In the middle of a dry California summer, stormwater runoff feels like the furthest thing from anyone’s mind. Yet for industrial facilities under the IGP, now is the time to prepare for what’s shaping up to be a wet El Niño year. According to NOAA’s Climate Prediction Center, El Niño is strengthening and, in NOAA’s assessment, carries a 97% chance of persisting through early spring 2027, with an 81% probability of a very strong event during October through December 2026. Southern California typically sees higher-than-normal precipitation during El Niño years, while Northern California’s correlation is less consistent, though a wetter-than-average outcome remains possible. For California industrial facilities subject to the Industrial General Permit (IGP), a strong El Niño means more rain, which generally means more Qualifying Storm Events (QSEs), more required sampling, and more data available to environmental groups that bring Clean Water Act (CWA) citizen suits.
The IGP requires most industrial facilities to collect stormwater samples from four QSEs each reporting year: two during the first half of the reporting year (July through December) and two during the second half of the reporting year (January through June). A QSE is a precipitation event that produces a discharge and is preceded by at least 48 hours with no discharge. In drier years, facilities often cite an insufficient number of QSEs as the reason for under-sampling.
Environmental groups frequently challenge that under-sampling explanation in 60-Day Notices, however, contending that NOAA rainfall data shows qualifying storms occurred during a facility’s operating hours. And in a strong El Niño year, that explanation may be harder to sustain. If the forecasted rainfall materializes, there should be no shortage of QSEs, and facilities that fall short of the four-sample requirement may face closer scrutiny.
Facilities should consider the following steps before the rains arrive:
After an extended dry period, pollutants — including dust, oils, metals, and sediment — accumulate on impervious surfaces. The first significant storm of the season, often called the first flush, can carry the highest pollutant loads of the year. It can produce Numeric Action Level (NAL) exceedances even at facilities with well-implemented Best Management Practices (BMPs).
Environmental groups appear to place particular weight on first-flush data for the reasons discussed above. And settlement agreements resolving citizen suits have, at times, gone further than the IGP’s own sampling schedule, with some consent decrees specifically requiring facilities to collect the first sample of the reporting year from the first QSE, with at least one referring to that requirement using the term “first flush” itself. Others have required sampling from each of the first several storm events of a wet season, rather than leaving the choice of which qualifying storms to sample to the facility.
California’s Stormwater Multiple Application and Report Tracking System (SMARTS) is the public database where facilities upload Annual Reports, sampling results, and Exceedance Response Action documentation. Environmental groups regularly review SMARTS records when preparing 60-Day Notices, cross-referencing self-reported data against NAL thresholds and Annual Report explanations.
A wetter year generally means more QSEs, more required sampling, and more self-reported data in SMARTS. More data may increase the likelihood that some results may show NAL exceedances, particularly first-flush samples, which could give plaintiff organizations additional material to review when evaluating potential citizen suits.
Facilities that prepare before the rains arrive are generally in a stronger position to respond if a notice does come. If your facility wants to review its SWPPP, sampling protocols, or overall IGP compliance posture ahead of the 2026–2027 wet season, contact the Allen Matkins environmental team.
California Stormwater Defense is published by Allen Matkins Leck Gamble Mallory & Natsis LLP. Nothing in this post constitutes legal advice. Contact us for a consultation specific to your facility.