanying blizzards can combine to create another danger. The wind chill factor is the amount of cooling one "feels" due to the combination of wind and temperature. During blizzards, with the combination of cold temperatures and strong winds, very low wind chill values can occur. It is not uncommon in the Midwest to have wind chills below -60F during blizzard conditions. Exposure to such low wind chill values can result in frostbite or hypothermia. For more information, go to the NWS wind chill web page.
Blizzards also can cause a variety of other problems. Power outages can occur due to strong winds and heavy snow. Pipes can freeze and regular fuel sources may be cut off.