Rain, storms are rolling in, but wet season is near end

Light showers are likely to fall across the Bay Area on Wednesday, but it could be a while before rain falls again, the National Weather Service said.

“It’ll be one of the last hurrahs of the season,” forecaster Diana Henderson said Tuesday.

A low-pressure system will move over the region by midday, bringing a chance of showers in the afternoon, evening and Thursday morning, Henderson said.

Some thunderstorms could pop up, too, because the clouds are “on the unstable side, kind of like my Aunt Margaret,” Henderson said.

Thursday and Friday will be partly cloudy, and by Saturday the skies should be clear.

“Saturday will be a Crissy Field day, and the Giants are back, so it’s a good time to go to the game,” Henderson said. “Don’t sit in the sun.”

While spring settles in around the bay, larger weather patterns are also shifting as a two-year La Niña phase comes to an end, probably by the end of the month, forecasters said.

During La Niña episodes, the Pacific Ocean is cooler than average near the equator on the eastern end and warmer than average on the western side near Australia. During El Niño episodes, the temperature patterns are reversed, “like a seesaw,” said Warren Blier, a science officer with the National Weather Service.

“It’s been a weak to moderate La Niña and now, over the last few months, the seesaw has been untipping,” Blier said.

Generally, a La Niña pattern makes a wet winter more likely in the Pacific Northwest and a dry winter more likely in Southern California.

“We’re kind of in the middle and could go either way,” Blier said of the Bay Area.

The 2010-11 winter saw higher-than-average rain and snowfall totals in California, but this past winter was far drier than normal until last month. In the last two weeks of March, more rain fell in the East Bay than in the previous eight months put together, according to the East Bay Municipal Utility District.

Enough rain and snow fell in March in the Mokelumne River watershed, where the district gets much of its water, to make drought-related restrictions on water usage unnecessary.

The ocean’s temperature balance should stay flat through the summer, Blier said. What comes after is hard to predict.

“Predicting the onset of one of these phases is more challenging than predicting how one that’s already there might evolve,” Blier said.