El Niño is coming. Here’s what it actually means for your gutters, roof and drainage on the Peninsula.

Forecast figures current as of NOAA’s October 8, 2026 update. NOAA’s seasonal outlook refreshes October 15 and its next El Niño discussion is November 12; we’ll update this page after each.

Every few years a winter gets a name before it arrives. This one has one already — “super El Niño” in the headlines, “historic” in NOAA’s own language — and the phone has started ringing with the same question in different words: how bad is it going to be, and what should I do about the house?

Here is the short version, and then the long one.

What NOAA is actually saying

It’s worth separating two things that get mashed together in news coverage: how strong the El Niño is, and how much rain it will bring here. The first is about as certain as seasonal forecasting gets. The second is a tilt, not a lock.

On strength: NOAA’s Climate Prediction Center has had an El Niño Advisory in effect through its October 8 update, reports the event “continues to strengthen,” and gives better than an 83% chance it stays strong-to-very-strong through January–March 2027. Its own word for what this could become is “historic” — an event exceeding every El Niño since 1950. UCLA climate scientist Daniel Swain wrote on October 12 that it is “the strongest on record by almost any metric.” Governor Newsom proclaimed a statewide El Niño preparedness state of emergency on September 21.

On rain: NOAA’s September seasonal outlook gives the California coast its highest odds — 50 to 60 percent — of above-normal precipitation for December–February and January–March. That is a meaningful lean toward wet. It is not a guarantee, and NOAA says so: the chances of El Niño impacts are “larger, though not guaranteed.” Swain’s research-model view is far stronger — over 90% odds of a wetter-than-average winter statewide, with the Bay Area inside his highest-confidence zone — while the local National Weather Service office has been careful to describe the Bay Area as a “transition zone” where the El Niño signal is weaker than in Southern California.

Read those together and the honest summary is: a wet winter is more likely than not, a very wet one is more likely than usual, and nobody serious is promising either.

Will it really hit the Peninsula? What the last three big ones did

The most useful thing you can know about El Niño and the Bay Area is that very strong events have produced both the second-wettest season in San Francisco’s 177-year record and a perfectly ordinary one.

Look at the last row. The storm sequence that flooded San Mateo Creek at El Camino Real, put Belmont Creek over its banks on four separate January days, and gave SFO its wettest 23-day stretch on record happened in a La Niña winter. February 2024’s hurricane-force gusts and 300-plus landslides statewide came during a modest El Niño that left San Francisco only slightly above normal for the season.

The reason is atmospheric rivers. A 2025 Scripps study found they supply up to 65% of Northern California’s annual rain, and that about a third of El Niño and La Niña years since the 1950s didn’t follow the script — mostly because of how many atmospheric rivers showed up. In the words of one of the authors, “Atmospheric rivers don’t dance to the tune of ENSO.” El Niño raises the odds that more of them come, and this year’s record-warm ocean means the ones that do come may carry more water. But a single well-aimed one in any winter is enough to overwhelm a clogged downspout.

That’s the whole argument for getting the house ready, and it doesn’t depend on El Niño doing what the headlines say.

When does it start, and when’s the deadline?

Two timelines matter. The first is El Niño’s: its storm-track signal usually doesn’t show up until December and is strongest in January and February, with the risk running through March. Autumn rain, or the lack of it, tells you nothing about what’s coming.

The second is the Bay Area’s ordinary calendar, which doesn’t wait for El Niño:

Last year’s first atmospheric river arrived on October 22. When it did, the National Weather Service told the Bay Area to “take it as a gateway to winter” and clear the gutters before the bigger storms. That’s the right instinct, one storm late.

So the practical deadline is the end of October. Get gutters, downspouts, roof valleys and yard drains cleared now, while everything is dry and the ladder work is safe. Then re-check after the first real storm, because November’s leaf drop will refill a gutter under a big oak or eucalyptus in weeks. Homes under redwoods, which shed needles all fall, usually need a second clearing in mid-winter.

Watch what’s coming

This map is live. The first tab shows real precipitation radar over the Peninsula right now; the second shows the forecast rain for the next ten days. When a band is sitting off the coast and moving east, that’s your window to check the downspouts closing.

What one clogged downspout does in a three-day storm

People picture a clogged gutter as water splashing over the edge. What it really is, during a multi-day atmospheric river, is a redirect: an entire roof section’s runoff, delivered to one spot at the base of the house, for days.

The U.S. Department of Energy’s building-science guide describes the chain plainly. Roof runoff that isn’t carried away “can quickly saturate the soil surrounding the building and wick through the foundation,” leading to mold and rot; expansive clay soils swell and “put extreme pressure on foundations”; and where roof edges meet walls without proper flashing, overflow “runs down the wall,” damaging siding and windows. On the Peninsula, that translates into the calls we get every January:

• Fascia rot. Overflow runs over the back of the gutter and soaks the painted wood it’s hung on. Two or three wet winters and the fascia is gone, and replacing it means pulling the gutter too.
• Crawlspace and garage water. A downspout is supposed to put roof water at least five feet from the house. A blocked one puts it at the foundation. On hillside lots in Hillsborough, the Millbrae hills and upper Burlingame, that water finds the garage or the lower level.
• Buried drain failures. Many Peninsula downspouts tie into underground pipes because the lot can’t meet the code’s grading rule (six inches of fall in the first ten feet). Those lines clog with leaves, silt and roots, and the symptom is water backing up and spilling out at the downspout base. The test is simple: run a hose into the downspout and watch where the line is supposed to come out.
• Sag and separation. A gutter full of wet debris is heavy. Hangers pull, seams open, and the gutter starts holding water permanently.

The Peninsula checklist, in order of effort

This weekend, no ladder required

• Walk the downspouts. Every one should discharge onto a splash block or extension that carries water at least five feet from the foundation. If one just ends at the dirt, add an extension — it’s a ten-dollar fix.
• Find your yard drains and clear the grates. Leaves and bark mat over them in a single storm.
• Hose-test any buried downspout line. If water doesn’t appear at the outlet, it’s blocked.
• Sign up for SMC Alert (San Mateo County’s emergency notifications) and check whether your home is in a flood zone at FloodSmart.gov.
• Know your sandbag site. City of San Mateo stations are open 24/7 from October 1 to April 30 (limit 10 per vehicle). The county serves unincorporated residents at four yards, including 752 Chestnut Road in Redwood City (limit 15). Millbrae has used the 600 block of Millbrae Avenue and Landing Lane. Other cities: call public works before you need them.

Before the first big storm

• Gutters cleared by hand, every downspout flushed until it runs clear. Blowing debris out of the open channel doesn’t touch the part that actually clogs.
• Roof valleys and behind chimneys cleared. Debris packs there like it does in gutters, and it backs water up under the shingles.
• Roof moss treated before it’s saturated. Moss holds water against shingles and tile all winter. It’s also what insurance companies are flagging in aerial photos. Low-pressure soft washing now, not a pressure washer, and not in the rain.
• Trees trimmed back from the roofline. Less debris in the gutters, and fewer limbs on the roof in a windstorm.
• If you’re creek-side in Burlingame, the city’s rule is that private owners must keep their section of creek clear of obstructions. Burlingame has five creeks running through it, mostly across private property.

Worth handing off

• Two-story and steep roofs. Ladder falls spike every fall. If the roof is high, steep, or wet, this is not the weekend to learn.
• Walkways, steps and driveways. The first rain on a summer’s worth of algae turns brick, aggregate and concrete into a skating rink. A hot-water surface clean before the rains is a slip-and-fall prevention job as much as a cosmetic one.
• A mid-winter re-clean under heavy canopy. Book it now for December or January; by then the phones are busy.

The insurance part nobody mentions

A clogged gutter doesn’t just cause damage; it can cause damage your policy won’t pay for. The California Department of Insurance notes that flood, earth movement and wear-and-tear are typically excluded from homeowners policies, and water damage from seepage, neglect or lack of maintenance commonly is too. Flood is a separate policy through the National Flood Insurance Program, which typically has a waiting period before it takes effect — so if you’re near a creek or in a mapped zone, that decision is a now decision, not a December one.

Two practical things follow. First, take dated photos of your gutters, downspouts and drains after they’re cleaned; Cal OES already recommends photographing valuables before storms for claims, and documented maintenance is the same idea. Second, if your insurer has sent you a roof letter — and a lot of Peninsula carriers are — see our insurance roof notice page. The before-and-after documentation we provide is exactly what they’re asking for.

One more thing, because the SF Standard quoted a Bay Area roofer saying it well: do not sign anything from someone who shows up at your door and says “El Niño is coming.” We don’t knock on doors. We’re licensed and insured, we quote in writing within 24 hours, and we’ll tell you honestly if what you have is a roofer’s job rather than ours.

If you’re in Burlingame, Millbrae, Hillsborough, San Mateo, Redwood City or anywhere on the Peninsula and the gutters haven’t been touched since spring, this is the window. Gutter cleaning details are here, roof moss removal here, or just call or text and we’ll get you on the calendar before the first storm.