What strong El Niños look like in California

Millions are waiting to see the effects of a strong El Niño, hoping that it brings a significant amount of rain and snow to California.

But many forecasters have spent months offering mixed signals about the potential for heavy precipitation this winter, particularly in Northern California, home to many of the state’s largest reservoirs.

The latest forecast from the National Oceanic and Atmospheric Administration says there’s an 80 percent shot that the Sacramento Valley will see normal precipitation this winter, and a 34 percent to 40 percent chance of above-average precipitation. But William Patzert, a climate expert at NASA’s Jet Propulsion Laboratory in Pasadena, told The Bee last week that this El Niño “is too large to fail” and chided his colleagues for being too tentative. “It’s a monster,” he said of this El Niño.

A lot of the differences in tone can be explained by looking at data from past El Niños. There is no real correlation between weak-to-moderate El Niños and high amounts of precipitation in Northern California, forecasters note. The data for strong-to-very-strong El Niños is more encouraging, but there’s just not a lot of it.

We’ve had only two very strong El Niños that match this year’s predicted magnitude since 1950, and both drenched the state. We’ve had several additional “strong” El Niños – the exact number depends on the index used to measure the phenomenon. They often resulted in above-average rainfall. But a few strong El Niños were busts in Northern California, delivering below-average precipitation.

A Bee analysis earlier this year based on the Southern Oscillation Index, a measurement of large air pressure changes between the western and eastern tropical Pacific, found that rainfall in Sacramento averaged 24 inches, roughly 130 percent of normal, during nine strong and very strong El Niño years since 1950. Six of the nine strong El Niño years resulted in more than 25 inches of rain in Sacramento. During weak-to-moderate El Niño years, precipitation averaged 18 inches – about normal for the city – and ranged from 7 inches to 31 inches.

One note of caution: Rain alone won’t stanch the impacts of California’s drought. Northern California needs Sierra snowmelt to sustain its reservoirs through spring and summer, and forecasters are saying this winter may be unusually warm, bringing more rain than snow.

The accompanying chart shows the amount of precipitation across California in the five strongest El Niño years according to the Oceanic Niño Index, which measures variation from normal sea temperature in the east-central Pacific Ocean.

This story was originally published October 16, 2015 at 3:46 PM with the headline "What strong El Niños look like in California."