A week-long barrage of Sierra Nevada snowfall buried the UC Berkeley Central Sierra Snow Lab under 111 inches of accumulation, a figure that captures both the raw power of California’s winter storms and the complex question of what all that snow actually means for the state’s water supply. The lab, perched near Soda Springs in the Donner Summit area, recorded the total through its manual measurement protocol, taken each morning at 8 a.m. PST. For millions of Californians who depend on Sierra snowmelt for drinking water, irrigation, and hydropower, the difference between headline-grabbing snowfall depth and the water content locked inside that snow is the detail that matters most.
What 111 Inches Looks Like at Donner Summit
The Central Sierra Snow Lab posted its seven-day snowfall total of 111 inches on its homepage, a figure derived from daily board measurements cleared and reset each morning. Because the lab uses a manual method rather than automated sensors alone, each reading reflects a trained observer’s direct assessment of new snow depth, a practice that dates back decades at the Soda Springs site. The lab’s data portal also offers downloadable datasets and seven-day condition plots through its Sky-to-Stream platform, letting anyone reconstruct the storm’s daily progression from raw numbers rather than relying on secondhand summaries.
That level of transparency matters because storm totals reported on social media or in casual weather discussions often conflate snowfall depth with snowpack gain or water content. A single 24-hour period during this event likely contributed well over two feet of accumulation, but snow that falls at different temperatures compacts at different rates. Without knowing the snow water equivalent, the raw 111-inch figure tells only part of the story. Independent verification is possible through NOAA’s Global Historical Climatology Network, which archives daily snow data from reporting stations across the country in near-real-time, offering a federal cross-check against any single lab’s readings and helping place the Donner Summit numbers in a broader regional context.
Snowfall Depth vs. Snow Water Equivalent
Most coverage of big Sierra storms focuses on the dramatic depth numbers, but California’s water managers care far more about snow water equivalent, or SWE, the amount of liquid water contained in a given column of snowpack. The California Department of Water Resources runs the state’s official monitoring network, using a combination of manual snow courses and automated snow sensors spread across the Sierra and other mountain ranges. The long-running Snow Surveys program tracks these measurements at hundreds of stations, and those data feed directly into statewide water supply forecasts, reservoir operations decisions, and seasonal flood outlooks.
The gap between snowfall totals and SWE is not trivial. Light, dry powder can pile up quickly but contain relatively little water per inch, while dense, warm-storm snow packs more moisture into fewer inches of depth. A storm that drops 111 inches of light snow might deliver less water than one that drops 60 inches of heavy, wet snow. DWR’s automated sensors, accessible through the California Data Exchange Center’s snow reports, capture SWE readings at individual stations, but those measurements can vary sharply from one basin to the next depending on elevation, aspect, and storm track. This uneven distribution is exactly why statewide averages can be misleading and why basin-level analysis is essential for both drought planning and the prevention of late-season flooding.
Why Basin-Level Modeling Changes the Outlook
The California-Nevada River Forecast Center, a division of NOAA’s National Weather Service, publishes basin snow analyses that translate point measurements like the Snow Lab’s into regional hydrologic forecasts. These products map SWE across entire drainage basins, showing where storms like this one have concentrated their load and where gaps remain. On a color-coded map, a week-long barrage over Donner Summit can appear as a bright, water-rich core surrounded by areas that received much less, underscoring that the impressive 111-inch total is part of a patchwork rather than a uniform blanket.
That fragmented distribution is the overlooked risk in storms of this magnitude. Much of the public conversation treats heavy snowfall as an unqualified positive for a state that has cycled through severe drought in recent years. But the speed and location of melt matter enormously. If the snow melts gradually over weeks, reservoirs can capture the runoff and downstream levees are less likely to be stressed. If a warm spell or rain-on-snow event triggers rapid melt across a basin that received disproportionate loading, the result can be localized flooding rather than orderly storage. The USDA’s SNOTEL network, which provides independent automated SWE readings at high-elevation sites, offers another data layer for assessing how evenly this storm distributed its moisture, and those observations are available through the NRCS SNOTEL portal for comparison with state-operated sensors.
From Snow to Water Supply Forecasts
California’s official seasonal water supply outlook, published as Bulletin 120 by the Department of Water Resources, integrates snow course data, automated sensor readings, and precipitation records into runoff projections for the state’s major river systems. A storm of this magnitude will almost certainly shift those projections upward, but the degree of change depends on how subsequent weather patterns treat the new snowpack. A string of cold, dry days would preserve the snow in place, allowing SWE to slowly increase as the pack settles and compacts. By contrast, follow-up atmospheric rivers could add moisture and accelerate compaction, raising SWE without adding much new depth, while also increasing the odds of midwinter melt events.
The practical consequence for California residents and water agencies is that a single week of extraordinary snowfall does not, by itself, resolve long-term supply concerns. Snowpack is a seasonal reservoir, and its value depends on the full arc of accumulation and melt from roughly November through June. What this storm does accomplish is a dramatic reset of the baseline. Basins that were running below average now have a substantial cushion, and the state’s monitoring infrastructure (from DWR’s snow courses to federal SNOTEL stations) will track how that cushion evolves as spring approaches. For policymakers and the public following along through official channels such as the statewide California portal, the key is to look beyond the impressive 111-inch headline and focus on how much water is actually stored in the snowpack, where it is concentrated, and how quickly it is likely to move from the mountains into rivers, reservoirs, and, ultimately, taps and fields across the state.
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*This article was researched with the help of AI, with human editors creating the final content.
