The Weather Signal Is Strengthening
The 2026–27 El Niño is no longer a speculative possibility. Its eventual consequences remain uncertain, but the warning signal has become difficult to ignore.
Above Ninety Percent
Something unusual is developing in the Pacific. On September 10, the National Oceanic and Atmospheric Administration reported that El Niño had strengthened again and now carries a greater than 90 percent chance of becoming a very strong event during the Northern Hemisphere fall and winter of 2026–27.
The latest observations show sea-surface temperature anomalies exceeding 3°C in parts of the eastern equatorial Pacific. The Niño-3.4 index reached +1.8°C in August, while unusually warm water remains widespread below the surface. Oceanic and atmospheric circulation are now coupled in a pattern consistent with a strengthening El Niño. NOAA also assigns a 75 percent chance that the October–December 2026 period will reach a three-month Relative Oceanic Niño Index value of at least +2.5°C, a threshold that would exceed previous El Niño events in its record dating to 1950.
The World Meteorological Organization is similarly emphatic. Its September update describes El Niño as firmly established and expected to intensify further, with a nearly 100 percent likelihood that it will persist through February 2027. WMO is warning governments and meteorological agencies to prepare for elevated risks of flooding, drought and extreme heat.
None of this establishes where a destructive flood will occur, whether Tennessee will experience a major ice storm, or whether California will endure a damaging sequence of Pacific storms. It establishes that a major planetary climate pattern is strengthening rapidly enough to change the probabilities of disruptive weather across large portions of the world, giving governments, infrastructure operators, farmers and households time to prepare before the most consequential effects become visible.
What the Signal Means
El Niño is not a storm. It is a large-scale disruption of the coupled ocean-atmosphere system in the tropical Pacific. Unusually warm water across the central and eastern equatorial Pacific alters where tropical convection and heavy rainfall develop, and those changes affect atmospheric circulation far beyond the tropics.
Over North America, one common result is a change in the position and strength of the jet stream, which in turn alters the routes storms tend to follow. El Niño winters commonly favor a more active southern branch of the jet, increasing opportunities for storms across parts of California, the southern United States and the Southeast.
Seasonal climate signals do not provide a weather itinerary. A stronger El Niño increases the likelihood of some characteristic effects, but NOAA explicitly warns that those effects remain more likely rather than guaranteed. Other ocean patterns, atmospheric variability and the timing of individual weather systems still matter. A season can finish wetter than normal without producing a catastrophic flood, while a relatively dry season can still contain one devastating storm. A mild winter can likewise contain several days of dangerous cold.
Those distinctions matter because public discussion of seasonal climate forecasts often collapses probability into certainty. The available science does not support that. It does support paying closer attention when the probabilities move substantially.
The Signal Has Strengthened Quickly
The progression during 2026 is notable. NOAA ended its La Niña advisory and issued an El Niño Watch in April, when ENSO-neutral conditions had returned and the probability of El Niño developing during May through July stood at 61 percent. At that point, NOAA described roughly a one-in-four chance of a very strong El Niño developing later in the year.
By May, NOAA estimated an 82 percent chance that El Niño would emerge during May through July and a 96 percent chance that it would continue through the following winter, although uncertainty about its eventual strength remained substantial.
El Niño conditions were present by the June 11 update. NOAA then estimated a 63 percent chance of a very strong event during November through January. By July, after further ocean-atmosphere coupling and warming, the agency's strength outlook had moved substantially toward the upper end of the range.
The September outlook uses an even more demanding measure for its most striking number. NOAA now assigns a 75 percent chance that the October–December event reaches a three-month Relative Oceanic Niño Index value of at least +2.5°C, a threshold that would exceed previous events in the record dating to 1950. It separately places the chance of a very strong El Niño during fall and winter above 90 percent.
Those probabilities should not be read as a single sequence in which one number simply rises or falls from month to month. NOAA's outlooks refer to different seasonal windows and strength thresholds as the event develops. The consistent development is increasing confidence that El Niño will be both persistent and unusually strong.
The United States Will Not Experience One El Niño Winter
El Niño does not impose one uniform weather pattern across the United States. Historically, stronger events have often favored wetter conditions across parts of the southern tier, including southern California and the Southwest, while portions of the Ohio and Tennessee Valleys have shown a tendency toward below-average winter precipitation. Even among strong El Niños, however, individual winters vary substantially.
The Tennessee Valley illustrates why seasonal totals and individual storms must be kept separate. A winter can finish drier than normal because relatively little precipitation falls across the season as a whole and still contain a short period in which a moisture-rich storm encounters sufficiently cold air to produce damaging snow, sleet or freezing rain. Seasonal precipitation describes accumulation across several months; it does not describe how that precipitation was distributed in time or the temperature profile present when it fell.
El Niño can increase opportunities for southern storm systems without determining when Arctic air will arrive. That makes the southern Plains, Tennessee Valley, Appalachians and portions of the Mid-Atlantic worth watching without pretending that a specific storm can already be forecast. If a moisture-bearing system coincides with a significant cold-air intrusion, the consequences can be substantial even if the season ultimately finishes near or below its normal precipitation total.
California Has a Different Exposure
California's concern is less about precipitation type and more about repeated Pacific storms. Strong El Niño winters have historically shown a relatively reliable tendency toward above-average precipitation in southern California and the Southwest, although the outcome still varies considerably from one event to another.
The danger increases when storms arrive in succession. One storm can saturate soil, another can raise rivers and reservoirs, and additional rainfall can destabilize slopes already holding large amounts of water. At higher elevations, the same sequence can produce substantial snow while lower elevations receive heavy rain.
The resulting hazard can therefore be cumulative rather than confined to a single storm. Flooding, debris flows, landslides, power failures and transportation disruption can build on one another when multiple systems affect the same region in a short period. NOAA's retrospective analyses of recent El Niño winters also show how El Niño-related jet-stream changes can accompany unusually wet conditions and atmospheric-river activity in California.
No forecast currently establishes that such a sequence will occur during the coming winter. The atmospheric conditions capable of supporting it are becoming more plausible, which is enough to justify watching where the Pacific storm track settles and whether systems begin repeatedly striking the same watersheds.
Severe Weather Remains Part of the Southern Risk
A more active southern storm track can also create environments favorable for severe thunderstorms during the cool season, although that relationship needs to be described carefully. El Niño does not translate cleanly into more tornadoes. NOAA research has found that El Niño events persisting into spring are associated with comparatively mild tornado-outbreak risk across much of the country, with an important exception along the Gulf Coast and in central Florida during February.
The Southeast also has vulnerabilities that can make tornadoes particularly dangerous when they do occur. NOAA-supported research identifies the frequency of nighttime tornadoes and the prevalence and distribution of mobile and manufactured housing as important contributors to the region's tornado risk. Separate work supported through NOAA's VORTEX program has found that nocturnal tornadoes are substantially more deadly than daytime events and that residents of mobile and manufactured housing face particularly high vulnerability.
A potentially historic El Niño therefore does not justify predicting a historic tornado season. It does justify including severe thunderstorms and cool-season tornadoes among the hazards that warrant attention in parts of the South, rather than treating the winter risk entirely as a question of snow and ice.
The Global Exposure Is Larger
North American winter weather is only one part of the developing story. The Food and Agriculture Organization and World Food Programme have already launched a joint anticipatory-action appeal covering June 2026 through March 2027. The effort seeks to protect nearly nine million people across 22 high-risk countries from potential El Niño impacts before droughts and floods further damage food security, livelihoods and agricultural production.
Agriculture is particularly exposed because El Niño redistributes rainfall rather than simply increasing or decreasing it everywhere. FAO and WFP expect strong El Niño conditions to intensify drought and flooding risks across parts of Africa, Asia and the Pacific, Latin America and the Caribbean, placing already food-insecure populations at additional risk.
One region may receive too little rain while another receives too much. Drought can damage crops and pasture, while flooding can destroy stored food, livestock, roads and agricultural infrastructure. Heat can add another source of stress, and marine ecosystems and fisheries can also respond to changing ocean conditions.
FAO Deputy Director-General Beth Bechdol described the logic behind acting before the impacts become severe:
"Experience consistently shows that early action is more effective and less costly than responding after a crisis has escalated." Beth Bechdol, FAO Deputy Director-General
FAO and WFP's June appeal includes measures such as cash assistance, drought- and flood-resistant seeds, livestock protection, water storage, flood infrastructure, agricultural advisories and early-warning systems. The purpose is to preserve assets and food production before households are pushed from vulnerability into crisis.
That lead time matters most in places where food systems already have little room to absorb another shock. A drought affecting a food-secure exporting country is damaging, but the same drought affecting a population already experiencing acute hunger is far more dangerous. Flooding across redundant transportation infrastructure creates disruption, while flooding across the only road connecting an agricultural district to market can destroy livelihoods. One poor harvest may be absorbed by global trade; simultaneous production losses across several vulnerable regions are harder to absorb.
Weather Can Become an Economic Event
The transmission does not stop at farms or flooded roads. Agricultural losses can move into commodity prices, while low river levels or flooding can interrupt inland shipping. Extreme precipitation can damage rail corridors and highways, changes in rainfall can affect hydropower production, and floods, severe storms and landslides can generate insurance losses and public disaster spending.
None of those outcomes follows automatically from El Niño. They are transmission pathways whose importance depends on where the climate shock lands, which systems are exposed and how much resilience those systems retain.
That is the same systems problem Horizon Accord examined in Where the Fertilizer Shock Went. The initiating event matters, but the broader consequences emerge through the networks connected to it. A climate disruption can therefore produce economic effects far from the places experiencing the most dramatic weather because agricultural production, transportation, energy, insurance and humanitarian systems connect local physical shocks to wider markets and institutions.
A crop loss becomes economically important when it affects supply in a market with little spare production. A flood becomes a logistics problem when it interrupts a corridor that cannot easily be bypassed. A regional drought becomes a humanitarian problem when it reaches communities that are already food insecure. The physical event and the systemic consequence are related, but they are not the same thing.
Other Climate Patterns Matter Too
El Niño is not operating in isolation. WMO expects a positive Indian Ocean Dipole to develop, with the September–October–November 2026 seasonal mean forecast around +0.9°C. The equatorial Atlantic is also expected to remain warmer than normal. WMO cautions that these conditions may reinforce, weaken or alter typical El Niño effects in different regions.
Those interactions make simple historical analogies less reliable. The atmosphere does not respond to El Niño alone, and regional outcomes will depend on the combined state of several oceanic and atmospheric systems as the season develops.
Historical Parallel — Documented Record
The strongest El Niño events in the modern observational record provide useful comparisons, but not templates. The 1982–83, 1997–98 and 2015–16 events each produced substantial climate disruptions, yet their regional effects differed significantly.
The 1982–83 and 1997–98 events brought heavy rainfall, flooding and landslides to California, while the 2015–16 event reached comparable strength by major ocean-atmosphere measures without reproducing the same California precipitation outcome. NOAA's retrospective work on these events is a useful reminder that the strength of El Niño does not map directly onto the strength or location of its impacts.
These events establish that a very strong El Niño can reorganize weather at continental and global scales. They do not establish that the 2026–27 event will repeat any particular historical sequence. Ocean conditions outside the tropical Pacific, atmospheric variability and the warmer background climate all make direct one-to-one analogies unreliable.
Historical comparison is useful for identifying the kinds of hazards that become plausible under a very strong El Niño. It is less useful as a map of where those hazards will occur.
Warning Is Not Prediction
There is an interval in an emerging hazard when the evidence is strong enough to justify preparation but not strong enough to describe the eventual outcome with confidence. The current El Niño has entered that interval.
No scientific basis currently supports announcing a nationwide flood, a guaranteed Tennessee ice storm, a catastrophic California winter or a worldwide agricultural crisis. There is substantial scientific basis for saying that one of Earth's dominant climate patterns is strengthening, has a significant chance of reaching historically exceptional intensity, and is expected to alter rainfall and temperature probabilities across large portions of the world into 2027.
Waiting until an individual storm appears on a seven-day forecast would provide much greater certainty and much less preparation time. Seasonal forecasting is useful precisely because meaningful information becomes available before the details of individual events can be known.
The distinction matters for public communication. Treating every probabilistic warning as a prediction produces unnecessary alarm when the most dramatic outcome does not occur. Refusing to act until an outcome becomes certain wastes the lead time that forecasting provides. Responsible preparation occupies the space between those two errors.
What Preparation Looks Like
Preparation should match both the hazard and the confidence of the evidence. Emergency managers can review drainage capacity, shelters, warning systems, backup power and flood-response resources. Utilities can examine vegetation management, ice-loading exposure, mutual-aid arrangements and vulnerable substations. Transportation agencies can identify roads, rail lines, mountain passes and bridges with known flood or landslide vulnerabilities.
Agricultural planners can track soil moisture, crop calendars, irrigation availability, livestock exposure and commodity risks. Humanitarian organizations can act before drought or flooding destroys the assets people depend on for survival. Household preparation is simpler and more local: maintaining weather alerts, medications, backup lighting and power, basic supplies, pipe protection where severe cold is possible, and evacuation routes where flooding is a known hazard.
None of these measures requires certainty about the eventual weather. They require enough evidence to justify inexpensive, reasonable preparation before the cost of being surprised becomes much higher.
What Horizon Accord Will Watch
Several indicators will sharpen the picture during the coming months. El Niño's actual peak strength will matter, but intensity alone will not determine its impacts. The position and persistence of the North American storm track will matter just as much. California's exposure will become clearer if Pacific systems begin arriving frequently or in clusters, while winter-storm risk farther east will depend heavily on the timing of cold-air outbreaks relative to southern storm systems.
Agricultural conditions will require a wider lens. The most consequential food-system effects are likely to emerge when climate stress reaches regions already dealing with food insecurity, weak infrastructure or limited capacity to absorb crop and livestock losses. Simultaneous problems across several producing regions would matter more to global systems than an isolated weather disaster.
The interaction between El Niño, the Indian Ocean Dipole and conditions in the Atlantic will also require continued attention because those patterns can modify the familiar El Niño footprint enough to make historical analogues imperfect guides.
These developments can be monitored as the season evolves and compared with earlier forecasts. The Pacific is not providing a precise description of the coming winter, but it is showing that the distribution of plausible outcomes has changed enough to deserve sustained attention while there is still time to prepare.