The rate at which snow falls is just as important for travel as the total amount that falls in a storm. On the evening of Saturday January 1, 2005, Duluth, Minnesota rang in the New Year with a fireworks display nature style. Thundersnow dumped 6 inches of dense moisture laden snow in two hours at the National Weather Service Office. Snowfall rates of 3 inches or more an hour are not unprecedented for strong convective snow bursts but are rare for any specific location. For about 20 minutes the snow was so intense and the visibility so low that the city government ordered snow removal vehicles off the road because of the weather conditions. The heavy snow fell from roughly 6:30 PM to 8:30 PM. If the snow had fallen Friday Evening New Year's Eve or between 6 AM and 6 PM on a week day the impact would have been much more significant.
The National Weather Service uses specific criteria to indicate how much snowfall is needed to cause major travel problems or economic disruption. The snowfall amounts and the length of time over which the snowfall occurs are both incorporated into the thresholds. In most of the Middle and Upper Mississippi Valley, snowfall of 6 inches or more in 12 hours or 8 inches or more in 24 hours triggers a warning for heavy snow. Slightly lighter amounts are covered by an advisory but may still trigger a warning if other elements like very strong winds are expected. Lesser amounts of snow, if combined with heavy accumulations of sleet or freezing rain, may also require the issuance of a warning.
Warning criteria vary greatly depending upon how much experience a region has in dealing with snow. Note the following list of threshold values used by the National Weather Service in different parts of the country. Snow totals would need to reach or exceed these numbers.
8 inches in 12 hours
10 inches in 24 hours
6 inches in 12 hours
8 inches in 24 hours
4 inches in 12 hours
6 inches in 24 hours
2 inches in 12 hours...Good grief, that's just flurries
4 inches in 24 hours
LiteratureCall, D. A., 2005: Rethinking snowstorms as snow events: A regional case study from Upstate New York. Bull. Amer. Meteor. Soc., 83, 37-51.
Kocin, P. J., and L. W. Uccellini, 2004b: A snowfall impact scale derived from Northeast storm snowfall distributions. Bull. Amer. meteor. Soc., 85, 177-194.