Western U.S. Grapples with Extreme Weather Swings: From Heavy Snow to Record Heat

The Western United States has experienced a period of extreme weather shifts, beginning with heavy snowfall in the Sierra Nevada in late February that was followed by a deadly avalanche. This temporary boost to snowpack was then largely offset by a record-setting heatwave across the Western U.S. in March 2026, leading to rapid and widespread snowpack depletion, particularly in California and the Colorado River Basin. Forecasts indicate a continued trend of warmer and potentially drier conditions through spring and early summer.

Late February Snowfall and Avalanche in Sierra Nevada

In late February, the Sierra Nevada mountain range received heavy snowfall, marking the first time this season that snow extended to lower elevations. This event followed a nearly 40-day period of dry and unusually warm conditions, occurring during what has been identified as the warmest winter on record for much of the Western U.S. The heavy snow led to widespread travel disruptions, including extended closures of major mountain highways.

The snowfall was followed by an avalanche near Castle Peak, west of Truckee, California, which resulted in the deaths of nine backcountry skiers.

This incident represents the deadliest avalanche in contemporary California history and the single deadliest in the United States since 1981.
Contributing factors to the avalanche included a weak and icy snow layer formed by previous warm temperatures and freeze-thaw cycles, combined with an intense blizzard that deposited 4-6 feet of snow on nearby peaks over 2-3 days. The National Weather Service and the Sierra Avalanche Center had issued advance warnings regarding a high backcountry avalanche risk.

During this period, the Central Sierra Snow Lab recorded its 10th snowiest 24-hour period and 9th snowiest 72-hour period. A notable accumulation was 111 inches over a 5-day period, ranking as the 3rd snowiest such period on record.

Western U.S. Snowpack Conditions and Water Outlook

The heavy snowfall in late February significantly increased snow water equivalent (SWE) in the central and southern Sierra Nevada, with many high-elevation sites reporting near to above-average levels. However, lower-elevation sites and areas in the Northern Sierra and Shasta Mountains remained below average. California's overall water situation was considered stable following a wet start to the season in central and southern California and this mid-winter increase in Sierra snow water.

Across the broader Western U.S., the February snowfall was primarily concentrated in California, with less significant amounts observed in the Cascades and Rockies. While overall snowpack conditions improved slightly across most of the West, only California and parts of the northern Rockies were near average. Extremely low snow conditions persisted in the Pacific Northwest, Great Basin, and central/southern Rockies. The Colorado River Basin continued to report period-of-record lows for Upper Basin SWE.

Early March Atmospheric River Forecast

Following the late February snowfall, a warm and moist atmospheric river was forecasted to affect Northern California and the Pacific Northwest. This event was expected to bring widespread rain on snow across most of Northern California’s mountainous areas, with snow levels projected to be above 10,000 feet, and at times over 12,000 feet.

Flood risk was anticipated to be localized and minor to moderate. This was due to the atmospheric river lacking strong synoptic support, meaning precipitation would largely depend on orographic lifting, resulting in moderate rainfall (1-3 inches total) in windward mountain slopes without intense downpours. Additionally, the thick snowpacks in the higher Sierra were expected to absorb most of the rain, temporarily increasing water storage and limiting runoff. Localized urban/street flooding was possible in some areas due to clogged drains. Widespread or severe flooding was not expected.

March 2026 Heatwave Across the Western U.S.

March 2026 was recorded as the warmest March for at least one-third of the continental United States. A heatwave lasting approximately 10 days, from March 15 to March 26, caused numerous locations across the western two-thirds of the U.S. to break all-time March monthly heat records, with some locations also breaking April or May records. Over 100 monthly records and more than 700 daily records were broken or tied nationwide during this period.

The event in the American Southwest was considered among the most statistically anomalous extreme heat events ever observed in the region at any time of year, based on departures from typical seasonal temperatures. Near-shore ocean surface temperatures along much of the central and southern California coast also reached record-high values. A substantial contribution to the heatwave's magnitude and persistence was attributed to anomalous upstream diabatic heating in the Central Pacific. Scientific research indicates a connection between record heatwaves and climate change, with baseline warming contributing to the intensity of extreme heat.

Impact on Snowpack and Water Resources

The March heatwave caused a significant decline in the Water Year 2025-2026 snowpack across nearly all of the American West. Snowpack was already below average in many areas before this event. In California, mountain snowpack fell to approximately 10% of the historical median for the calendar date by March 27, tying for its lowest value on record for late March (previous record 2015), despite the major Sierra snowstorm in February. Most remaining snow water equivalent (SWE) was at very high elevations. Projections indicated a potential for the earliest SWE melt-out on record for California if the extreme melt rate were sustained.

Major reservoir levels in California remained at or above average seasonal levels, allowing reservoir operators to manage the early snowmelt and ensuring sufficient water for the current year. However, a significant divergence between spring reservoir levels and rapid draw-down later in the season is anticipated due to a projected lack of new snowmelt inflows beginning in late April.

In the Colorado River Basin, which faces a multi-decadal drought, the heatwave reached high elevations of the Rocky Mountains, leading to a decline in snowpack during what is typically the peak accumulation period. While Colorado Basin SWE was already near or below modern record lows for most of the season, it reached all-time record lows for the calendar date by March 28.

Upper Colorado Basin SWE was approximately 4.3 inches, compared to a calendar date median of 14.3 inches and a SNOTEL-era record low of 10.3 inches, representing a nearly 60% reduction below the previous record low for this time of year.

Near-Term Weather Outlook

Following the March heatwave, a pattern change was expected to bring cooler and locally wetter conditions. A transient trough was projected to replace the persistent Western ridge, resulting in temperatures near or slightly below average for a few days. Widespread, modest precipitation (generally under 1-2 inches of liquid equivalent over 7-10 days) was anticipated for the Pacific Northwest, northern and central Rockies, the California North Coast, and northern/central Sierra Nevada. Southern California was expected to remain largely dry. The initial wave of this system was expected to be relatively warm, potentially resulting in rain on mountaintops, with a subsequent colder wave possibly bringing modest accumulating snowfall at higher elevations.

Long-Term Forecast and Drought Status

The general trend for mid-April and into early summer indicates warming and drying conditions across the American West. Forecast models suggest a moderate likelihood of another, weaker ridge developing across the Western U.S., leading to warmer than average temperatures and lower-than-typical odds of April precipitation. A strong consensus points to much warmer than average conditions across the entire American West, including the immediate California coast, through the remainder of spring and early summer.

California has exited its multi-year drought for the time being and is currently the only Western U.S. state not experiencing acute drought. However, this warming and drying trend is not favorable for the ongoing severe snow drought across the Intermountain West and the Colorado River Basin, where snow water equivalent is expected to decrease rapidly relative to seasonal norms.