They called it Godzilla. Then it arrived.

This year’s El Niño was envisioned by many as a monster that might shift California from drought watch to flood watch. But while the pattern’s distinctively warm Pacific waters might send one more storm through the state this weekend, whatever comes will be too little and too late to salvage the promise of an exceptionally wet year.

Despite forecasts for above-average rain and snow, the state saw winter conditions that were remarkably average, with Southern California experiencing severe dryness — all of which ensures that the state’s four-year drought is far from over.

Weather watchers are now chalking up the overestimation of El Niño’s impact not to any problem reading forecasting models but simply to a lack of historic data on the event and how it might perform.

“We’re not happy with what our forecasts were,” said Mike Halpert, deputy director of the Climate Prediction Center, the division of the National Oceanic and Atmospheric Administration that provides the country’s foremost weather opinions. “But our forecast was justified given what we knew, and we would do it again.”

As Halpert and other climate experts note, just two El Niños that resembled the size of this year’s unusually large event occurred in the prior 65 years. That made it tough for modern meteorology to get a handle on what to expect.

Back to Gallery Why El Niño left hopes for a very wet winter doused 4 1 of 4 Photo: Paul Chinn, The Chronicle 2 of 4 Photo: Paul Chinn, The Chronicle 3 of 4 Photo: Paul Chinn, The Chronicle 4 of 4 Photo: Paul Chinn, The Chronicle







Moreover, forecasting comes with a multitude of variables, so even if this year’s El Niño were better understood, other less predictable ocean and atmospheric conditions could have interfered with how it played out.

“Looking at two cases (of El Niño) and expecting the third to be the same — there’s just no guarantee of that,” Halpert said.

Like the powerful El Niños of 1982-83 and 1997-98, which delivered drenching storms to the Golden State, this year’s event was similarly characterized by the pattern’s signature heat in the equatorial Pacific. This warm ocean water is what typically drives moisture into the atmosphere, bringing rain and snow to California and influencing weather across the globe.

Some predictions right

For all the limitations that forecasters faced, the projections for this year’s El Niño were not as far off as they could have been. In fact, beyond California, many were spot-on.

Much of South America saw the heavy rain associated with strong El Niños of the past as east-moving storms blew in from the tropics. Tens of thousands of people in Paraguay, Argentina and Brazil were left homeless because of record flooding.

On the other side of the Pacific, El Niño’s telltale dry conditions prevailed. Parts of Indonesia, Vietnam, Thailand and the Philippines were socked with drought. Even in the United States, the Southeast experienced the wetter-than-average weather associated with El Niño.

“On the larger scale, El Niño looks just like we expected,” said Daniel Swain, a climate researcher at Stanford University.

What was off kilter on the West Coast, Swain explained, was the path of incoming storms. And despite the big deviation between the amount of precipitation that fell and what was forecast, the storm track did not vary all that much from the models.

Instead of landing in California, where the wet systems were expected, they touched down in Oregon and Washington, which have typically been dry during El Niños. Seattle and Portland both received about 50 percent more rain than average this winter, according to the National Weather Service.

“This points out just how susceptible California can be to these subtle shifts,” Swain said.

Northern parts of the state fared OK, with Redding seeing about 20 percent more rain than normal and San Francisco about 3 percent more. Southern California, however, had as little as half as much.

The National Weather Service is calling for a chance of showers in the Bay Area on Thursday night and continuing into the weekend as a wet front approaches from the southern Pacific. Rainfall totals, though, are not expected to be significant.

Making matters worse for the Golden State this year, El Niño’s tropical influence has been felt through balmier temperatures, meaning that the precipitation that did fall arrived largely as rain instead of snow.

Snowpack is important because it holds water until spring and summer, when it melts and provides a reserve for Californians to tap during the driest months. Statewide snowpack on Tuesday measured 79 percent of what is normal for this time of year.

Changed pattern

Climate experts are still trying to figure out exactly why the storm track didn’t move south as it has in the past.

“It’s likely because of how big this Niño was, how broad it was,” said Jan Null, meteorologist for Golden Gate Weather Services. “That was a big factor.”

With the equatorial Pacific warmer than usual as far north as Hawaii, Null said the flow of moisture out of the tropics was bound to be different.

Warming ocean

Null and others point to higher ocean temperatures all over the Pacific, the result of climate change, as part of the reason the storm track might not have landed where it did in other El Niños.

Other competing climate patterns, such as the Arctic Oscillation, which is known for pushing chilly air masses south, may have been at play.

“We spent a lot of time looking at the similarities of this Niño with the two past events. It wasn’t until February that we started looking at the differences,” Null said. “Researchers will look at this for a couple years, I expect, and then write the paper.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander