Strongest El Niño in 140 Years

Weather

Apr 22

Strongest El Niño in 140 Years? What the April Forecast Means for San Joaquin County and Lodi

Summary

A rare triple-cyclone cluster near the equator in early April triggered what atmospheric scientists are calling possibly the most powerful westerly wind burst in the equatorial Pacific in a century. The April seasonal forecast from the European Centre for Medium-Range Weather Forecasts (ECMWF) shows near-universal model agreement that El Niño conditions will arrive by mid-to-late summer. A subset of models — about half of the ECMWF ensemble — projects sea-surface-temperature anomalies above 2.5°C by October.

NOAA’s Climate Prediction Center takes a more measured view: its April 9 update maintains an El Niño Watch with a 61% probability of emergence by May–July, and cautions that strength remains genuinely uncertain. NOAA also now uses a newer index (RONI) that tends to produce smaller El Niño numbers than the traditional one, complicating any “strongest in 140 years” comparison.

For San Joaquin County and Lodi, the picture is further complicated by this year’s unusual water-year volatility, a record-breaking marine heatwave off the California coast, and a Sierra snowpack currently tied with 2015 for the lowest on record to date. History shows that a strong El Niño label does not guarantee a wet winter here — but when conditions align, the Mokelumne and San Joaquin river systems, the Delta levees, and Lodi’s vineyards absorb the consequences together.

What Changed Since Our March Report

When LodiEye published its El Niño outlook on March 17, forecast models were already pointing to a strong event. The story has intensified since — though with an important caveat about which institutions are saying what.

On April 8, the San Francisco Chronicle reported that Paul Roundy, an atmospheric scientist at the University at Albany, described the developing setup as “real potential for the strongest El Niño event in 140 years.” Roundy was responding to a cluster of three tropical cyclones that formed simultaneously on both sides of the equator in the western Pacific, generating a westerly wind burst that is now pushing unusually warm subsurface water eastward at an accelerated pace.

“Real potential for the strongest El Niño event in 140 years.” — Paul Roundy, atmospheric scientist, University at Albany, quoted in the San Francisco Chronicle, April 8, 2026

That wind burst matters because of where it sits. It is positioned directly west of the warmest water currently in the Pacific, meaning it can continue driving that warm pool eastward toward the central and eastern equatorial Pacific — the region whose temperatures define an El Niño event.

What NOAA Actually Says

It is worth separating the Chronicle’s reporting from NOAA’s official line. NOAA’s Climate Prediction Center issued its latest ENSO Diagnostic Discussion on April 9, maintaining an El Niño Watch but placing the probability of El Niño emergence at 61% for May–July 2026. NOAA explicitly notes that if El Niño forms, the potential strength remains very uncertain, with roughly a one-in-three chance that it would be classified as strong during October–December 2026. That is a meaningfully more conservative position than the ECMWF ensemble midpoint or the individual scientist commentary that drove the Chronicle piece.

The IRI (International Research Institute for Climate and Society) mid-April plume, released April 17, tilts slightly more aggressive than NOAA: a 70% probability of El Niño developing in April–June, with probabilities climbing to 88–94% through the rest of 2026.

Institutional Model Agreement on El Niño Emergence

Percent probability of El Niño conditions by mid-to-late 2026 across major forecasting institutions, as of their April 2026 updates. Sources: Australian BOM (2026-04-04); ECMWF April seasonal; IRI Plume (2026-04-17); NOAA CPC ENSO Diagnostic Discussion (2026-04-09).

The RONI Caveat

A technical development matters here. In February 2026, NOAA transitioned its official ENSO tracking from the traditional Oceanic Niño Index (ONI) to the Relative Oceanic Niño Index (RONI). RONI adjusts for long-term global ocean warming; in most cases, it dampens the measured strength of El Niños and amplifies La Niñas relative to the older index. Any “strongest in 140 years” claim anchored to ONI-era records is therefore not a direct apples-to-apples comparison with how this event will be officially characterized.

In plain terms: the event does look strong. Whether it ends up labeled as the single strongest on record depends partly on which index the label is attached to.

Australia and ECMWF Remain Bullish

Australia’s Bureau of Meteorology, running its seasonal forecast on April 4, put the probability at 100% by June and projected the event crossing into super territory (anomalies above 2°C) by August. The April ECMWF run shows virtually all of its ensemble members reaching El Niño thresholds by mid-to-late summer. A Columbia University climate scientist told the Chronicle that the unusual feature this year is the level of cross-model agreement among independent institutions.

Projected Peak Sea-Surface-Temperature Anomaly (Niño 3.4)

Historic event peaks vs. April 2026 ECMWF ensemble midpoint projection. A Super El Niño is defined as anomalies exceeding 2.0°C. Source: NOAA CPC ERSSTv5 historical; ECMWF April 2026 seasonal forecast.

The California Conditions El Niño Would Arrive Into

An El Niño label describes the ocean. California’s actual winter depends on the atmospheric conditions the warm Pacific couples with. Several unusual factors are already in place as of late April.

A Volatile Water Year

UCLA climate scientist Daniel Swain, writing on his Weather West blog on April 19, characterized Water Year 2025–26 as genuinely strange: record Oct–Dec rainfall on the central and south coast (wettest ever observed in Santa Barbara, Ventura, Kern, and parts of LA County), followed by oscillation between exceptional dryness and one of the largest single Sierra snowstorms in recent decades in January–February, then the hottest and in some areas driest March on record across much of the West. April has flipped again to unusually active, with some Sierra locations on track for a top-three wettest April on record.

Snowpack: Tied for Lowest on Record

Despite April’s wet pattern, the Sierra Nevada snowpack remains in a virtual dead heat with 2015 for lowest on record to date — though unlike bone-dry 2015, this year has actually featured much higher precipitation totals. The problem is timing: the record March heat melted accumulation almost as fast as it arrived. The Upper Colorado River basin snowpack, which matters for the broader Western water system, remains the lowest on record for the calendar date.

This has direct implications for how an El Niño winter would play out. Heavy precipitation falling on depleted snowpack runs off as water, not snow. Reservoirs that would normally benefit from a slow April–July snowmelt instead have to absorb storm-by-storm inflows. The 1997 New Year’s flood unfolded under similar conditions: warm rain on limited snowpack, overwhelming the Mokelumne and San Joaquin systems simultaneously.

Record Marine Heatwave

A second unusual factor: the northeastern Pacific is experiencing an extreme marine heatwave. According to Swain, a vast triangular region between Hawaii, the Pacific coast of Mexico, and central/southern California is recording its warmest-ever sea surface temperatures, with the Scripps Pier in San Diego logging nearly continuous record-warm days through 2026. C3S multi-model projections suggest this warm pool will expand to cover essentially the entire Pacific Coast of North America by September.

The southwest-to-northeast axis of this warmth aligns with the positive phase of the Pacific Meridional Mode (PMM), which climate research increasingly treats as a potential precursor to El Niño. Combined with the equatorial Pacific signals, it reinforces the case that the broader climate system is shifting. For California specifically, Swain expects a muted May Gray / June Gloom marine layer season, warmer-than-average coastal overnights, an elevated chance of tropical remnant moisture reaching the state in late summer, and potentially more active Eastern Pacific hurricane activity.

Why California Residents Should Take This With Calibrated Caution

The memory of 2015–16 is still fresh. That winter was forecast to be a Godzilla El Niño strong enough to end a multi-year drought. The strong El Niño arrived on schedule. The drought-breaking rain did not. The Bay Area finished near normal precipitation. Southern California came in at roughly 72% of normal — a dry winter despite one of the strongest Pacific warming signatures on record.

Historical California rainfall outcomes during strong El Niños tell a mixed story:

Lodi-Area Rainfall During Historic Strong El Niño Winters

Water-year precipitation totals for the Lodi / northern San Joaquin Valley area during strong El Niño winters, expressed as percent of 30-year normal. Source: NOAA NCEI COOP station records (Lodi, Stockton Metro).

The takeaway: 1982–83 and 1997–98 produced genuinely wet winters with serious flood consequences across the Central Valley. 1976–77 was one of California’s driest years on record despite an active El Niño. 2015–16 delivered near-normal precipitation but did not refill the system. A strong El Niño is a probability shift — not a guarantee.

San Joaquin County: The Delta and the Levees

If this El Niño does produce a wet winter, San Joaquin County sits at one of the most exposed points in California’s water system. Four major rivers — the Stanislaus, Tuolumne, Merced, and Mokelumne — plus smaller tributaries feed into the San Joaquin River, which drains through the Delta. When the Sierra watershed is saturated and reservoirs are near capacity, water arrives in the Delta faster than the levee network was designed to absorb.

Stockton’s Structural Exposure

Stockton has been repeatedly flagged by federal studies as one of the most under-protected major urban areas in California. A 2018 U.S. Army Corps of Engineers assessment found that much of the city could flood to depths of 10 to 20 feet if its main levees were breached. The primary levee protecting the city is rated to only a 100-year flood standard — and according to the National Levee Database, that single levee protects more than 129,000 residents and roughly $16 billion in property value. Rural levees elsewhere in the county have lower thresholds, with some privately owned structures rated to only 10-year flood protection.

The Delta itself adds a compounding risk. Many of the roughly 60 Delta islands sit 10 to 25 feet below sea level, held dry only by aging levees. The ARkStorm scenario modeled by the USGS estimated that in an extreme atmospheric river sequence, roughly half of those islands could flood, with removal of the water taking up to 18 months. That scenario is not forecast for this winter — but the combination of a strong El Niño, any lingering tropical moisture plume, and saturated soils is precisely the cocktail that stresses the levee network.

The 1862 and 1997 Precedents

Historical memory in San Joaquin County runs deep on this. The 1862 Great Flood turned the Central Valley into an inland sea for weeks; Mokelumne City (the third-largest community in the county at the time, seven years before Lodi was founded), Woodbridge, and Lockeford were all overrun when the Mokelumne, Cosumnes, and Calaveras rivers broke their banks on Christmas night. The 1997 New Year’s flood — during the strong 1997–98 El Niño winter — caused multiple levee breaks along the Mokelumne River, inundating Thornton and causing billions of dollars in damage across the Sacramento, Feather, and San Joaquin systems.

The Lodi Focus: Mokelumne, Vines, and Reservoirs

For Lodi residents specifically, three systems deserve close watching over the next nine months.

1. The Mokelumne System — Pardee, Camanche, and WID

Lodi’s municipal water supply and a significant share of agricultural water in the district flow from the Mokelumne River, stored at East Bay Municipal Utility District’s Pardee and Camanche reservoirs, and distributed through the Woodbridge Irrigation District. Pardee and Camanche together hold a maximum of roughly 615,000 acre-feet. The operating question in a strong-El-Niño scenario is not whether enough water will arrive — it is whether the reservoirs will need to make controlled flood-control releases, and when.

The risk geometry is this: if the Sierra snowpack remains thin (as it has for much of the 2025–26 water year) and an El Niño then delivers heavy rain onto already-saturated ground, runoff arrives fast without the natural buffer of a slow-melting snowpack. That is the pattern that stressed the Mokelumne system in 1997.

2. Vineyards — Bloom, Mildew, and Harvest Timing

Lodi harvests roughly 82,000 acres of wine grapes, delivering approximately 600,000 tons in a normal year — about 20% of California’s total. A wet El Niño winter is a mixed proposition for the appellation:

Positive: Deep winter soil moisture during dormancy supports healthy budbreak and vine vigor, as reported across the region after the wet 2022–23 season.
Negative: Wet spring conditions extending into April–May drive mildew and botrytis pressure, force more aggressive canopy management, and crowd fruit zones — exactly what Lodi growers contended with in 2023. Growers who kept vineyards clean and thinned aggressively were rewarded; those who could not saw fruit losses.
Timing risk: Late-season rain arriving during veraison or harvest forces expedited picks, compressed labor windows, and sorting costs. For Lodi’s signature old-vine Zinfandel, rain during harvest can degrade the concentration that defines the vintage.

Against the backdrop of an industry already stressed by oversupply, weak demand, and ongoing vineyard removals, a disease-pressured harvest would arrive at a particularly unforgiving moment for the local wine economy.

3. Urban Flood Infrastructure — The Downtown and the Mokelumne Corridor

Lodi’s storm drainage, the Mokelumne River levees north of town, and the overflow behavior at Lodi Lake are the three structural elements that determine whether heavy rain arriving in rapid pulses translates into localized flooding. The city has invested in drainage upgrades over the decades, and a 100-year rain event remains the design standard that most urban infrastructure is sized against. An El Niño winter with multiple back-to-back atmospheric rivers tests that standard directly. Residents in District 5 and in the lower-lying neighborhoods near the Mokelumne should know their evacuation routes and consider whether flood insurance — which is separate from standard homeowner’s coverage — is worth reviewing before October.

What the Forecast Does — and Doesn’t — Tell Us

The unusual feature of this forecast cycle is the degree to which international modeling centers on different continents are producing the same answer. ECMWF, the Australian BOM, the IRI plume, and NASA all converge on high probabilities of El Niño emergence. NOAA’s more measured 61% posture reflects the institutional convention of weighting human forecaster judgment alongside models, and the known difficulty of seasonal ENSO forecasts made before June — what meteorologists call the spring predictability barrier. The tropical Pacific signals suggest that barrier may be weakening this year, but it has not fully broken.

What remains genuinely uncertain is the California-specific rainfall response. El Niño modulates the probability of where the Pacific storm track sets up, but the actual winter depends on the number and strength of atmospheric rivers that reach land — a detail no seasonal model resolves well. Southern California historically sees a stronger El Niño rainfall signal than Northern California. The San Joaquin Valley sits near the hinge, and outcomes there have been the least predictable of any part of the state.

What to Watch Between Now and October

NOAA CPC May 8 ENSO update: The next monthly Diagnostic Discussion is likely when NOAA upgrades from El Niño Watch to El Niño Advisory if the Pacific keeps warming on schedule.
Sierra snowpack April 1 & May 1 readings: The April 1 measurement, already tied with 2015, largely determines this year’s runoff baseline. Any late-April snow accumulations will change the picture only marginally.
Pardee and Camanche storage curves through summer: Reservoirs that enter fall near capacity have far less room to absorb heavy autumn rain.
Marine heatwave persistence: Whether the northeastern Pacific warmth expands or fades will influence atmospheric river moisture loading.
Delta levee assessments: San Joaquin County OES and the Department of Water Resources typically update fall flood preparedness by September.
Vineyard bloom and late-spring disease pressure: An early signal of what a wet fall harvest could look like in Lodi.

Practical Steps for Lodi Residents

Check whether your property sits in a FEMA Special Flood Hazard Area at msc.fema.gov.
Flood insurance has a 30-day waiting period — purchasing in fall is too late if storms arrive in November.
Know your evacuation route: San Joaquin County maintains maps at sjmap.org/evacmaps.
Sign up for San Joaquin County Office of Emergency Services alerts.
Growers: review vineyard drainage, canopy-management plans, and sorting labor contingencies before budbreak next spring.

LodiEye will continue tracking ENSO forecast updates, Mokelumne system storage, and fall flood preparedness through the 2026–27 water year. If you have questions or local observations to share, reach us at editor@lodi411.com.

About This Report: AI-Assisted Research and Analysis

This LodiEye civic climate brief was produced using artificial intelligence tools under the direction and editorial review of Lodi411’s human editor. Lodi411 uses multiple AI platforms in its research and publication workflow, including Anthropic’s Claude (primarily Opus and Sonnet models) and Perplexity AI across a variety of large language models offered by each. These tools were used in the following capacities:

Source Discovery: AI-assisted search and retrieval identified over a dozen primary and secondary sources, including the San Francisco Chronicle article (via Apple News), NOAA Climate Prediction Center diagnostic discussions, the IRI / Columbia University ENSO plume, ECMWF and Australian BOM seasonal forecasts, Daniel Swain’s Weather West analysis, Yale Climate Connections coverage, San Joaquin County flood protection documentation, Wine Institute harvest reports, and historical flood records from the Lodi News-Sentinel and Wikipedia. Perplexity AI was used for initial source discovery and real-time data retrieval; Claude was used for deeper analysis of identified sources.

Credibility Validation: AI cross-referenced forecast claims across five independent institutions (ECMWF, Australian BOM, NOAA CPC, IRI / Columbia, NASA GMAO), prioritizing government datasets, peer-reviewed research, institutional analysis, and news reporting in that order. Historical flood references were verified against primary San Joaquin County government documentation and USGS ARkStorm scenario materials. Multiple AI models were used to independently verify key data points and flag inconsistencies — most notably the divergence between NOAA’s official 61% probability and the higher international modeling consensus, and the technical implications of the new RONI index.

Analysis and Synthesis: Claude Opus assisted in structuring the Lodi-specific risk framework around the three systems most relevant to residents: the Mokelumne water supply chain (Pardee, Camanche, WID), the vineyard economy, and urban flood infrastructure. The article synthesizes national forecast data with localized historical context from the 1862 and 1997 floods, and integrates the current Water Year 2025–26 context (record-low snowpack, marine heatwave, volatile precipitation) that shapes how an incoming El Niño would actually play out.

Presentation: Claude assisted in drafting and structuring the report for clarity and readability, including the three inline Kendo UI chart visualizations (institutional model agreement, historic SST anomaly comparison, Lodi-area rainfall during past strong El Niños), the historical framing, and the narrative flow from forecast context to local implications to resident-facing guidance.

Final Review: Multiple AI models reviewed the completed draft for factual consistency, source attribution accuracy, logical coherence, and balanced presentation — particularly the separation between Chronicle reporting and NOAA’s official position. All editorial judgments, analytical conclusions, and publication decisions were made by Lodi411’s human editor.

Lodi411/LodiEye believes transparency about AI use in journalism serves both readers and the profession. We use multiple AI platforms — including Anthropic’s Claude (Opus and Sonnet) and Perplexity AI — as research, analysis, and presentation tools, not as autonomous authors. All editorial judgments, analytical conclusions, and publication decisions are made by Lodi411’s human editor, who directs and reviews all AI-assisted work.

References

Contact: editor@lodi411.com

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Don Bradford