Where the Oakmont portfolio stands on flood, going into a record El Niño winter

Every Oakmont community — all 119, across 30 counties in four states — was run against the FEMA National Flood Hazard Layer at its own coordinates. That is the same mapping your insurers and lenders use, and it is the right place to start — but it answers one narrow question, and this briefing is mostly about what it leaves out.

How to read these zones

The vocabulary on FEMA's maps is unhelpfully named, and two of the terms mean close to the opposite of what they sound like. This is the whole of it.

"100-year flood" is the worst phrase in the field. It does not mean once a century, and a site is not safe for 99 years after one. It means a 1% chance every year, independently — which compounds to roughly a one-in-four chance across a 30-year horizon. The 0.2% flood works the same way and lands near 6% over 30 years. Neither is rare in the sense people hear.

Zone A and Zone AE are not the same, and A is the worse one to have. Both are the 1% floodplain. AE means FEMA ran the detailed study and published a Base Flood Elevation — a number you can design, insure and plan against. A means they mapped the extent but never ran the study, so no elevation exists. That is why Bonita is a harder problem than Mariner Point, not an easier one — even though the zone letter looks milder.

Across 119 sites, only two carry a usable number. Mariner Point has a base flood elevation of 10.0 ft NAVD88 and Simi Valley a flood depth of 2.0 ft. Those are the only two places in this portfolio where the map tells you how deep. Escondido Hills is Zone AE, but the polygon at the site carries no static elevation — on an AE panel the elevations are drawn as contour lines rather than stored per area, so that one has to be read off the FIRM itself.

Being in a Special Flood Hazard Area has legal weight. It triggers mandatory flood insurance on a federally-backed mortgage and brings building and substantial-improvement rules through the local floodplain administrator. The other zones carry no such requirement, which is exactly why a levee-reduced or 0.2% site can quietly carry real exposure with no paperwork prompting anyone to look at it.

Five sites in a floodplain

Five of the 119 sit inside a Special Flood Hazard Area — the regulatory 1% floodplain, where flood insurance is mandatory on a federally backed mortgage and where local floodplain rules bite. They are not five versions of the same problem. Two have a published number to plan against and three do not, and that distinction matters more than the zone letter does.

And one site sitting on the line

Eight sites that are dry because a levee is holding

These are mapped as reduced risk because a structure is there. The map shows the world in which the levee performs. That is a reasonable assumption most winters and a poor one to leave unexamined in this one. Eight sites now sit in this category, and three of them — two in Sacramento and one in Stockton — are behind the Central Valley system, which is the largest and most consequential levee network any of these communities depends on.

Four sites nobody has studied

Zone D does not mean low risk. It means FEMA has performed no flood hazard analysis for that area at all, so the hazard is undetermined rather than determined to be small. It is the one result on this list that is easy to misread as a clean bill of health.

Ten sites in the 0.2% floodplain

Outside the regulatory floodplain, inside the mapped 0.2%-annual-chance flood — the "500-year" event. No insurance requirement attaches and no flood elevation is published, but these are the sites where an exceptional winter is most likely to produce water the maps did anticipate. Three of the ten are in the South Bay, on the creek network that flooded San Jose in February 2017, and three of the additions are in the Central Valley and inner East Bay.

The other 91, and why "minimal" is not "none"

All 91 came back Zone X, minimal mapped hazard — outside both the 1% and the 0.2% floodplains. That is a real and welcome result, and it is worth being precise about what it covers: river and coastal flooding that has been studied and mapped. It says nothing about the flooding that actually closes buildings and roads most often.

What a record El Niño changes

NOAA's Climate Prediction Center has an El Niño Advisory in force. Its 10 September discussion puts a greater than 90% chance of a very strong event this winter and a 75% chance that October to December exceeds any El Niño since records began in 1950.

The detail that matters for your sites is where the warm water sits. Niño-1+2, hard against South America, is at +3.4 °C while Niño-4 in the central Pacific has fallen to +0.1 °C. That is an east-Pacific event, and the east-Pacific flavour is the one historically tied to wet, damaging winters in Southern California — where 57 of the 119 communities are. The other 53 California sites are in the north and Central Valley, which is where the snow-level story below applies.

El Niño does not cause floods. Atmospheric rivers do, and El Niño shifts the odds on how many arrive and how far south they track. It also makes storms warmer, which raises snow levels and turns what would have been snowpack into immediate runoff — that is the Sacramento and Placer story rather than the Los Angeles one.

The other thing a long wet winter does

Flooding is not the only way sustained rain causes damage, and the two hazards run on different clocks. Flash flooding responds to one intense hour. Deep-seated landslides respond to cumulative saturation over weeks — groundwater rising through a slope until the material gives. A long, repeatedly wet winter is the condition that produces them, which is why this belongs in an El Niño briefing rather than a separate one.

Seventy-eight of the 110 California sites stay below class 5 everywhere we sampled. Thirty-two reach class 5 or higher somewhere within 40 m:

Twenty of these thirty-two swing by five classes or more, so the ones worth attention are the consistent scores rather than the single highest readings. Ivy Park at Mission Viejo returns class 10 at every sample point, which is the strongest and most stable signal in the portfolio. Sea Bluffs reads 9 at the building and 9 to 10 around it, and Oakmont of Santa Clarita is flat at 9 — both new to this review and both in the same category as Mission Viejo rather than the noisy ones. Oakmont of San Antonio Heights is steady at 7, as are Otay Ranch, Sabre Springs and The Ivy at Wellington at 5.

By contrast Oakland Hills, Hayward, Fullerton, Oakmont of Redding and Oakmont of Valencia read 0 at the building and reach 8 to 10 within 40 m. Those are sites with steep ground at the property edge, which is a different problem from a building on a slope — it is a question about what could come down onto a car park, a path or a boundary wall rather than about the building itself.

Two sites appear in both halves of this briefing. Oakmont of Simi Valley is in Zone AO with a 2.0 ft flood depth and reads class 7 for landslide, and Oakmont of San Antonio Heights is in the 0.2% floodplain and steady at 7. Both sit on alluvial fans below steep ground, which is the terrain that produces the two hazards together.

Glen Cove returns no value at the site coordinate but ranges 0 to 10 nearby. It is not unassessed, as we previously reported — it sits on the edge of the mapped data.

Source. California Geological Survey, Susceptibility to Deep-Seated Landslides, Map Sheet 58 — a statewide map first published May 2011, combining rock strength and slope by the method of Wilson and Keefer (1985) as implemented by Ponti and others (2008), scored 0 to 10. Queried per site against the CGS map service at gis.conservation.ca.gov/server/rest/services/CGS/MS58_LandslideSusceptibility_Classes. You can view the same layer, with individually mapped landslides drawn over it, in the CGS Landslide Inventory and Deep Landslide Susceptibility map ↗ — still marked beta by CGS, and worth zooming to the parcel rather than judging from a regional view.

What has actually happened here before

Every one of the 119 sites sits in a county with a declared flood history — 30 counties, between 4 and 21 separate federal disaster declarations each, the earliest from December 1964 and the most recent from April 2026. There is no county in this portfolio without one. Flooding near these communities is not hypothetical.

Five events stand out for how much of the portfolio they covered at once, and two of them are the direct analogues for this winter. 1983 (DR-677, coastal storms and floods) covered 19 of the 30 counties you operate in — that was the 1982–83 El Niño. 1998 (DR-1203, severe winter storms and flooding) covered 19 — that was 1997–98. March 1995 (DR-1046) covered 22, the widest single footprint in this record, and January 1995 (DR-1044) covered 19 on its own. The 2023 atmospheric river sequence covered 20 in January and another 17 in April.

The two winters most like the one NOAA is forecasting each produced a federal disaster declaration across nearly every county where Oakmont operates. That is a stronger argument than any seasonal outlook.

What we would do next

Where this came from

Flood zones are point determinations against the FEMA National Flood Hazard Layer at all 119 sites' own coordinates, run 27 September 2026 and re-run across the full portfolio on 29 September 2026. County assignment is by coordinate against the US Census Bureau county boundaries rather than the county stored with each site, which corrected 12 sites. Site maps show those zones over the Esri ArcGIS Light Gray Canvas basemap (Esri, HERE, Garmin, © OpenStreetMap contributors). The El Niño status and Niño index values are from the NOAA Climate Prediction Center ENSO Diagnostic Discussion issued 10 September 2026. The disaster history is the FEMA declarations database. Landslide susceptibility is the California Geological Survey, Susceptibility to Deep-Seated Landslides, Map Sheet 58 (2011), sampled at each of the 110 California sites and at four points 40 m out from each. Every site links through to the FEMA Flood Map Service Center ↗, which is the authoritative source for effective panels and printable FIRMettes.

A determination is taken at the site's stored coordinate, so a large campus can straddle a zone boundary. This is decision support, not a substitute for a site-specific flood study or an official FEMA determination for insurance or lending.