NOAA's National Centers for Environmental Information
Approximately 25 percent of the contiguous U.S. (CONUS) was covered by snow at the beginning of December according to NOAA's National Snow Analysis. Snow was present in the Sierras and from the Pacific Northwest to the central Rockies and into the northern Plains and Upper Midwest and portions of the Great Lakes and Northeast. The minimum extent for the month—23.3 percent—occurred on the 5th, prior to several notable winter storm events. One such event peaked on the 17th with 44 percent snow cover across much of the mountainous West, northern Plains, Midwest, Great Lakes and Northeast. Snow cover remained steady over the next week and increased to more than 55 percent on Christmas Eve — the maximum snowcover for the month. Snow covered much of the contiguous U.S., from the mountainous West to the Tennessee Valley and from the Tennessee Valley to the New England. During the final week of the month, warmer temperatures reduced the snow cover across much of the central Plains, Great Lakes and in the Ohio and Tennessee valleys and in parts of the Northwest, northern Rockies and Northeast. By December 31, snow cover was reduced to 32.9 percent coverage.
According to NOAA data analyzed by the Rutgers Global Snow Lab, the December snow cover extent was 1,327,000 square miles, 146,000 square miles above the 1981-2010 average. This was above the 30-year mean and the 14th-highest value in the 57-year satellite record. Above-average snow cover was observed across much of the Cascades, Sierras, Great Basin, Rockies, northern Plains, western Great Lakes and parts of the Tennessee Valley and Northeast. Below-average snow cover was observed from the southern Rockies to the southern Great Lakes and in parts of the Mid-Atlantic, Northeast, Pacific Northwest Coast and southern Sierras.
Melting of winter and spring mountain snowpack provides a crucial summer water source across much of the western United States. The total annual water budget for agriculture and human use in the mountainous West is highly dependent on the amount of snow melt that will occur in spring and is proportional to the amount of snow on the ground, which can be approximated by a measure of the snow water equivalent (SWE).
On December 31, SWE values were at or above 100 percent of median across most of the western U.S. with portions of the Sierras and Great Basin reporting SWE values in excess of 200 percent of median. By the end of December, SWE values across much of the southern Rockies and Washington Coast were less than 75 percent of median at the end of the month.