NOAA's National Centers for Environmental Information
It was a snowy beginning to the season for the Rocky Mountains and Plains states as a high-amplitude trough brought record cold temperatures and accumulating snowfall deep into the region over the last half of the month. On October 11-12, snow covered small portions of the northern Cascades, Bitterroots and parts of the northern and central Rockies. According to NOAA's National Snow Analysis, this was the beginning of the 2020-21 snow season across the contiguous U.S. (CONUS). Snow cover ebbed and flowed somewhat over the ensuing week. By October 19, the northern Rockies, Plains and western Great Lakes received snowfall covering approximately eight percent of the CONUS. A major cold air event during the last half of October brought significant and unusually early snowfall to much of the Northern Tier as well as the Rockies and central and southern Plains. Over the course of this event, many locations set records for the most snow so early in the season and for the snowiest October on record. Over the course of the following week, additional rounds of snow fell and spread across the mid-section of the country as cold air continued to plunge southward into parts of New Mexico and Texas. The earliest ice storm on record for Oklahoma occurred on October 27 and impacted more than 350,000 residents as foliage-filled trees brought down powerlines. Approximately 35 percent of the CONUS was blanketed in snow on the 28th and was at peak coverage for the month. Warmer temperatures over the last few days of the month helped to erode snow cover to nearly 8 percent of the CONUS.
According to NOAA data analyzed by the Rutgers Global Snow Lab, the October snow cover extent was 305,000 square miles, 233,204 square miles above the 1981-2010 average. This was the highest value in the 53-year satellite record. Above-average snow cover was observed across much of the Rockies, northern and central Plains as well as in portions of the Northeast. Below-average snow cover was observed over part of the Sierra Nevada range in California.
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). At the end of October, with October marking the beginning of the snow season across the Lower 48, SWE values in excess of 150% of median were observed across a large portion of the northern Cascades, the Bitterroots, and the northern Rockies as well as across portions of the southern Rockies. Below-average SWE values existed across the Sierra Nevada Range, the Great Basin and from the southern Bitterroots into Utah and western Colorado.