A La Niña climate pattern has officially arrived, meteorologists said Thursday, which is linked to weather trends characterized by warmer, drier conditions across most southern states with increased precipitation and cooler conditions in other regions.
An El Niño weather pattern—La Niña’s counterpart—brought the warmest winter on record last year.
La Niña conditions emerged in December and will likely persist through April, though the phenomenon will likely be a weak one and have a lesser impact on precipitation and temperature compared to La Niñas in the past, according to the latest projections from the National Weather Service.
La Niña is a climate pattern marked by cooler-than-average sea surface temperatures in parts of the Pacific Ocean and is the opposite of El Niño, which brings warmer-than-average temperatures.
Because of cooler sea surface temperatures, the jet stream—a current of air flowing from west to east—is pushed to the north, decreasing the odds of precipitation in the southern U.S. while increasing the chances of heavy flooding in the Pacific Northwest and Canada, according to the National Oceanic and Atmospheric Administration.
During a La Niña winter, temperatures are usually higher than average across southern U.S. states and cooler than normal in the North, NOAA said.
Despite likely being a weaker event, the National Weather Service said La Niña has already produced wetter conditions in the Pacific Northwest and the Great Plains from October through December, in addition to drier conditions across the East Coast, Southwest, Northeast and Southern California, and above-average temperatures in the Southwest and Mountain West.
La Niña has also resulted in more snowfall across the Pacific Northwest and the Northeast, according to the agency.
Get Forbes Breaking News Text Alerts: We’re launching text message alerts so you'll always know the biggest stories shaping the day’s headlines. Text “Alerts” to (201) 335-0739 or sign up here.
NOAA expects drier conditions in the Southeast, Southwest and some states in the Great Plains and Mountain West, including Utah, Nevada, Colorado, Wyoming, New Mexico and Oklahoma, between November and January. Above-average temperatures are also more likely in the Southwest, Southeast and Mountain West, along with Southern California, states along the East Coast and the Northeast up to Maine.
The Pacific Northwest, Midwest and Northeast are expected to face above-average precipitation, according to NOAA. States with a higher chance of increased precipitation between November and January include Washington, Oregon, Idaho, Montana, Wisconsin, Michigan, Illinois, Indiana, Ohio, Pennsylvania and New York.
La Niña can also result in more severe hurricane seasons in the Atlantic Ocean by expanding the area of low vertical wind shear, which increases the number of hurricanes that develop and allows stronger storms to form.
Last winter—affected by El Niño—was the warmest on record in the U.S., with temperatures across the lower 48 states measuring 5.4 degrees higher than average, according to NOAA. Several states recorded their warmest winters ever, NOAA said, including North Dakota, Minnesota, Iowa, Wisconsin, Michigan, New York, Vermont and New Hampshire.
The La Niña and El Niño weather patterns are created by interactions between ocean surfaces and the atmosphere in the Pacific, and both occur about every three to five years, according to NOAA. Both weather patterns are part of the El Niño Southern Oscillation (ENSO), a climate phenomenon that emerges from a shift in winds and sea surface temperatures over the Pacific. The agency previously warned climate change may increase the frequency of extreme El Niño and La Niña events with amplified rainfall and increasingly warmer temperatures. The two patterns don’t always follow each other, though it does happen. A La Niña that follows an El Niño winter typically results in warmer summers, and the National Weather Service warned earlier this year La Niña would likely result in record-breaking temperatures. The average La Niña pattern lasts about 15.4 months while El Niño typically lasts 9.5 months, though the longest La Niña on record spanned 37 months between 1973 and 1976.