A Very Strong El Niño Is Coming This Winter — I'm Buying or Already Own a Bay Area Home. What Do I Do Before the Rains Start?

Quick answer

Three things, in this order: confirm the FEMA flood zone for your exact street address, buy flood coverage before the rainy season starts (an NFIP policy generally carries a 30-day waiting period), and book the inspection for a day when it is actually raining.

Here is the size of the forecast. NOAA's Climate Prediction Center issued an El Niño Advisory on August 13, 2026: the chance of a "very strong" event this winter is above 90%, and the chance that the October-through-December average exceeds +2.5°C — which would make it the strongest event since 1950 — is 69%. A week later, the seasonal outlook raised the odds of above-normal December-through-February precipitation along coastal California to above 50%, holds it there through February to April, and named 1998 and 2024 as the analog years, both late-winter storm years. Columbia University's IRI model ensemble is blunter still: every season through March 2027 carries a 100% El Niño probability.

But a Bay Area house is not a question of whether it rains. It is a question of which water. Creek overtopping, bay-side tide and saturated hillside soil are three separate sets of due diligence — and this winter, the flood-control work covering all three happens to be either a construction site or a set of drawings. What follows breaks the decision into three dimensions: the forecast, the geography, and the money and the contract.

Who this is for

• Buyers touring the creek-side neighborhoods of Palo Alto and Menlo Park, or the bay-side blocks of Foster City and the Menlo Park Bayfront, who expect to be in escrow this winter. Before you write an offer you need to know which kind of water this address faces, when a flood policy actually takes effect, and which day to schedule the inspection.
• Families who already own in Woodside, Portola Valley or Los Altos Hills, and sellers preparing to list during the rainy season. In the hills the things to watch are the soil and the road. For a seller, the question is drainage prep and exactly what goes into the disclosure package.
• All-cash buyers and cross-border buyers. With no lender, nobody runs a flood determination on your behalf and nobody requires you to carry flood coverage before closing. That entire review layer is absent by default in an all-cash purchase, and you have to supply it yourself.

Three things that decide the answer

One: what the forecast actually says, and what it does not

Start with the official language. The Climate Prediction Center's ENSO discussion of August 13 reported the July Niño-3.4 sea surface temperature anomaly at +1.4°C, with the eastern equatorial Pacific already above +2.0°C. It put the chance of a very strong event this winter above 90%, and the chance of an October-through-December three-month average above +2.5°C at 69% — a threshold that, if reached, would exceed every event on record since 1950. In IRI's multi-model summary of August 19, 15 of 26 models put the October-through-December Niño-3.4 anomaly at or above +3.0°C.

Then the layer that lands in California. The Climate Prediction Center's seasonal outlook discussion of August 20 states that the probability of above-normal December-through-February precipitation along coastal California has been raised significantly, to above 50%, holding above 50% from December–February through February–April, with 1998 and 2024 as the historical analogs — both years that produced substantial late-winter rain. Put differently, the analog years put the surplus in late winter, not at the start of the season.

Now what the forecast does not say. The NWS Sacramento ENSO page is deliberately restrained: Northern California precipitation correlates weakly with most ENSO patterns, and it is Southern California where the El Niño signal is clear. In 2015-16, an event also labeled a "super El Niño," the warm pool shifted west during the winter, and California finished the season near normal, with much of the northern half of the state on the dry side. Between "a good chance of a wet winter" and "severe flooding is coming" sits that piece of history.

So the correct reading is this: probability is not rainfall, but the deadlines are fixed. The Climate Prediction Center issues its next ENSO discussion on September 10. NOAA publishes its official winter outlook in mid-October. The first king tide arrives on November 24. What you need finished before those three dates has nothing to do with how much rain eventually falls.

Two: which kind of water your house faces

Creek overtopping. The best-documented creek on the Peninsula is San Francisquito Creek. On February 3, 1998, after days of heavy rain coinciding with a king tide, the creek overtopped its banks; Palo Alto, Menlo Park and East Palo Alto together saw roughly 1,700 properties damaged and about $40 million in losses, and Crescent Park, Duveneck/St. Francis, Community Center, Triple El and the Willows in Menlo Park have been on the high-risk list ever since. It happened again on December 31, 2022: the Stanford gauge gave a preliminary reading of 7,420 cfs that day, widely reported at the time as a record above 1998, which USGS later revised to 6,340 cfs — below the record on the corrected figure. What the JPA's later review did establish is that the channel's real conveyance capacity is roughly 25% lower than the original model assumed. On the East Palo Alto side the same night, more than 20 vehicles were flooded, and most of the affected residents carried no flood insurance.

Two facts about that creek are worth holding onto this winter. The old Newell Road bridge was closed and demolished in May 2026, with the replacement due in spring 2027, so the bridge site is an active construction zone for this entire rainy season. And the upstream reach from 101 to Middlefield is still in alternatives evaluation, with construction no earlier than 2027; the full reach-by-reach timeline is in the flood-zone article linked below. The larger South Bay creek is Coyote Creek: the February 2017 flood caused roughly $73 million in damage and forced about 14,000 people to evacuate. Valley Water's phase 1 flood protection work was accepted in June 2025; phase 2 was awarded on July 24, 2026 at roughly $124 million, with construction running from August 2026 to October 2028 — so the 9-mile reach between Montague Expressway and Tully Road is also a construction site this winter. For the FEMA letter zones, NFIP premiums and below-grade restrictions that creek-side buyers have to price out, see The Palo Alto House I Want Is in a FEMA Flood Zone — Should I Still Buy It?; this article does not repeat that ground.

Bay-side tide. The trigger for the 1998 flood was rain landing on top of a king tide, and that combination is what makes an El Niño winter distinctive: the creek's peak and the high tide arrive in the same hour. Speaking to KQED in June, UC ANR climate scientist Daniel Swain put it plainly — El Niño can temporarily raise sea level by 6 inches to 2 feet, with storm surge capable of adding another 1 to 2 feet on top. By August 10, KQED's follow-up — quoting Swain alongside a National Weather Service meteorologist and Scripps Institution of Oceanography — was more direct still: the single highest sea levels are quite possible this winter. The California Coastal Commission's King Tides Project has already published the 2026-27 dates: November 24-26, December 23-25 and January 21-22, with some Northern California locations also listing December 22, January 20 and January 23.

Bay-side communities fall into two groups by how far their shoreline work has progressed. Foster City is the finished kind. Measure P passed in 2018 with nearly 81% support, authorizing a $90 million bond; construction to raise and widen the levee began in October 2020 and finished in February 2024, and the city submitted a Letter of Map Revision to FEMA that same month. The city's own FAQ states that Foster City remains in Zone X, with no mandatory purchase requirement. The Menlo Park Bayfront and Belle Haven, along with the East Palo Alto shoreline, are the unfinished kind: the final environmental review for the roughly 7-mile SAFER Bay levee was certified on May 28, 2026, and phase 1 design does not finish until January 2027. Nothing new stands on that shoreline this winter.

Saturated hillside soil. The California Geological Survey's note on El Niño and landslides comes down to one sentence: landslides occur in any ENSO phase, provided the rainfall is enough to saturate the soil and rock — and the two strong El Niños of 1982-83 and 1997-98 both brought well-above-normal statewide precipitation accompanied by widespread landsliding. The most recent local example is close at hand. After the run of atmospheric rivers between December 31, 2022 and January 18, 2023, USGS compiled a reconnaissance landslide inventory for the Bay Area; in Woodside, SR-84 at the Portola Road intersection was fully closed in March 2023 by a 250-foot-wide landslide and did not reopen to two-way traffic until May 31, 2024 — 14 months. For a hillside property, three things need checking before the rains: whether there is an alternate route in and out, whether the policy excludes earth movement, and what the town's files say about the parcel's slope zoning and geotechnical history. For how the zoning and geologic review actually work, see our guide to buying a Portola Valley or Woodside house that sits on a fault zone.

Three: the three clocks running on money and contract

Clock one: 30 days for flood insurance. FEMA's own language is that an NFIP policy "generally takes 30 days to go into effect," unless the insurance is required by a government-backed lender in connection with a loan, or is related to a community flood map change. Translated into a calendar: if you already own the house and want coverage in force before the first king tide on November 24, the policy has to be bought by October 24. If you are closing with financing, the lender-required policy takes effect at funding, which is the exception. The Palo Alto and Menlo Park NFIP averages, the gap between A/V and X zones, and why Risk Rating 2.0 forces an address-level quote are all laid out in the Palo Alto flood-zone article; one number is enough here: voluntary coverage in the moderate-to-low-risk B, C and X zones averaged $622 a year statewide (Policygenius, June 2024). The waiting period, not the premium, is the thing to race.

Clock two: the review layer that vanishes in an all-cash deal. The legal machinery behind mandatory purchase is that the property sits in a Special Flood Hazard Area and the loan comes from a federally regulated or insured institution. Remove the lender and nobody runs the flood determination, and nobody makes you buy a policy before closing. In the second quarter of 2026, 74.2% of Woodside closings, 64.5% of Atherton closings and 36.0% of Palo Alto single-family closings were all cash, so that review layer was already thin across the Peninsula's upper market. Replacing it is straightforward: run your own address search at the FEMA Flood Map Service Center and save the screenshot, then ask the seller for any existing elevation certificate and for the in-force NFIP policy — an NFIP policy is assumable with the house, and taking one over saves you a waiting period. The cost and turnaround of a certificate are covered in the flood-zone article linked above.

Clock three: contract and disclosure. On the buy side, write flood zone, drainage and below-grade space as their own line in the due diligence contingency rather than burying them inside a general property investigation. Palo Alto's median days on market last quarter was just 8, so the inspector has to be booked before the offer goes out, not after acceptance. On the sell side, California Civil Code §1103.2 requires the first item of the NHD statement to disclose whether the property lies within a FEMA-designated Special Flood Hazard Area, and the Transfer Disclosure Statement (§1102 et seq.) requires disclosure of known seepage and water intrusion history — an as-is clause in the contract does not release you from it. For which defects have to be written down and what happens when they are not, see our guide to what a Bay Area seller must put in the disclosures. The insurance, disclosure and project material above is a summary of public sources and is not insurance, legal or engineering advice; confirm your own situation with an insurance broker, a real estate attorney and licensed professionals.

Six towns, last quarter's closings — and why the kind of water changes the first move

The headline numbers first. In the second quarter of 2026, Palo Alto recorded 139 single-family closings at a $4,100,000 median with a median 8 days on market, and faces both creek and bay water. Woodside recorded 31 closings at a $4,500,000 median, 74.2% of them all cash, with a median 21 days on market, and faces saturated hillside soil. Foster City recorded 16 closings at a $2,250,000 median, and is the levee-protected bay-side case. Same rainy season, three different first moves.

What to remember. Two differences carry the decision. First, days on market sets the order of your due diligence: Palo Alto and Foster City both ran a median of 8 days and Los Altos Hills 9, so the inspector and the flood zone search have to be finished before the offer, while Woodside's 21 days and Portola Valley's 18 leave room for a real geotechnical review — but only that much room. Second, the all-cash share decides who is checking on your behalf: 74.2% of Woodside's closings had no lender involved, so the flood determination and mandatory purchase machinery never engaged at all, while Foster City's 12.5% cash share means most buyers there did have a lender — except that the city's Zone X status means mandatory purchase does not apply anyway. Two different routes to the same place: the insurance decision is yours to make. (Source: MK Bay Area Pulse 2026 Q2, MLSListings single-family closings, 2026-04-01 to 2026-06-30.)

What we see in the field

In July 2026, walking two active listings in Palo Alto, Marie Wang and Kevin Mo put a set of on-site numbers on the basement question: rebuilding on the same lot runs roughly $4 million in construction cost without a basement, and roughly $6 million with a basement and full landscaping. Inside a Special Flood Hazard Area, the $2 million option does not exist, because the zone requires the lowest floor to sit above the base flood elevation. So checking the zone before the rainy season is not only ahead of insurance; it is ahead of design. What the lot can hold is settled before you hire an architect.

The other end of the range is speed. In May 2026, MK Group handled a roughly $11 million all-cash purchase in Los Altos Hills, bought through a corporate entity and locked in two weeks. That deal had nothing to do with the rainy season, but its structure applies to every all-cash purchase: with no lender there is no third-party review, and the due diligence contingency becomes the buyer's only guardrail. Put the same structure into a hillside rainy season and slope zoning, road access and policy exclusions all have to be worked through by the buyer's own advisors inside an equally tight window. Paying cash removes the loan process, and it removes the review with it. Winter only widens the gap.

Which is why a buyer going into escrow in winter has one nearly free move available: pick a rainy day for the inspection. On a dry day you cannot see whether the yard grading pushes water toward the foundation, whether the downspout discharges next to a crawlspace vent, or whether the sump pump comes on at all. In the rain, ten minutes settles all three. On a dry day you are guessing.

Common Misconceptions

Misconception one: "A very strong El Niño is coming, so the Bay Area is guaranteed a flood winter."

Not guaranteed. NWS Sacramento's position is that Northern California precipitation correlates weakly with most ENSO patterns, and that the region with a clear El Niño signal is Southern California. The comparable 2015-16 event ended with California's seasonal precipitation near normal. What the Climate Prediction Center has issued this time is a greater-than-50% probability of above-normal December-through-February precipitation along coastal California, and sustained through February to April. That is a tilt in the odds, and it is a long way from a promise of rainfall. The preparation, though, does not depend on the forecast verifying: the zone search, the insurance waiting period and the king tide dates are all independent of how much rain finally falls.

Misconception two: "I'll wait until the rainy season starts and buy flood insurance when it looks serious."

That is too late. FEMA's position is that an NFIP policy generally carries a 30-day waiting period, with exceptions only where a government-backed lender requires the coverage in connection with a loan, or where a community flood map change is involved. The first king tide is November 24, and the analog years put the rain in late winter. Counting from the day NOAA publishes its official winter outlook in mid-October, the window left to you is a matter of weeks. Existing owners should buy now. Buyers in escrow should confirm that the lender-required policy is in force at funding. All-cash buyers have no exception available to them at all.

Misconception three: "Foster City just finished its levee, so bay-side houses don't have to think about tides anymore."

What the levee changed is the FEMA zone designation and whether mandatory purchase applies. Whether to carry the insurance is still the owner's own call. Foster City's levee was completed in February 2024, the city submitted a LOMR to FEMA, and the city remains in Zone X with no lender purchase requirement — that is the good news. But the sea level anomaly in an El Niño winter is a temporary rise of 6 inches to 2 feet, with storm surge capable of adding another 1 to 2 feet, while voluntary Zone X premiums sit around the $622 statewide average. Put the two figures side by side and the answer is usually clear. The Menlo Park Bayfront and the East Palo Alto shoreline need a sharper distinction still: the SAFER Bay levee only cleared its environmental review on May 28, 2026, so there is no new protection standing on that shoreline this winter.

Misconception four: "Winter is a bad time to be in escrow — you can't see anything in the rain."

The opposite is true. Winter is the only time you get to watch the drainage work in real rain. Whether the yard grading pushes water toward the foundation, where the downspouts discharge, whether the crawlspace holds standing water, whether the sump pump runs, whether the back of the retaining wall weeps — on a dry day all of it is inference, and in the rain or within 24 hours after it, all of it is visible. The genuinely bad timing is different: with a median 8 days on market in Palo Alto, you start looking for an inspector after acceptance, and then you walk the whole process on a sunny day.

Misconception five: "The house is up in the hills, so it can't flood and the rainy season doesn't concern me."

In the hills, the rainy season is about soil and roads. The California Geological Survey's note is that landslides occur in any ENSO phase, provided rainfall is enough to saturate the soil and rock, and that the strong El Niños of 1982-83 and 1997-98 both brought widespread statewide landsliding. The closest example is SR-84 at the Portola Road intersection in Woodside, closed for 14 months from March 2023 by a 250-foot-wide slide. Three checks belong on a hillside property before the rains: whether there is an alternate route in and out, how the policy words its earth movement exclusion (most standard homeowners policies do not cover landslide, so go through it line by line with your broker), and what the town's files hold on this parcel's slope zoning and geotechnical reports.

What to do next

1. Search the zone by street address today, not by neighborhood name. Enter the specific address at the FEMA Flood Map Service Center and save a screenshot; the two sides of one street can fall in different zones. If you already hold an NHD report, check that its first item — whether the property is within a Special Flood Hazard Area — matches what the FEMA search returns.
2. Back-date the policy from the 30-day waiting period. Existing owners should buy NFIP or private flood coverage by October 24 so it is in force before the first king tide on November 24. Buyers closing with a loan should confirm the lender-required policy takes effect at funding. All-cash buyers should ask the seller for any existing elevation certificate and for an assumable in-force NFIP policy.
3. Schedule the inspection for a rainy day or within 24 hours after one, and write flood and drainage as their own contingency. The checklist: yard grading, downspout outlets, crawlspace standing water and vents, sump pump and backup power, roof and gutters, retaining wall seepage. In a market running a median 8 days on market like Palo Alto, the inspector has to be booked before you write the offer.
4. Run three checks on a hillside property before the rains. Clear culverts and drainage ditches, confirm an alternate route in and out, and read the earth movement exclusion in your policy line by line. If you are planning an addition, pull the parcel's slope zoning and any existing geotechnical reports from the town first.
5. Sellers: finish the drainage prep before listing, and put the history into the disclosures. Repair gutters, extend downspouts and correct the ground grading first, and keep the invoices. Any location that has taken water or seeped, and the repairs made, goes into the TDS and NHD as required. On a house listed during the rainy season, the buyer's inspector will be looking at exactly those items in the rain.
6. Put three dates on the calendar. The Climate Prediction Center's ENSO discussion on September 10, NOAA's official winter outlook in mid-October, and the first king tide on November 24-26. Re-check your own list against each forecast update, rather than opening it once the rain has already started.