La Niña, the cooler counterpart of El Niño, has returned for the second year in a row.
It is a natural event associated with the larger El Niño-Southern Oscillation (ENSO) climate pattern and is marked by cooler-than-average ocean water in the central and eastern tropical Pacific. More specifically, a La Niña event is declared when sea surface temperatures in that region are at least 0.5°C below average and accompanied by a consistent atmospheric response. This includes more clouds and rain over the western Pacific and less over the central part of that basin. Stronger than average upper-level wind is also a key atmospheric indicator.
The relatively cool water associated with La Niña creates an area of high pressure over the Pacific and pushes the jet stream northward. This, in turn, affects weather patterns around the globe.
In the US, a La Niña event typically brings the southern states conditions that are warmer and drier than normal. The northern tier, on the other hand, usually experiences below average temperatures with the northwest and Great Lakes regions receiving above average precipitation.
That said, every La Niña event is different. There are also other atmospheric factors, such as blocking patterns, which can influence the weather.
According to NOAA, there is a 65%-75% chance of weak La Niña conditions continuing through the winter months.