About El Niño Tracker: Data Sources and Methodology

What this site does

El Niño Tracker follows the El Niño-Southern Oscillation (ENSO) and what it could mean for temperatures in 25 countries where El Niño and La Niña have well documented effects. For each country it shows a plain-language seasonal outlook, the current state of El Niño, and a 7-day forecast for major cities compared with the same week last year.

Where the data comes from

NOAA Climate Prediction Center: the Oceanic Niño Index and the monthly ENSO diagnostic discussion, which set the current phase and strength.
Copernicus Climate Data Store: seasonal forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF SEAS5 model).
IRI, Columbia University: how each country has typically responded to past El Niño and La Niña events.
Open-Meteo: 7-day city forecasts, and the historical weather API (ERA5 reanalysis) for the same dates last year.

How El Niño is measured

NOAA measures El Niño and La Niña with the Oceanic Niño Index (ONI), the 3-month average sea surface temperature anomaly in the Niño 3.4 region of the central tropical Pacific. An El Niño needs the ONI at +0.5°C or more, and a La Niña at −0.5°C or less, held for five overlapping 3-month seasons. By size, events are weak (0.5 to 0.9), moderate (1.0 to 1.4), strong (1.5 to 1.9) or very strong (2.0 or more). The latest ONI is +1.8°C for June to August 2026, a strong El Niño.

How the seasonal outlook is made

For most countries the outlook uses the ECMWF ensemble mean forecast of 2 m air temperature for the next three complete months, compared with the model's own 1993 to 2016 average. The value is taken at the model grid point nearest the middle of the country, so it describes that area rather than every part of the country.

The chances of a cooler, typical or warmer season are estimated by El Niño Tracker from the size of that difference, because the ensemble mean product does not include the model's own odds. Confidence is low for differences under 0.5°C, moderate from 0.5 to 1°C, and high at 1°C or more. When no model forecast is available, the outlook falls back to the country's historical pattern in past El Niño or La Niña events.

7-day weather and last year

Each country page lists up to four major cities with forecast lows and highs, conditions and the average of the daily mean temperatures for the next 7 days. The same calendar dates last year come from observed ERA5 data. Differences under 0.5°C are shown as about the same, since they are within normal forecast error.

How often it updates

All sources are checked twice a day, at 00:00 and 12:00 UTC, and every page is rebuilt with the new data. The ECMWF seasonal forecast itself is issued once a month, and NOAA updates the Oceanic Niño Index monthly.

Limitations

Seasonal outlooks describe likelihoods over a whole season, not the weather on a given day. Short-range forecasts can change several times a day, so for day-to-day decisions use your local weather service. Nothing on this site is an official warning.