Strong El Niño likely this winter but its impact to Sierra remains unclear

Government weather forecasters have said there is now a 90 percent chance of a strong El Niño this winter and an 80 percent chance it will last until early spring of 2016. The weather phenomenon, which is essentially warmer than normal water in the tropical Pacific, has been shaping up since spring.

With California and Nevada experiencing four years of drought, an El Niño winter may be the kind of boost needed to rebuild snowpack in the Sierra and replenish dwindling water supplies in both states. Or not.

While there's been many El Niño winters since record keeping began in the 1930s, there’s been two notable El Niño events, the winter of 1982-83 and winter of 1997-98 that produced heavy snow in the Sierra, according to the National Weather Service in Reno. While those two winters were extreme and there have been several El Niño events since, a strong one doesn’t necessarily guarantee a snowy winter for 2015-16, said Edan Weishahn, a meteorologist for the weather service.

Other El Niño events — whether they've been weak or moderate ones — have come and gone in the form of snow in the Sierra and rain in Northern and Southern California and in Nevada. Or in the case of this previous winter there was an El Niño but it barely registered.

Lake Tahoe, Reno and Carson City sit in an area — an undefined line — where an El Niño impact could go either way. It could hit straight on as it did in the winters of 1982-83 or 1997-98 or it could stay to the south, impacting the southernmost Sierra and Southern California.

It could bring cold weather systems capable of dumping several feet of snow within hours or warm systems bringing several inches of rain that could produce damaging floods such as the one experienced New Year’s 1997 that affected much of the west coast including Northern Nevada.

“Even if we do end up with above average precipitation, it doesn’t necessarily mean it will fall as snow and snowpack is what we ideally need,” she said. “It took four years to create the precipitation deficit that we have now and it will take more than one wet winter to end the drought.”

While longterm government forecasts say it is highly likely the El Niño will happen and that it may be the largest ever on record, there’s nothing definitive right now as to how it will define itself and which direction it may take, she said.

What will happen depends where the jet stream sets up. The unusually warm water in the Pacific could bring the subtropical jet north or south. And it depends on the storms too and what’s coming out of the Gulf of Alaska.

“The thing about this El Niño, if it does pan out to be as strong as they think, is that there is already warm water, that we refer to as a ‘blob’ sitting off the Gulf of Alaska Coast. It presented itself last winter and is still around. Climate scientists are wondering how it will figure in the scenario,” she said. “We will be going into unchartered territory with this.”

In the near future the weather service in Reno will release an informational video to discuss El Niño and impacts, specifically for the Sierra and Western Nevada.

Across the contiguous United States, temperature and precipitation impacts associated with El Niño are expected to remain minimal during the Northern Hemisphere summer and increase into the late fall and winter. El Niño will likely contribute to a below normal Atlantic hurricane season, and to above-normal hurricane seasons in both the central and eastern Pacific hurricane basins, according to NOAA.

In the meantime, all we can do is wait. There is no set time of arrival. NOAA’s winter El Niño outlook is a peak between December and February.

As it stands now, weather service confidence will increase as we get closer to winter.

“It’s hard to predict the impact of this El Niño this far out,” said Weishahn. “Messages or predictions at this point are premature. We do know there’s an El Niño. We just don’t know what it will do.”