La Niña conditions are currently forming in the Pacific Ocean, however forecasters expect that if these conditions persist through the winter, this will only be a mild episode, compared to La Niña that have occurred in the past.
La Niña, Spanish for "little girl", occurs when sea surface temperatures drop to below normal along a band following the equator in the Eastern Pacific, the counterpart to the phenomenon of above-average sea surface temperatures known as El Niño. These two phenomena are part of the El Niño-Southern Oscillation (ENSO), one of the strongest drivers of climate variation in North America and around the world.
The current La Niña conditions are forecast to have a 65 to 75 percent chance of persisting through the next few months. If this is the case, cooler conditions with heavier precipitation would be expected for the northern US, while the south would see warmer, drier conditions.
"That’s our best guess, but there are a lot of exceptions," explains Tony Barnston, with Columbia’s International Research Institute for Climate and Society. "Even in a weak La Niña, occasionally you can get pretty strong impacts on the climate." While the cooler conditions tend to suppress hurricane formation in the Pacific, the opposite tends to be true in the Atlantic, with the possibility of the current La Niña conditions having contributed to hurricanes Harvey, Irma and Maria.
But such a forecast isn’t guaranteed, as some strong La Niña occurrences in the past had little effect on local and worldwide weather patterns. "You get a lot of other influences besides La Niña," Barnston continues, "and a lot of different variations within those general tendencies."