As predicted, La Niña has arrived and is likely to stick around through the winter, according to NOAA's Climate Prediction Center.
The Center issued a La Niña Advisory Sept. 10 reporting that La Niña conditions were present in August and have a 75 percent chance of continuing through the Northern Hemisphere this winter.
A NOAA blog on the El Niño/Southern Oscillation advisory notes three conditions are present that caused forecasters to declare La Niña's presence.
First, monthly sea surface temperature in August at a specific location along the equator was at least 0.5 degrees Celsius (0.9 degrees Fahrenheit) colder than average. According to the discussion, below-average sea surface temperatures extended across the central and eastern equatorial Pacific Ocean, with the largest departures in the east-central Pacific, from the surface to 200 meters below the surface.
Second, forecasters also think the sea surface temperatures will stay more than a half-degree Celsius cooler than average for the next several months.
Finally, the atmosphere in the Pacific is showing signs of responding to the cooler-than-average sea surface.
La Niña's atmospheric circulation over the Pacific Ocean affects global weather and climate, notes a NOAA blog posting by Emily Becker, an associate scientist with the University of Miami's Cooperative Institute for Marine and Atmospheric Studies.
"While every ENSO event (and every winter!) is different, La Niña can make certain outcomes more likely," Becker said. La Niña often shifts the Pacific jet stream northward. "Generally, La Niña winters in the southern tier of the U.S. tend to be warmer and drier, while the northern tier and Canada tend to be colder," Becker noted.
Forecasters have been predicting a La Niña winter for the past couple of months.
In the West, La Niña winters bring a greater chance for cooler and wetter weather in the Pacific Northwest. California is typically in a transition zone, with wetter-than-normal conditions more likely in Northern California, and drier-than-normal conditions more likely in Southern California.
The last La Niña winter was in 2017-2018.