Forecasters are warning that a powerful El Niño weather pattern is building, with significant consequences for California industrial facilities managing stormwater obligations.
According to NOAA’s Climate Prediction Center, El Niño carries a 97% chance of persisting through early spring 2027, raising serious concerns for facility operators across the state.
NOAA also puts the probability of a very strong El Niño event at 81% during the October through December 2026 period, which covers the critical early wet season.
Southern California typically experiences higher-than-normal precipitation during El Niño years, while Northern California’s correlation is less consistent but a wetter outcome remains possible.
For industrial facilities operating under California’s Industrial General Permit, known as the IGP, increased rainfall means more Qualifying Storm Events and more mandatory stormwater sampling.
The IGP requires most industrial facilities to collect stormwater samples from four Qualifying Storm Events each reporting year, split evenly across two six-month periods.
In drier years, some facilities cite an insufficient number of qualifying storms as a reason for falling short of sampling requirements, but environmental groups routinely challenge that explanation using NOAA rainfall data.
A strong El Niño season could make under-sampling arguments far harder to sustain, and facilities falling short of the four-sample requirement may face intensified scrutiny from plaintiff organisations.
Legal experts at Allen Matkins Leck Gamble Mallory and Natsis LLP are urging facilities to review their Storm Water Pollution Prevention Plans, confirm laboratory contracts, and train personnel ahead of the wet season.
A particular concern is the so-called first flush, which occurs when the first significant storm of the season washes accumulated pollutants including dust, oils, metals, and sediment from impervious surfaces.
First-flush events can produce Numeric Action Level exceedances even at facilities with well-implemented Best Management Practices, making early-season storms especially consequential for compliance records.
Settlement agreements arising from Clean Water Act citizen suits have, in some cases, specifically required facilities to collect their first sample of the reporting year from the first qualifying storm event.
California’s Stormwater Multiple Application and Report Tracking System, known as SMARTS, serves as the public database where facilities upload annual reports, sampling results, and exceedance documentation.
Environmental groups regularly review SMARTS records when preparing 60-Day Notices, cross-referencing self-reported data against Numeric Action Level thresholds to identify potential violations.
A wetter year driven by El Niño will generate more self-reported data in SMARTS, increasing the likelihood that some results will show exceedances and giving plaintiff organisations additional material to assess.
Facilities that prepare before the rains arrive are generally in a stronger position to respond if a 60-Day Notice is received, according to the Allen Matkins environmental team.