A developing historic El Niño, with Niño-3.4 sea surface temperature anomalies already exceeding +2.1°C and forecasts showing an 83% chance of becoming the strongest event on record through late 2026, serves as the dominant driver tilting trader sentiment toward wetter-than-average conditions for California’s 2026-27 winter. This pattern typically shifts the Pacific jet stream southward, increasing storm frequency and precipitation across the state, with model ensembles such as ECMWF SEAS5 indicating 150-250% of normal rainfall in many areas from December through March. NOAA’s Climate Prediction Center and recent dynamical runs reinforce elevated odds of above-average totals, though atmospheric variability and potential late-season weakening introduce realistic uncertainty that caps implied probabilities below 90%.