El Nino Update - Temperatures up another Tenth of a Degree.

The El Niño signal is getting stronger

A potentially historic El Niño is developing

The latest numbers from our 4070weather.com ENSO dashboard continue to show an exceptionally strong El Niño, and this is becoming one of the most important signals to watch as we head toward winter.

The latest Niño 3.4 chart, initialized October 1, shows the observed anomaly reaching +3.30°C as of October 12. The North American Multi-Model Ensemble (NMME) projects further warming toward late fall or early winter, with the ensemble mean approaching +3.8°C. The CFSv2 forecast is somewhat lower, but it also maintains a strong El Niño signal before temperatures gradually decline during 2027.

The relative Niño 3.4 index, or RONI, provides another important piece of the puzzle. The latest chart shows RONI at +1.69°C for July–September, up 0.33°C from the previous month. This index measures Niño 3.4 warming relative to the rest of the tropical oceans. Because the broader tropics have also warmed, the relative index is considerably lower than the absolute Niño 3.4 anomaly.

That distinction matters. The +3.30°C value on our dashboard is an exceptionally large regional temperature anomaly, while RONI helps us evaluate how unusual that warming is compared with the surrounding tropical ocean. NOAA's October 8 diagnostic discussion also confirms that El Niño is strengthening, with a strong-to-very-strong event favored through early 2027.

What does this mean for the coming winter?

The important question is how the atmosphere responds to this warming. El Niño influences the position and strength of the Pacific jet stream, the distribution of tropical thunderstorms, and the larger-scale circulation patterns that help determine where storms track across North America.

For the United States, a strong El Niño can favor a more active southern storm track, increasing the potential for repeated precipitation events across California and parts of the southern tier. The northern United States can experience very different conditions depending on the placement of the jet stream, blocking patterns, and the availability of cold air.

For our winter outlook, I am particularly interested in how the El Niño signal interacts with the Arctic Oscillation, North Atlantic Oscillation, Pacific-North American pattern, and the development of blocking across the higher latitudes.

If the pattern becomes favorable for cold-air delivery into the eastern United States while the storm track remains active, the potential for significant snowstorms could increase across the Appalachians, Great Lakes, and Northeast. But El Niño alone does not guarantee a cold or snowy winter in those regions. The atmospheric indices and actual storm tracks will help determine whether that potential materializes.

The October pattern is our next major clue

October will be an important month for evaluating how the developing El Niño translates into the weather pattern over North America.

We will be watching for changes in the jet stream, stronger disturbances moving across the country, and increasing opportunities for colder air to spill south from Canada. The timing of those cold-air intrusions relative to developing storms will be critical for determining where rain changes to snow and whether coastal storms can produce significant snowfall inland.

The NMME and CFSv2 forecasts both support the idea that El Niño remains strong into the colder months, although they differ on the magnitude of the peak. Their agreement on the broad signal is meaningful, but the precise strength and timing remain uncertain.

The bottom line

The tropical Pacific is sending a powerful signal, but the atmospheric response will determine the winter we actually experience.

With Niño 3.4 at +3.30°C on our dashboard and RONI at +1.69°C, this is an exceptionally strong event. The next step is to watch how the jet stream and atmospheric indices evolve through October and November.

I continue to see reasons to prepare for a potentially volatile winter, with opportunities for major storm systems and significant snowfall in parts of the country. The exact locations and timing will depend on how the larger pattern develops.

This is why we keep watching the indices, the models, and the evolving pattern. The ocean establishes the background signal; the atmosphere decides how that signal plays out.

WeatherMadness Brief™

Making Weather Make Sense.

I'm Meteorologist Henry Margusity with your WeatherMadness Brief™.

The Madness continues.