National Snow and Ice

Northern Hemisphere Snow and Ice

Approximately 26 percent of the contiguous U.S. (CONUS) was covered by snow at the beginning of December according to NOAA's National Snow Analysis. Snow covered much of the mountainous West, Northern Tier, Great Lakes, Northeast and parts of the Midwest. Snow cover decreased steadily over the subsequent week and reached a minimum extent for the month of 17 percent on the 9th. An active winter weather pattern brought several snow-producing systems to the central Plains, Great Lakes, Ohio Valley and Northeast. Snow cover reached a maximum extent for the month on the 26th with nearly 28 percent coverage across the CONUS. During the last week of December, strong warm air advection brought warm temperatures to much of the eastern half of the CONUS. This allowed the snow extent from the northern Plains to the Northeast to recede before a final blast of cold dipped into the northern Plains on the 31st, bringing the CONUS snow coverage to 24 percent.

According to NOAA data analyzed by the Rutgers Global Snow Lab, the December snow cover extent was nearly 840,000 square miles, 342,000 square miles below the 1991-2020 average, and the ninth-lowest value in the 59-year satellite record. Below-average snow cover was observed across much of the Sierra Nevada range, southern Rockies, Plains, Midwest, southern Great Lakes and Appalachians. Above-average snow cover was observed across parts of the Cascades, central Rockies, Northern Tier and Northeast.

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 November 30, SWE values were above normal across most of the Cascades, Sierras, Great Basin, Bitterroots and parts of the central and southern Rockies. Much of the northern and portions of the central Rockies had SWE values that were 25 to 75 percent of median at the end of the month. By the end of December, SWE values were above normal across the Cascades, Sierra Nevada and Bitterroot ranges. SWE values were less than 70 percent of median across portions of the northern and southern Plains.