Recent Flooding Impacts California

Written by Aaron Studwell, Ph.D. for IIR News (Sugar Land, Texas)

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Summary

Late-December storms in Southern California produced rapid runoff, flooding, outages and pipeline risks, boosting reservoirs via rain-driven inflows while leaving water-year and energy reliability highly sensitive to snowpack development and the late-winter storm track.

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Late-December Deluge

California's late-December onslaught of storms has produced a significantly different impact than earlier-season precipitation, driven less by rainfall totals and more by the compounding effects of antecedent moisture, terrain-focused enhancement and the storm's slow movement.

Basin-average rainfall of several inches across the Los Angeles Basin masked significantly higher accumulations along the south- and southwest-facing slopes of Southern California's Transverse Ranges, where repeated orographic lift converted high moisture efficiency into rapid runoff. While surface soils had partially dried since the Christmas event, subsurface moisture and river flows remained elevated, allowing even moderate rainfall rates to translate quickly into stream rises, roadway flooding, and debris flows in steep terrain and burn scar environments.

Hydrologically, the response was fast and nonlinear. Reservoir inflows across the South Coast increased sharply during the event window, forcing reservoir operators to manage both flood control and storage optimization simultaneously. Lake Cachuma provides a clear illustration of this inflow response with levels rising on the order of 15 feet over a five-day period as runoff accelerated out of the Santa Ynez watershed. High-rate releases through Bradbury Dam were initiated to manage additional forecast inflows, underscoring that this was not merely a supply-positive rain event but one that carried downstream flood implications as well.

Click on the image at right for the National Oceanic and Atmospheric Administration Weather Prediction Center's excessive rainfall outlook map.

Energy Impacts: Power Infrastructure and Pipeline Exposure

Electrical infrastructure impacts were concentrated at the distribution level and operationally meaningful. Thousands of customers experienced outages across the Los Angeles metro area during peak rainfall and wind periods, with the most persistent disruptions occurring in hillside, canyon and foothill communities. Failure modes followed a familiar winter-storm pattern: pole and conductor damage from wind and saturated soils, localized substation exposure where drainage capacities were exceeded, and repeated feeder faults tied to debris and slope instability.

For power grid operators, restoration timelines were driven less by damage severity and more by access constraints, as washouts, mudslides and road closures limited crew mobility and increased labor hours per outage.

Natural gas infrastructure faced a higher risk. Prolonged rainfall elevated geotechnical instability along pipeline corridors across the Transverse Ranges and northern Los Angeles County, where cut-and-fill slopes and older alignments are particularly sensitive to ground movement. A reported major gas line rupture near Castaic, likely associated with rainfall-induced slope failure, highlighted the dominant hazard pathway for Southern California pipelines: landslide and soil displacement rather than direct fluvial scour. Even isolated ruptures of this type can force precautionary pressure reductions, service curtailments, or extended repairs, introducing asymmetric supply risk during periods of elevated winter demand.

Changes in the 2025--2026 California Water Year Outlook

From a water-year perspective, the past month has produced a meaningful shift in expectations for 2025--2026. Early outlooks reflected caution following a dry autumn, with uncertainty around reservoir recharge and snowpack development. Since then, successive Pacific systems have delivered substantial precipitation, in some cases several hundred percent of normal water year-to-date. This pattern has improved reservoir storage trajectories and short-term supply indicators across Southern and Central California.

As of Wednesday, provisional California Snow Telemetry (SNOTEL) measurements showed a mixed early-season snowpack picture with important implications for spring runoff and water supply reliability. Snow water equivalent (SWE) at many Sierra Nevada sites varies around or below median levels, with basin indexes near the high-80s to mid-90s percent of median, even while precipitation totals remain well above average given recent storms.

These values reflect a season that has seen significant rain-driven moisture but limited accumulation of deeper snow at mid-elevations, particularly compared with historical early-season peaks. Independent media snow surveys corroborate that statewide snowpack sits below average in terms of total water content, a function of warm storms yielding rain instead of snow at lower elevations despite decent depth figures at select high-elevation SNOTEL sites. For energy and water planners, this means reservoirs benefit now from rain-driven inflows, but the lack of robust SWE at SNOTEL sites raises forward risk, since spring melt and peak runoff typically depend on a robust mountain snowpack later in the season.

Data from the U.S. Bureau of Reclamation show multiple key reservoirs entering late December well above historical averages for the date, materially reducing near-term drought risk compared with expectations a month earlier. Statewide reservoir storage was being characterized as well above seasonal averages with several major reservoirs materially above historical average for the date. The practical interpretation is that the market should distinguish between (1) immediate operational resilience improved by high reservoirs and active flood-control releases and (2) forward allocation and summer reliability still dependent on snowpack evolution and the late-winter storm track.

Renewed Flooding and Severe Weather Threats Through the End of the Week

California's near-term risk environment remains elevated into the upcoming weekend. A slow-moving, upper-level low and coastal surface low focused another round of moderate to heavy rainfall across Southern California, with the most efficient rainfall expected late Wednesday through Thursday morning. Southerly flow ahead of the low transitions to west-southwesterly flow, maximizing upslope enhancement during the period of strongest forcing along the Transverse Ranges.

While rainfall was expected to diminish Thursday as the system weakened inland, soils and channels were already primed, raising confidence in renewed runoff and debris-flow concerns during the heaviest rainfall window. In addition, a marginal severe weather risk exists along the immediate Southern California coast, where modest low-level moisture and cooling aloft may support isolated thunderstorms capable of locally damaging winds or a brief tornado. Any convective enhancement during peak rainfall would increase outage risk and further complicate restoration in terrain already stressed by hydrologic impacts.

Key Takeaways

• Late-December storms in Southern California produced rapid runoff, flooding, outages and pipeline risks.
• Thousands of customers experienced outages across the Los Angeles metro area during peak rainfall and wind periods.
• Prolonged rainfall elevated instability along natural gas pipeline corridors.

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