La Nina’s impacts were felt before NOAA declared pattern underway
Weather patterns across the U.S. and the world from October through December resembled patterns from previous La Niña events. La Niña is considered to be the cool phase of the El Niño-Southern Oscillation (ENSO) and is characterized by lower-than-average sea-surface temperatures, with anomalies of at least -0.5 degrees Celsius over critical parts of the central and eastern Pacific.
