|December 24 2021|
Western U.S. Heavy Snow Event: (12/22 - 12/28)
By: Bryan Jackson, WPC Meteorologist
The apex of a particularly active December weather pattern for the West Coast was a multi-day series of waves that brought heavy snows (including several records) and particular cold to the western U.S. from December 22 to 28, 2021. A rather zonal pattern that brought Pacific moisture and mountain snows to fairly narrow corridors of the Pacific Northwest and northern Rockies December 18 to 21 gave way to a much more meridional pattern starting on December 22 as a positively tilted upper trough off the West Coast shifted east, reaching the Pacific Northwest by the 23rd and southern California on the 24th. A series of powerful shortwave troughs originating from the Gulf of Alaska then ran down the length of the West Coast December 24 to 26 before shifting inland east across the northern Rockies December 27 and 28. Despite the main trough axis now inland, additional reinforcing shortwave troughs moved down the West Coast into December 29. This made for a nearly continuous weeklong winter event that featured snow levels down to sea level for several days across the Pacific Northwest and into northern California, extreme snows (particularly over the Sierra Nevada), and bitter cold over the Pacific Northwest through the northern Great Plains.
Three consecutive waves produced the bulk of the winter weather through this prolonged period with each bringing subsequently colder conditions to the region and directing considerable moisture inland over the Coast Ranges, Cascades/Sierra Nevada, and across the Intermountain West through the Rockies: 1) the initial amplifying trough that shifted into the West Coast December 23 and 24. 2) a subsequent trough that pushed south down the West Coast from a low over Vancouver Island to the central California Coast later on December 24 through December 25. 3) a reinforcing shortwave trough that rounded an even deeper low over Vancouver Island on December 26 into December 27.
This prolonged period of snow and cold that centered on Christmas made for multiple extreme impacts to travel and infrastructure across the western U.S
Storm total snowfall through this stretch was notable with 10 of the 11 western states (all but Arizona) having at least one reporting site with a foot of snow and seven western states with at least one report of two feet of snow: Washington, Oregon, California, Idaho, Nevada, Wyoming, and Colorado. Along the crest of the northern Sierra Nevada snowfall totals ranged from 6 to 10 feet between December 21 and 28. This sequence of storms contributed 118” of the 212” of snow recorded at the Central Sierra Snow Lab at Soda Springs, CA during December, which is their third highest monthly total dating back to 1970. This repeating heavy snow brought the seasonal snow total at that location to 70% of average annual snowfall which is beneficial given continued drought and especially since there were rather dry conditions across California in November 2021 and January/February, 2022.
The heavy snow along with subsequent avalanches closed most if not all of the mountain road passes through the Cascades and Sierra Nevada. Interstate 80 was closed over the crest of the Sierra Nevada from early December 25th to the afternoon of the 28th. According to Caltrans, nearly $5M of commercial goods are transported each hour through this road corridor, so the multi-day closure of just this stretch of highway likely had economic impacts in the hundreds of millions of dollars. The first avalanche fatality of the season in Colorado was a skier on December 24 in Jackson County. Two snowmobilers were killed in an avalanche in Montana on December 26. Sporadic power outages were noted through the storm with approximately 90,000 customers without power in Washington, Oregon, Nevada, and California with many of the outages in the Sierra Nevada lasting for several days.
On December 26, 3 to 7” of snow fell across the Puget Sound area which was rather impactful to road and airport travel with hundreds of flight delays and cancellations. Also, snow bands enhanced by the Strait of Juan de Fuca on northeasterly flow produced 12 to 16” of snow on the northeastern Olympic Peninsula including Port Angeles. Particularly cold air spilled in during this period with record low temperatures recorded across much of western Washington on December 26 and 27. The low of 17 degrees at Sea-Tac Airport on the 27th is the lowest temperature there since 2010. The high temperature of 23 degrees on the 27th was the lowest maximum temperature since 1990. One cold weather-related death was reported in Seattle on the 27th.
Power Outages: ~9000 customers in WA, ~7000 customers in OR, ~69000 customers in CA, ~5000 customers in NV.