Every storm season, some Marin homes take on water: a roof leak during an atmospheric river, a flooded crawlspace near a creek, or a burst pipe. The water is the first problem. Mold can be the second, showing up days or weeks later in walls, under floors, or in closets. The good news is that mold needs moisture, and how fast you dry things out makes a big difference.
As of the Climate Prediction Center's October 8, 2026 ENSO discussion, an El Niño Advisory is in effect, and NOAA says a strong-to-very strong El Niño is likely through January–March 2027. NOAA is also careful to say that a stronger event makes El Niño-like impacts more likely, not guaranteed. El Niño winters have historically tilted Northern California toward wetter conditions, but individual seasons vary, and no outlook can tell you when or where a specific storm will hit. Watch the CPC seasonal outlooks and the National Weather Service Bay Area office for forecasts as the season unfolds.
The CDC advises cleaning up and drying out your home fully and quickly, within 24–48 hours, after a flood (CDC). The EPA puts it simply: the key to mold control is moisture control (EPA). Mold spores are present nearly everywhere; what turns them into a problem is wet material that stays wet.
Our sister site covers the first 24 hours after water damage in detail.
According to the CDC, mold can be removed from hard surfaces with household products, soap and water, or a bleach solution of no more than 1 cup of household laundry bleach in 1 gallon of water. Never mix bleach with ammonia or other cleaners, which produces toxic gas. Wear rubber gloves, goggles and boots, and ventilate (CDC). The EPA's Mold Cleanup in Your Home page has more detail, including when to consider professional help.
The EPA says homeowners can usually handle areas under about 10 square feet themselves. For larger areas it points to its remediation guidance, it advises not running an HVAC system suspected of mold contamination, and it recommends a professional when sewage or other contaminated water was involved (EPA). Professional post-flood mold prevention and remediation can include containment, removal of affected materials, and verification.
Even with good drying, check for musty odors, discoloration or new staining in:
Crawlspaces in the Ross Valley. Homes in San Anselmo, Fairfax, Ross and Kentfield often have crawlspaces that stay damp long after a storm. See our guide to crawlspace mold in older Marin homes.
Shaded canyon homes. In Mill Valley and Larkspur's Baltimore Canyon, walls and lower floors dry slowly with little sun.
Bay-edge homes. In Corte Madera, Greenbrae and the Canal area of San Rafael, high groundwater keeps lower areas damp.
Closed-up homes. Second homes in Stinson Beach and West Marin may sit wet for days before anyone notices.
Mold coverage varies. Some policies cover mold resulting from a covered sudden water event, often with limits, while excluding mold from long-term moisture. Fast action and good documentation help. See water damage insurance claims on our sister site.
A few common mistakes make post-storm mold worse. Don't paint over water stains or visible mold before the material is dry and the cause is fixed, because mold can keep growing underneath. Don't close up walls or reinstall flooring until moisture readings confirm materials are dry. Don't run central heating or air conditioning if ducts were flooded, as that can spread contamination. Have them evaluated first. And don't mix cleaning chemicals: the CDC specifically warns never to mix bleach with ammonia or other cleaners. Finally, don't assume that because surfaces feel dry the job is done. Wall cavities and subfloors can stay wet long after the surface is dry, which is why professionals use moisture meters.
If you're seeing mold after a leak or storm, submit a request to connect with an independent local mold remediation provider.
The CDC recommends drying fully and quickly, within 24–48 hours.
The CDC says a solution of no more than 1 cup of bleach per gallon of water can be used on hard surfaces. Never mix bleach with ammonia or other cleaners.
The EPA says areas under about 10 square feet are usually manageable yourself; for larger areas it points to its remediation guidance, and it recommends a professional after sewage or contaminated water.
Not before removing it, drying the material and fixing the moisture source.
If ducts or equipment got wet, have them evaluated first; the EPA advises not running an HVAC system you suspect is contaminated with mold.
Moisture meters give objective readings; surface feel isn't reliable.
This article is general information, not legal, insurance or medical advice. Sources are linked in the text.