The short answer
CPC classifies ENSO strength using the Relative Oceanic Niño Index (RONI). In its current strength table, a very strong El Niño is the category with an index of +2.0°C or higher. The September 2026 outlook assigns very high probabilities to that category through late fall and early winter.
Strength is not the same thing as impact
This is the most important distinction on the site. A stronger tropical Pacific signal can make classic El Niño patterns more likely, but it does not force the atmosphere to produce the same result in every state. CPC explicitly warns that stronger ENSO events do not guarantee stronger local impacts.
That is why the local checker keeps temperature, precipitation and snow separate. A region can tilt wetter while snow odds fall because temperatures are also warmer. A mountain region can benefit from extra storm moisture while nearby low elevations mostly see rain.
How to read the 2026–27 probabilities
The official September CPC tables keep the probability of El Niño at 100% from August–October through January–March. The probability of the very strong category is 98% for October–December, 93% for November–January and 75% for December–February. Those values describe ENSO strength, not local temperature or precipitation probabilities.
What a stronger event changes for planning
A very strong event gives more reason to pay attention to the familiar large-scale El Niño tendencies: a more active southern storm track, wetter odds across parts of the southern U.S. and California, and warmer tendencies across much of the northern tier. But you should still use CPC's seasonal temperature and precipitation outlooks for your region and switch to normal weather forecasts for specific dates.
What it cannot tell you
- How much snow your city will receive.
- Whether a specific ski weekend will be good.
- Whether one storm will be rain, ice or snow.
- Whether a particular road or mountain pass will be safe.
- How many severe-weather events a state will get.
Best way to use the signal
Use ENSO strength as one layer of confidence, then look at the current CPC seasonal outlook for temperature and precipitation, and finally use short-range forecasts when your decision becomes date-specific. The local checker is designed around that hierarchy.
Primary sources: NOAA CPC ENSO Diagnostic Discussion, ENSO Strength Probabilities, and ENSO Probabilities.