Temperature difference from normal over the tropical Pacific Ocean. Blue areas indicate cooler than normal, signaling La Niña. (Climate.gov)

La Niña has arrived and is likely to linger through the winter, the National Weather Service announced Thursday morning.

The opposite of the more famous El Niño phenomenon, La Niña describes a natural cooling of the tropical Pacific Ocean which affects weather all over the planet.

Typically, during La Niña events, cold and stormy periods assault the north central and northwest United States repeatedly during the winter, but have a hard time penetrating too far south and southeast, and enduring. The southern United States tends to end up mild and dry, on balance.


So far this cold season, since late October, the weather has behaved true to La Niña form, with bitter cold invading the Dakotas, northern Rockies, and Pacific Northwest, but only briefly skirting through areas farther south and east.

La Niña is not the only control on winter weather, however. In the eastern United States, the character of the winter is often defined by whether and how frequently areas of strong high pressure develop near Greenland, sometimes referred to as blocking patterns, which force cold air in Canada to spill southward. These patterns usually can’t be predicted more than one to two weeks ahead of time.

The Weather Service says there is a 65 to 75 percent chance La Niña will persist through the winter, and probably through at least April.

Should La Niña last into the spring, it could portend more violent thunderstorms across the nation. La Niña conditions tend to trigger ingredients in the atmosphere that lead “to an increase in tornado and hail reports,” wrote researchers Michael K. Tippett and Chiara Lepore for Climate.gov last spring.

Like the La Niña that developed at exactly the same time last year, this latest event is a weak one. Unless it strengthens, its effects on the weather will probably be somewhat subtle compared to a more intense event.

Even though La Niñas have a history of accompanying winters that unleash punishing cold waves in the northern United States, the intensity of the cold during these events has waned over time due to climate change.


In other words, the odds of a La Niña event bringing about a cold winter for the balance of the nation has declined.

The cooling of the tropical Pacific Ocean associated with La Niña tends to result in cooler years for the planet as a whole compared to El Niño, when the tropical Pacific is warmer than normal. However, over time, La Niña years have trended warmer globally (consistent with what we see with La Niña winters in the Lower 48).

Temperature difference from normal for Earth since 1980. Blue shades indicate La Niña periods, red shades indicate El Niño periods and gray shades indicate neutral periods, neither La Niña or  El Niño (NOAA.)

Despite La Niña events to begin and end this calendar year, climate scientists predict 2017 to rank among the top three warmest years in recorded history for the planet.