A La Niña watch remains in effect, according to the latest outlookfrom NOAA, with weak La Niña conditions expected to develop by this winter.
La Niña is the periodic cooling of the equatorial eastern and central Pacific Ocean. When sea-surface temperatures are cooler than average by at least 0.5 degrees Celsius for at least three consecutive months, along with consistent atmospheric indications, a La Niña is considered to be in place.
The peak response in the atmosphere to La Niña or El Niño typically occurs during winter in the Northern Hemisphere. This response also depends on the strength of the La Niña; this year is currently expected to be weak.
The map below from NOAA shows the overall winter pattern during a La Niña winter.
The jet stream tends to drive the storm track into the Pacific Northwest, resulting in wetter-than-average conditions during the December through February period. This pattern also steers low-pressure systems to track through parts of the Midwest into the Ohio and Tennessee valleys, meaning above-average precipitation in those areas.
Meanwhile, a ridge of high pressure can mean drier-than-average conditions for much of the southern tier of the U.S. This can also bring warmer-than-average temperatures from Texas into the Southeast during the winter.
The best chance for colder-than-average conditions during a La Niña winter is from Montana into the northern Plains, due to the southward dip in the jet stream over that region, which can allow colder conditions to surge farther southward. The Upper Midwest into New York and New England can also see colder-than-average temperatures for this reason.
Although not depicted on the map above, the West Coast can also see cooler conditions.
Last winter, a weak La Niña was in place and portions of the West and Upper Midwest had one of the wettest winters on record, while areas from the East into the South and Midwest had one of the warmest winters.
It's important to remember that La Niña, El Niño or ENSO-neutral conditions are only one factor that can play a role in the winter's weather.
A La Niña watch is issued when conditions are favorable for the development of La Niña conditions within the next six months; a watch was issued in September. A La Niña advisory means La Niña conditions have been observed and are expected to continue.
From Sept. 10 to Oct. 7, the sea-surface temperatures were generally below average in the eastern Pacific, indicating possible La Niña conditions.
Additionally, there are several ways the atmosphere is exhibiting patterns expected during La Niña.
However, over the past week, sea-surface temperatures have warmed closer to average, and several computer models suggest a period of near-average sea-surface temperatures for the next several weeks before trending toward below average.
Consequently, NOAA said the ocean-atmosphere system did not meet the criteria for a La Niña advisory. In other words, La Niña is not solidly established just yet. Computer models, however, are in general agreement that by winter, the La Niña threshold will be met and a weak La Niña is likely.