|December 31 2022|
West Coast to Upper Midwest Snowstorm: (12/31/22 - 1/4/23)
By: Josh Weiss, WPC Meteorologist
The start to 2023 brought a long duration and impressive winter storm which began as an atmospheric river (AR) event moving onshore California, and ended with an impressive winter storm which spread heavy snow as far east as the Great Lakes. Snowfall amounts reached as high as 4-6 feet in the Sierra Nevada mountains of California (CA), with 1-2 feet common across much of the higher terrain of the Great Basin and Four Corners states. East of the Continental Divide, an impressive swath of snowfall exceeding 6 inches stretched from the Central High Plains northeast through the Upper Peninsula of Michigan.
This event began as a 500mb shortwave trough which amplified while dropping along the Pacific Coast on Dec 31 and then moving onshore and into the Great Basin during the day on Jan 1. This shortwave continued to deepen as it advected eastward through Jan 2, becoming a closed low over the Four Corners. Downstream of this trough, and throughout its evolution, a Pacific jet streak at 250mb intensified to more than 150 kts, overlapping with the confluent mid-level flow southeast of the shortwave to drive copious moisture onshore Dec 31 - Jan 1. This manifested as a strong AR pivoting into CA, with moisture spilling over into the Great Basin Jan 1-2 as noted by precipitable water (PW) anomalies reaching +3 standard deviations above the climatological mean for CA and the Great Basin. Although snow levels rose within the accompanying warm air advection, a southward advancing cold front beneath the primary shortwave moved across the region Jan 1-2, causing snow levels to crash while also providing additional forcing for ascent through the associated baroclinic zone. Additionally, W/SW 850-700mb flow from the Pacific through the Four Corners not only contributed to the moist advection, but also resulted in enhanced orographic lift into the terrain helping to maximize snowfall in the higher elevations.
During this first phase of the system, this evolution resulted in heavy snow across much of the terrain from CA through the Great Basin and into the Four Corners, with the highest snow amounts generally above 7000 ft in the Sierra, as well as across the Wasatch, the Mogollon Rim, the San Juans, and into portions of Wyoming. Lighter snows did reach down into the foothills and valleys as well as snowfall persisted during the period of falling snow levels.
A stronger and consolidated low pressure system began to develop from this same upper level evolution as it moved into the Central Plains Jan 2-3. The closed low across the Four Corners continued to deepen as it shifted into Nebraska (NE), reaching as low as -3 standard deviations below the climatological mean at 500mb, with additional deepening continuing as it moved towards the Great Lakes on Jan 4. Aloft, this was accompanied by increasing diffluence within the left-front quadrant of a northward arcing subtropical jet streak, and the overlap of the primary mid-level height falls with the upper diffluence allowed the surface low to deepen quickly as it moved from the lee of the Colorado Rockies into the western Great Lakes. Downstream of this wave, meridional moisture transport out of the Gulf of Mexico intensified as 290-295K isentropic upglide surged into the Plains and Midwest, with the associated theta-e ridge lifting into an impressive trough of warm air aloft (TROWAL) rotating cyclonically around the surface low.
This warm advection allowed a warm nose at 850mb with temps above 0C to punch northward, resulting in an axis of moderate freezing rain accretion in Iowa (IA) and southern Minnesota (MN). However, it also provided additional instability to support an intense snow band north of the surface low which manifested as snowfall rates of 2-3”/hr with isolated thunder across the Central Plains and Upper Midwest. This low and associated precipitation would be heaviest from late Jan 2 through early Jan 4, with weakening occurring thereafter as the surface low occluded to a triple point across the southern Great Lakes and became vertically stacked beneath the 500mb trough. Moderate to heavy snow continued across the Great Lakes during Jan 4, including some lake enhanced snowfall across the Upper Peninsula of Michigan (MI). Continued weakening through disconnect of the upper ascent and the eastward shift of the best moisture advection away from the surface low caused this event to wind down by the morning of Jan 5.
By the time this storm was over, a swath of snowfall of more than 6 inches continued, almost without any breaks, from the coast of CA through the Upper Peninsula of MI. The heaviest snowfall occurred in the Sierra of CA where the Central Sierra Snow Lab recorded almost 58” of snow, with additional heavy snow accumulating at Mammoth Mountain (54”), South Lake Tahoe (39”), and Truckee (28”.) Other impressive snowfall amounts occurred across much of the Intermountain West terrain, including 52” at Sundance, Utah (UT), 40” at Diamond Peak, Nevada (NV), 35” in Rockwood, Colorado (CO), and 12.1” at Flagstaff, Arizona (AZ). Farther to the east, as the low consolidated, heavy snow measured 27” in Fulton and Lake Andes in South Dakota (SD), 18” in Thedford, NE, 15.1” at the Minneapolis, MN airport, 13” in Rock Rapids, Iowa (IA), and 11” in Hawthorne, Wisconsin (WI).
As the storm moved into CA, it was accompanied by extremely strong winds which exceeded 100 mph in the Sierra, but even reached 50-80 mph in the lower elevations, including 64 mph at Sacramento. This combined with the wet grounds led to significant tree damage and resulted in more than 60,000 power outages across the state. Farther to the east, the heavy snow produced an enhanced avalanche threat which resulted in a long duration closure of Cottonwood Canyon, CO, with an “interlodge” requirement put in place around Alta, UT during the height of the storm, meaning residents and tourists were not allowed to leave their buildings due to the avalanche concerns. The heavy snow and gusty winds also resulted in many road closures due to snow covered roads and multiple jackknifed tractor trailers across central and northern Arizona according to the AZ DOT.
Across the Central Plains and Upper Midwest, blizzard conditions resulted in a 130 mile stretch of I-90 being closed across SD, with almost all roads across central and northern NE declared “impassable” on Jan 3. The Sioux Falls, SD airport was closed during the storm, while more than 200 flights were canceled at Minneapolis. Additionally, the MN DOT reported more than 350 crashes across the state due to the heavy snow rates, which reached 3”/hr at times and was accompanied by thunder and lightning in Albert Lea and Waseca, MN.