|January 27 2021
Western U.S. Atmospheric River Event (1/26 – 1/29)
By: Rich Otto, WPC Meteorologist
A powerful winter storm affected the western United States between 26 and 28 January 2021, with the greatest impacts from the storm occurring across California in the form of heavy mountain snow and flooding rainfall. Widespread snowfall totals in excess of 48 inches were observed in the central to northern Sierra Nevada while 10 to 15 inches of rain was reported along the central to southern Coastal Ranges of California. There were numerous reports of damage as a result of debris flows and strong winds across the state during the event.
On 26 January, a large closed mid-level low/trough began approaching the West Coast of the United States along with an accompanying cold front at the surface. A leading mid-level shortwave trough axis and the accompanying surface front reached the coasts of Oregon and northern California shortly after 0000 UTC 27 January. After the shortwave trough move inland, colder air behind the cold front temporarily changed rain to snow for lower elevations of the northern Sacramento Valley. A 160 to 180 kt jet maximum at 250 mb, located on the west side of the longwave trough axis over the Pacific, allowed for amplification of the broader mid-level trough toward the south. Amplification of the trough axis off of the West Coast resulted in either unchanged or slightly higher heights along downstream locations of the central to northern coast of California. This evolution of flow aloft supported a slowing and temporary stalling of the cold frontal and heavy rainfall progression from roughly 1200 UTC 27 to 28 January along the central coast of California.
Ahead of the cold front, precipitable water values near the central coast of California peaked near 1 inch for short periods of time, but were more commonly below 1 inch for the majority of the event. Southwesterly wind speeds between 50 and 60 kt were present at 850 mb ahead of the slow moving cold front, directed into the central coast of California, with upslope flow downstream into the higher terrain providing orographic enhancement to the lifting of the relatively warm and moist Pacific air. The strength of the low level wind helped to compensate for precipitable water values near the coast that were only 1 to 2 standard deviations above the mean. The combination of Pacific moisture and low level winds supported Integrated Water Vapor Transport (IVT) values ranging between 500 and 700 kg/m/s (4 to 5 standard deviations above the mean) for 24 to 48 hours along portions of the central coast of California. The duration of IVT resulted in a category 2 Atmospheric River for California, using a recently developed scale that ranks Atmospheric Rivers from 1 to 5 (Ralph et al. 2019). Rainfall rates above one-half inch per hour were common along the Coastal Ranges and occasionally peaked over 1 inch per hour. While instability was largely absent during the event, which is notable given the relatively modest moisture and high rainfall rates, lift was enhanced within the entrance region of a 130 kt upper level jet streak which move inland across central California near 0000 UTC 28 January.
As the mid-level trough located just offshore of the West Coast began to translate eastward after 1200 UTC 28 January, the cold front started to advance to the southeast with the moisture plume reaching the Transverse and Peninsular Ranges of southern California near 0000 UTC 29 January. During the 24 hour period between 1200 UTC 28 and 29 January, wind speeds at 850 mb weakened into the 30 to 50 kt range and the plume of moisture thinned and lessened in magnitude as well. However, heavy rain and mountain snow continued for southern California with locally heavy snowfall in the San Gabriel, San Bernardino and San Jacinto Mountains.
Snowfall accumulations for the multi-day storm totaled over 48 inches for a broad section of the central Sierra Nevada. Localized storm totals over 84 inches were recorded with 94 to 107 inches reported at Mammoth Mountain Ski Area. In addition, accumulations of 88 inches were reported at both Dodge Ridge and Kirkwood Mountain, California. Strong winds accompanied the storm system with peak wind gusts of 50 to over 100 mph which caused blizzard conditions. Winds at Alpine Meadows (on the west side of Lake Tahoe) peaked at 125 mph and 103 mph just south of Kirkwood Meadows, California.
They heavy snow resulted in the closure of multiple roadways, including the major thoroughfares of Route 50 near the crest of the Sierra Nevada, I-5 to the north of Redding, CA and I-80 through Donner Pass during the storm. Strong winds also contributed to reports of over 400,000 power outages during the peak of the storm according to poweroutage.us. Portions of El Dorado County made the decision to cancel COVID-19 vaccinations due to the inclement weather conditions on 28 January, at a time where the state of California was experiencing over 500 deaths per day due to the Coronavirus.
Lower elevation snow was also reported from the storm, with Redding, California measuring 1.5 inches of accumulation on 27 January. While this was not a daily record for the date, 1.5 inches tied for the airport’s thirteenth highest daily snowfall for a 24 hour period since record keeping began in 1893. However, snowfall totals for the storm in the city itself ranged between 4 and 7 inches given the elevation of the city is higher than the airport, as much as 300 feet higher for western portions of Redding.
Heavy snowfall fell across other portions of the western United States during the storm as well with portions of Idaho seeing over two feet of accumulation. McCall, Idaho picked up 25.5 inches of snow ending 28 January which was the city’s fifth highest 2-day snowfall total since record keeping began in 1905.
The storm will also be remembered for heavy rainfall as 10 to 15 inches of rain was reported in the Santa Lucia Range over the 72 hour period ending 1200 UTC January 29. Rainfall rates over 1 inch per hour at times contributed to numerous reports of flooding, flash flooding and debris flows, extending from near San Francisco Bay to the Peninsular Ranges of southern California. A segment of Highway 1 near Big Sur, CA collapsed into the ocean after a debris flow originated from the Dolan Burn Scar. Other debris flows were reported in the Coastal Ranges resulting in damage to homes and neighboring infrastructure. Heavy rain also set numerous daily maximum rainfall records across California, including 2.49 inches at Modesto, California on 27 January along with 2.94 inches at Paso Robles Airport and 2.24 inches at Santa Barbara Airport on 28 January.
A severe to extreme drought was affecting much of California on 26 January, leading up to the heavy precipitation event. While improvement was noted on the United States Drought Monitor a week later on 2 February, changes to the drought classification categories were small, suggesting that a prolonged period of precipitation would be needed to have any significant improvement on California’s drought. Heavy snowfall added to the Sierra Nevada during the 26-28 January event at least helped to mitigate the below average mountain snowpack, a significant source of California’s water supply.