El Niño Lifts Odds of Wetter South, Drier North This Winter

The El Niño chatter has been going on for months, but we’re now entering the time of year when the climate phenomenon has noticeable impacts on U.S. weather. Government forecasters announced Thursday that they expect this year’s strong event to set up a north-south divide in both temperature and precipitation: While the northern tier of states is more likely to see milder and drier weather, the southern tier is primed for a cooler, wetter winter.

Areas of the U.S. where precipitation is most likely to be above or below normal during the winter of 2015-2016.

Credit: NOAA

Nowhere is that news more welcome than in California, which is suffering from a years-long drought and is desperately in need of rain and, more crucially, mountain snow.

“This winter will be extremely important for the region’s water resources,” Alan Haynes, the service coordination hydrologist with California Nevada River Forecast Center, said during a National Oceanic and Atmospheric Administration press teleconference.

Of course, there are no guarantees. El Niño merely tips the odds in favor of certain weather regimes, but it doesn’t mean that there won’t be snowstorms and cold outbreaks in New England or that there will be sustained rains and snow in California.

“The climate system is far more complicated than just El Niño, even a strong one,” Mike Halpert, deputy director of NOAA’s Climate Prediction Center, said.

Overall, though, winters across the U.S. are trending warmer, as global temperatures rise due to the buildup of heat trapped by increasing amounts of greenhouse gases in the atmosphere. Last winter was the warmest on record globally, in fact, and 2015 as a whole is very likely to be the hottest year in the books.

While El Niño is a cyclical climate phenomenon marked by unusually warm waters in the eastern tropical Pacific, it can affect the weather around the globe because it alters the transfer of heat from the ocean to the atmosphere. This shifts around wind patterns there and has a domino effect through the atmosphere; over the U.S., it causes a shift in the position of the wintertime jet stream, which controls how storms move across the country.

The winter weather effects associated with El Niño are most pronounced during strong events, and the El Niño that has taken shape this year is rivaling some of the strongest events on record, including the blockbuster 1997-1998 event.

That year featured torrential rains across Southern California that caused deadly mudslides and flash floods. So while Californians have been waiting for El Niño rains for months, there are worries that they could cause such extreme floods again this year, especially given that drought-baked and wildfire-scarred soils are more impervious to water, making runoff more likely.

More important to California’s water situation are mountain snows, particularly in the northern half of the state, as these are a store of water that melts out slowly in the dry spring and summer seasons and helps top up reservoirs. The end of last winter saw the lowest snowpack on record in California, driven both by a lack of winter storms and by record-high temperatures that caused what snow was on the ground to melt out early. Reservoirs in Northern California are running between 17 and 33 percent capacity currently, Haynes said.

The link between El Niño and wetter weather isn’t a very strong one for Northern California, though, and with temperatures more likely to trend warmer this winter, it’s very uncertain how much snow the crucial northern mountain areas might see.

Areas of the U.S. where temperature is most likely to be above or below normal during the winter of 2015-2016.

Credit: NOAA

And even if California does see an extremely wet year, that won’t fully erase their drought, as the state is running a deficit of about 2.5 to 3 years of rain, Haynes said, but some improvement is expected in the southern parts of the state.

Drought across the Southwest and Southern Plains is also expected to improve or disappear, as those areas are also expected to be wetter. The drought that developed this year in the Pacific Northwest is expected to persist though, as El Niño is tipping the odds there toward drier and warmer conditions.

The highest chances for a wetter-than-normal winter are in the Southeast, particularly Florida, where NOAA forecasters have said the odds are around 70 percent.

“Probabilities this high are rare to see” in seasonal forecasts, Halpert said.

Parts of the Southeast, including Florida are more likely to see cooler than normal temperatures, as are the Southern Plains. Warmer weather is likely from the West up across the northern tier of the Lower 48, as well as across Alaska and Hawaii.

How the week-to-week weather turns out this winter, though, remains to be seen.

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