Your questions about the SoCal storm, answered

A powerful storm rolled through Southern California this week, triggering evacuations and flood warnings in areas that have been hit by wildfires and debris flows in recent months. Record-setting rainfall came down in parts of San Luis Obispo, Santa Barbara and Ventura counties.

You told us what you needed to know about the storms and we worked to find answers. Here are some of the questions you asked and what we found out:

When will the storm end?
How full is Cachuma Reservoir with this storm activity?
Do I need to evacuate? Is the hillside behind my home safe?
How important is localized rain, versus rain in the Sierra Nevada mountains, for local water districts?
Why was the spillway into Lake Casitas near Ojai closed?

Q: When will the storm end?

A couple of members on the Facebook page "Ojai Community Thomas Fire Network" wanted to know when they can expect the storm to end and the sun to come out again. 

The Los Angeles/Oxnard office of the National Weather Service forecasts rain will continue until mid-morning on Friday. But don't expect blue skies until later this weekend. 

"It looks like it'll probably be more like late Saturday or Sunday when we see skies actually clear out and temperatures start to rise," said NWS weather specialist Stuart Seto.

Heavy rains, with intensity up to half an inch per hour, will linger through Thursday night, Seto said. 

Flash flood watches for recent burn areas throughout Southern California were still in effect as of Thursday afternoon. The National Weather Service issues a flood watch when conditions may develop that lead to dangerous flooding and mudslides.

—David Wagner

Q: How full is Cachuma Reservoir currently with this storm activity?

Cachuma Reservoir was at 39.6 percent of capacity as of 8 a.m. Thursday morning, according to the Santa Barbara County Flood Control District. You can check updated rainfall and reservoir summaries each morning at this link. 

Susan Klein-Rothschild with the county's Joint Information Center said the reservoir's level is still low for this time of year, and the county isn't expecting this storm alone to leave the region flush with water. 

"We've been in a drought situation for a few years now. One or two rains does not make up our need of water," she said. 

Automated sensors at Cachuma Dam have measured 2.2 inches of total rainfall during the storm of the past two days, according to the county's Thursday morning rainfall summary.

Lake Cachuma is a crucial source of water for residents in Santa Barbara County. A chart on a website from the Santa Barbara County Water Agency shows that in 2016, water providers produced 9 percent of their water from Cachuma.

But that figure can be much higher for water users in certain cities. For instance, Montecito got 57 percent of its water from Cachuma. 

—David Wagner

Q: Do I need to evacuate? Is the hillside behind my home safe? How do I know how much rain my area has received?

We're seeing a lot of questions about specific canyons, hillsides, rivers and communities. The reality is that we don't have reporter eyes and ears everywhere during this storm, so we aren't in a great position to answer every one of your questions about specific neighborhoods.

With that, we want to ensure you have all the information you need to be prepared. For starters, check out this post from KPCC's Sharon McNary and Emily Guerin. For a deeper dive:

• Is my community at risk for a mudslide? Check out this map from the USGS, which shows which areas are most prone to debris flows after recent fires. There are county-specific maps as well — for Los Angeles County, for Santa Barbara County, and for specific zones of the Thomas Fire burn area in Ventura County.
• Do I need to evacuate? Check for evacuation orders from your county: Los Angeles | Ventura | Santa Barbara
• Sign up for emergency alerts for your community: L.A. County | L.A. City | Orange County | Ventura County | Santa Barbara County | Riverside County | San Bernardino County

Q: How important is localized rain, versus rain in the Sierra Nevada mountains, for local water districts?

Part of the answer depends on where you live.

Most of the rain that falls on Los Angeles washes over the city’s pavement and into the sea.

As UCLA’s Mark Gold told KPCC last year, during a storm, “you see all the water that ends up going through the L.A. River and Ballona Creek and Dominguez Channel, and you say, "Wow. That could have been our water supply for the next year," Gold said.

A fraction of that rainwater will end up in groundwater aquifers. But the L.A. Department of Water and Power got only 10 percent of its supply from groundwater in 2016-17.

For now, water imported from elsewhere — including the Sierra Nevada Mountains — is much more critical to Southern California.

The Metropolitan Water District, which serves communities from Oxnard to L.A. to San Diego, gets roughly 30 percent of its water from Northern California.

In 2016, the City of L.A. drew nearly half of its water from the Metropolitan Water District. It drew another 42 percent from the L.A. Aqueduct, which depends on runoff from the Eastern Sierras.

But for some communities, localized rain is heavily important. The city of Santa Barbara, for instance, depends largely on water from Lake Cachuma and the Gibraltar Reservoir — both of which are fed by rainwater.

“Runoff generated by average rainfall is generally enough to fill Gibraltar,” officials note in the city’s most recent water supply report. “However, it typically takes above-average rainfall to produce any significant inflow to Cachuma” — and because the last six years have been dry, Cachuma was less than half full at the end of last year.

– Kyle Stokes

Q: Why was the spillway into Lake Casitas near Ojai closed?

The Ojai Valley, parts of the city of Ventura and some beach communities in western Ventura County rely on Lake Casitas for their water supply.

The lake, just west of Ojai, is filled entirely by runoff from nearby watersheds — and by water siphoned from the Ventura River via the nearby Robles Diversion Dam.

The drought has left Casitas only 34 percent full, which is why two separate question-submitters were puzzled to find the spillway into the lake closed early on Wednesday, leaving water to flow downstream. "Much needed water rushing to ocean instead of lake," one of the questioners noted.

The spillway was closed earlier Wednesday, said Ron Merckling, who handles public affairs for the Casitas Municipal Water District. But around 1 p.m., the spillway into Lake Casitas opened, allowing water to flow both into the lake and down the Ventura River.

Merckling said the Ventura River must be flowing at 30 cubic feet per second at the diversion point before water can be diverted into the lake. The river must be kept moving at a certain rate so as not to jeopardize flows for water rights-holders downstream from the lake, and also to preserve runs of endangered steelhead, he said.

The diversion dam opens to Lake Casitas automatically, Merckling said, and water levels are now rising.

"This could perhaps be one of the largest storm events in 10 years and we hope that it continues through this evening for water supply purposes," he said. "We also encourage people to take necessary action to protect themselves and their property."

Merckling said he hoped a steady rain could raise lake levels to above 40 percent full.

– Kyle Stokes