Western United States snowpack generally remains below average, with exceptions in the central and southern Sierra Nevada in California and parts of the northern Rockies. Record or near-record low snowpack persists across much of the rest of the West, including the entire Colorado River basin.
Sierra Nevada Snowfall and Avalanche
Last week, the Sierra Nevada received heavy snowfall, extending to lower elevations for the first time this season. This followed a nearly 40-day period of dry and unusually warm conditions, occurring during what is identified as the warmest winter on record for most of the Western U.S., including many California mountain areas. The snowfall caused widespread travel disruptions, resulting in the closure of all major mountain highways in the Sierra Nevada for extended periods.
The weather shift was followed by a tragic avalanche near Castle Peak, west of Truckee, which claimed the lives of nine backcountry skiers.
This incident marks the deadliest avalanche in contemporary California history and the single deadliest in the United States since 1981.
Risk factors contributing to the event included a weak and icy snow layer formed by warm temperatures and freeze/thaw cycles in preceding weeks. This was combined with an intense blizzard-like snowstorm that dropped 4-6 feet of snow on nearby peaks over 2-3 days. This combination led to a high risk of snow slab failures. The National Weather Service and the Sierra Avalanche Center had issued advance warnings regarding the high backcountry avalanche risk.
Snowfall Statistics and Water Outlook
The Central Sierra Snow Lab recorded its 10th snowiest 24-hour period and 9th snowiest 72-hour period during this event. The most notable statistic was a 5-day accumulation of 111 inches, ranking as the 3rd snowiest such period on record.
This heavy snowfall substantially bolstered central and southern Sierra snowpack, with many high-elevation sites now reporting near to above-average snow water equivalent (SWE). However, many lower-elevation sites and areas in the Northern Sierra and Shasta Mountains remain below average. California's overall water situation is stable given a wet start to the season in central and southern California and this mid-winter increase in Sierra snow water.
Western US Snowpack Conditions
The recent snow deluge was concentrated primarily in California. While snow also fell in the Cascades and Rockies, amounts were less significant. Overall snowpack conditions improved slightly across most of the West, but only California and parts of the northern Rockies are near average. Extremely low snow conditions persist in the Pacific Northwest, Great Basin, and central/southern Rockies, with many locations remaining at or near record low SWE. The Colorado River Basin continues to report period-of-record lows for Upper Basin SWE. Dry and warm conditions are forecast to return to much of the West in the coming days.
Upcoming Warm Atmospheric River
A very warm and moist atmospheric river is forecast to affect Northern California and the Pacific Northwest this week. While the integrated water vapor transport is significant over the ocean, a lack of a strong low-pressure system near the California coast is expected to attenuate its impact inland, reducing the potential for severe flooding.
This event will bring widespread rain on snow across most of Northern California’s mountainous areas. Although total column water vapor over California will be near record-high levels for this time of year, weak storm dynamics are anticipated to prevent exceptionally heavy precipitation. Snow levels are forecast to be above 10,000 feet, and at times over 12,000 feet, meaning precipitation will primarily fall as rain even on many mountain peaks.
Two factors are expected to mitigate flood risk:
- The atmospheric river lacks strong synoptic support, meaning precipitation will largely depend on orographic lifting. Rainfall is expected to be light outside of windward mountain slopes, and moderate in those areas (1-3 inches total) over several days, without intense downpours.
- Thick snowpacks in the higher Sierra have a substantial capacity to absorb rain. It is probable that most rain will be absorbed into and refrozen within existing snowpack where more than 2-4 feet of snow remains, temporarily increasing water storage and limiting runoff. Lower elevations with thinner snowpacks are expected to receive more modest rainfall.
Overall, this warm but dynamically weak atmospheric river is likely to result in a widespread but relatively modest-impact rain-on-snow event in Northern California. Some localized and minor-to-moderate flood risk is possible along the North Coast, in the lower foothills of the central/northern Sierra, and in larger Sierra Nevada towns (e.g., Truckee, South Lake Tahoe) due to potential urban/street flooding from clogged drains. Widespread or severe flooding is not expected. The entire state is projected to become increasingly warm and humid.
Warming and Drying Trend
A robust and sustained warming and drying trend is expected to begin in Southern California and then spread to Northern California, with spring-like temperatures anticipated by late February. Southern California is forecast to experience well above-average temperatures, reaching into the 70s and potentially 80s. Northern California coastal and valley temperatures are expected to reach the 60s and 70s. Mountain snowpack will likely begin to melt more rapidly.
A broad ridging pattern is likely to develop across the continental U.S. by mid-March, potentially bringing a quieter weather period compared to the active pattern observed from late January through late February. This trend is not favorable for the ongoing severe snow drought across the Intermountain West, where snow water equivalent is expected to decrease rapidly relative to seasonal norms. However, California has exited its multi-year drought for the time being and is currently the only Western U.S. state not experiencing acute drought this month.