Methodology
This page documents, in full technical detail, how Nino34 turns raw NOAA data into the classifications, comparisons, and confidence labels shown across the site. It's the deepest page here on purpose — if you want the plain-language version of any one piece, the linked explainers throughout are written for a general audience. This page is for readers who want to see the actual rules.
a. Classification thresholds and strength bands
Nino34 uses NOAA's standard threshold convention for calling ENSO phase and strength. As of NOAA's February 1, 2026 operational switch, classification is RONI-forward: the Relative Oceanic Nino Index is the primary basis for the phase/strength label shown, with the classic ONI value displayed alongside it at all times rather than suppressed. See ONI vs. RONI for why the two indices can differ for the same season, and why we never hide one in favor of the other.
Phase threshold: El Nino or La Nina is called at a seasonal (3-month running mean) anomaly of at least ±0.5°C. Below that magnitude, the season is classified ENSO-neutral.
| Band | Anomaly magnitude |
|---|---|
| Weak | 0.5°C to 0.99°C |
| Moderate | 1.0°C to 1.49°C |
| Strong | 1.5°C to 1.99°C |
| Very strong | 2.0°C and above |
These bands apply symmetrically to El Nino (positive anomaly) and La Nina (negative anomaly, same magnitude thresholds). This is the same convention NOAA's Climate Prediction Center has used for ONI for years; we've carried it forward unchanged onto RONI so that the strength-band language stays legible against decades of prior ENSO reporting, while the underlying index feeding it now corrects for basin-wide warming as described in the RONI explainer.
Note that this NOAA convention is not universal. The Australian Bureau of Meteorology (BOM) calls El Nino at +0.8°C on its own Nino 3.4 index — a materially different threshold on a differently-constructed index. Every classification shown on this site is explicitly the NOAA/ONI/RONI convention; see the glossary entry for why the two shouldn't be blended.
b. "Official event" vs. a single season's "conditions"
This site distinguishes two related but different claims:
- Conditions describes a single 3-month season's anomaly crossing the ±0.5°C threshold. A single season past threshold is enough to say "El Nino conditions" or "La Nina conditions" are present.
- Official event requires five consecutive overlapping 3-month seasons, each at or beyond the ±0.5°C threshold, all on the same side (warm or cool). Because ONI/RONI seasons overlap (DJF, JFM, FMA, ...), five consecutive qualifying seasons corresponds to roughly seven months of sustained anomaly, not five independent months.
The distinction matters at the edges of an episode: a season or two can cross 0.5°C and then fall back below it without ever accumulating the five consecutive seasons needed to be logged as an official event. This site is careful to say "conditions" rather than "event" in that situation, and to only apply an event label (with its associated peak strength and analog comparisons) once the five-season criterion is actually met by the data — not projected in advance.
c. Analog-comparison methodology
Analog comparisons — overlaying the current episode against historical ones of similar strength — are one of the more failure-prone parts of ENSO communication if done sloppily. Two design choices here are meant to keep them honest.
Alignment by onset season, not calendar month
Different El Nino and La Nina episodes ramp up over different lengths of time, and they don't all start in the same calendar month. Comparing "January 2026" to "January 1997" would misalign two events that are actually at very different stages of development. Instead, every analog episode is aligned by its onset season: the first 3-month season in which that episode's anomaly crossed the ±0.5°C threshold, found by walking backward from a known point inside the episode (its peak or its most recent season) until the anomaly drops back under threshold. That onset season becomes offset zero for that episode. Every other season in the episode is then expressed as an offset from onset (onset+1, onset+2, and so on), and analogs are overlaid on that shared offset axis rather than on the calendar. This lets a fast-ramping event and a slow-ramping event line up at the same developmental stage even though they started in different months of different years.
"Percent of strongest event at this stage"
Where the site shows a figure like "current episode is at X% of the strongest analog at this stage," that percentage is computed with two deliberate constraints, both aimed at avoiding a misleadingly rosy or scary number:
- The comparison set is restricted to analog events that have actually reached the same offset as the current episode. A historical event that fizzled out before reaching this stage is excluded from the denominator at this stage — it isn't compared against a stage it never got to.
- The "strongest" reference at each stage is whichever qualifying analog is strongest at that specific offset — not each analog's all-time peak anomaly (which might occur at a different offset, often later in the episode). This keeps the percentage a fair, stage-matched comparison rather than comparing today's early-stage reading against some other event's eventual peak.
This figure is recomputed from the real underlying ONI/RONI series each time the page builds — it is not a hardcoded claim baked in once and left stale. If the composition of qualifying analogs or the current episode's offset changes, the percentage changes with it. Deliberately, this makes the number conservative: it can only ever describe "how this episode compares to analogs that got at least this far," never "how it compares to every El Nino ever," which would inflate or deflate the comparison depending on which historic episodes happened to be strongest overall.
d. Regional confidence tiers
Each of the site's ten US regions carries a confidence tag: strong-tendency, moderate, or weak-mixed. These tiers describe how consistent that region's historical composite signal has been across past ENSO episodes of a given phase — in other words, how often past events of that phase actually produced the expected precipitation/temperature lean in that region, versus how often they didn't.
A "strong-tendency" tag means the historical composite for that region has been fairly consistent across past events. A "weak-mixed" tag means past events have shown a noisier, more mixed record for that region — the average lean might still point one direction, but individual events have deviated from it often enough that the tendency is weak evidence at best. These tiers are explicitly about the region's historical consistency, not about how confident CPC or NOAA is in the current forecast, and not a promise about what will happen this year. Every El Nino is unique; regional pages describe tendencies, not guarantees, regardless of tier. See each region's page for its full composite detail — this page intentionally doesn't re-describe individual regional tendencies, to avoid two sources of truth drifting apart.
e. Data staleness and "data as of" timestamps
Every live number on this site — index values, advisory status, probability distributions, regional tendencies drawn from current classification — carries a real fetch timestamp showing when that value was actually pulled from its source, not when the page happened to render. Each upstream source (CPC's ONI table, the ENSO Diagnostic Discussion, IRI's probability plumes, and so on) has its own staleness threshold based on that source's normal publication cadence, since CPC's monthly discussion and a weekly SST update don't go stale on the same clock.
When a source's data is older than its staleness threshold, the site says so on the page — an explicit notice, with the age of the data and which source is delayed — rather than silently continuing to display the old value as if it were current. We treat this as a non-negotiable discipline: showing a stale number without a flag is functionally the same as showing a wrong number.
f. What this site is, and isn't
Nino34 is an independent, educational ENSO tracker. It is not an official NOAA forecast product and is not affiliated with NOAA, the Climate Prediction Center, or IRI. Every figure here is built from publicly published NOAA/CPC and IRI data, processed through the rules on this page, and presented as probabilistic tendency language — "loads the dice toward," "historically tends to," "tendency, not a guarantee" — never as certainty. For official forecasts, watches, and warnings, always consult CPC's ENSO Diagnostic Discussion directly.
For the raw source list and each feed's publication cadence, see Sources. For answers to common questions about how to read the numbers on this site, see the FAQ.