Will El Niño bring winter rain to California?
It may not be the kind of news growers like to hear, but rain during the period when California farmers harvest cotton and tree nuts this year is more likely than not, as the Pacific Ocean transitions into a short-lived El Niño pattern.
Forecasters are growing confident in the likelihood that El Niño will impact weather patterns this winter across California. Just how much and to what extent is uncertain. El Niño does not always portend wetter-than-average years.
WeatherBell, a private weather forecasting service, and the National Weather Service (NWS), suggest the likelihood of an El Niño will be 70 percent by September and closer to 80 percent by November.
What does it mean for California agriculture?
According to WeatherBell Chief Meteorologist Joe D’Aleo, the chances of an early rainy season in central and southern California are more likely than not, with a good chance that the weak to moderate El Niño event will peak by late in the calendar year. D’Aleo expects the rainy season to stretch into early 2015.
D’Aleo expects the short-lived El Niño could expand to bring above-normal precipitation to all of California by the January-February period before ocean temperatures cool below the El Niño threshold and West Coast weather patterns return to near normal.
The potential September start to this year’s rainy season could impact California’s cotton harvest plus tree nuts and rice, though D’Aleo says northern California could remain in a drought pattern until later in the year, thus possibly sparing California’s rice crop from rain.
“We’ll likely see our first Pacific El Niño system by October,” he said.
Conditions and early indications of a purported “super El Niño” are just not there at this time, D’Aleo says.
El Niño generally starts with the movement of warmer water eastward from the western Pacific. Warmer water in El Niño events move eastward along the equatorial region to western South America and spread up the eastern Pacific to California.
Scientific consensus defines El Niño as at least three months of sea surface temperatures at or above 0.5 degrees Celsius higher than average.
This push of warmer water can be assisted by a shift in wind patterns. D’Aleo says forecast models are not showing this push, which makes him believe this season’s El Niño will be on the weak to moderate side.
“There were some who thought this would be a super El Niño because the first push of warm water to the east was very strong and the water was very warm underneath,” he said.
California may rest assured that this season’s El Niño will likely not be the barn-burner compared to the 1997-1998 when a series of warm storms rolled into California and caused wide-spread flooding. Sea surface temperatures that year were as much as four degrees Celsius above normal.
Ocean temperatures are currently much closer to normal but about half-a-degree Celsius above normal, according to recent figures published by the NWS. It is the half-degree threshold that forecasters use over several consecutive months to declare an El Niño.
D’Aleo bases this on ocean temperatures that are cooler than the 1997-’98 El Nino, plus the depth at which the warmer water resides. Wind patterns are also not as favorable.
“We don’t see that happening; we don’t find the pressure patterns favorable for that,” D’Aleo said.
However, the warmer water does make the eastern Pacific riper for tropical storms and hurricanes, D’Aleo said.
An early example of this are the the two Category 4 hurricanes that appeared in the eastern Pacific within the past month. Both storms eventually met their demise in the cooler waters off the west coast of Mexico before heading out to sea.
Hurricane formation in the western Pacific during El Niño events can also push storms up the Baja California peninsula and into the Gulf of California. Rain can push into Yuma, Ariz. and southern Arizona, and southern California.