What ‘Very Strong' El Niño Odds Mean for California's Largest Reservoirs

A developing El Niño is forecast to become unusually powerful this winter, which could hold major implications for California’s water supply, reservoirs and flood risk.

The odds of a “very strong” El Niño-defined by exceptionally warm sea-surface temperatures in the equatorial Pacific-have increased dramatically as the event develops. The National Oceanic and Atmospheric Administration’s (NOAA) latest outlook, released Thursday, shows a 75 to 81 percent chance of a very strong event between November and January, the heart of California’s wet season.

While weak and moderate El Niño events do not reliably produce dramatic precipitation, California Institute for Water Resources climate scientist Daniel Swain told Newsweek, very strong El Niño events tend to provide one of the clearest early signals available about how California’s rainy season may unfold.

“It is arguably the single most important predictor this far in advance of what California’s rainy season might look like,” Swain said.

Why El Niño Matters

El Niño is a naturally occurring climate pattern that develops when ocean temperatures in parts of the tropical Pacific become warmer than average. Those changes can alter global weather patterns, influencing storm tracks, temperatures and precipitation across the United States.

In a typical El Niño winter, the southern tier of the U.S. often becomes wetter than average, while parts of the northern U.S. can trend warmer and drier. For California, however, the relationship is not always straightforward.

“El Niño does matter significantly for California winter precipitation,” Swain said. “But really only matters in a reasonably consistent way if it is particularly strong.”

That caveat is important because many previous El Niño events have failed to deliver widespread benefits across the state. But when an event reaches the strongest category, the historical signal becomes much more pronounced.

According to Swain, a very strong El Niño “tilts the odds greatly in favor of a wetter-than-average winter throughout most of California.”

The stronger the event becomes, the broader those wetter signals tend to be across the state.

Wet Winter Doesn’t Necessarily Mean Water Security

At first glance, a wetter winter sounds like welcome news for California’s water system, which has long struggled with drought. But water managers face a much different situation than they did during recent drought years.

California is not currently experiencing the type of severe statewide drought conditions that dominated much of the early 2020s. Instead, the state’s major reservoirs have been replenished by several consecutive wet winters.

Most of the state’s major reservoirs have water levels that are at 100 percent or higher of their historical average for this time of year, including Lake Shasta, California’s largest reservoir, and Lake Oroville, California’s second largest reservoir, which are at 100 and 113 percent of their historical average, respectively. Several of the state’s other major reservoirs are near capacity, even with the warm, dry conditions of summer.

That means there is less room available to store additional water if heavy storms arrive come winter.

“Almost all of them are at or above average capacity right now,” Swain told Newsweek. “If this still looks like elevated odds of a wet winter, reservoir operators will start drawing down the levels in them to make room for potential storms to come.”

In practical terms, that could mean reservoirs releasing water to create flood-control space. If major storms subsequently materialize, some of that precipitation may not be stored and instead will flow downstream.

Another complication is that not all precipitation provides the same benefit.

California’s water system relies heavily on Sierra Nevada snowpack, sometimes referred to as the state’s “frozen reservoir.” Snow accumulates during winter and slowly melts during spring and summer, providing a steady source of water long after storms have ended.

That slow release is crucial because it effectively expands California’s storage capacity beyond what reservoirs alone can hold.

A winter dominated by rain can still refill reservoirs, replenish groundwater and improve soil moisture. But rain runs off much more quickly than snow and is harder to store over long periods.

Swain stressed that it is not El Niño behind the chance that more rain will fall than snow this winter, but climate change.

“It will still snow in California’s higher mountains regardless of El Niño or not this winter,” he said. “But it’s more likely that more of the precipitation will fall as rain rather than snow at higher elevations than before, and that has less to do with El Niño and more to do with climate change.”

The concern for water managers is that a very wet, very warm winter could generate large amounts of runoff without producing a corresponding increase in long-lasting snow storage.

Flood Risk Growing More Important

Because California’s reservoirs are already relatively full, a very strong El Niño shifts the conversation from drought relief to flood preparedness.

Historically, some of California’s biggest floods have occurred during exceptionally wet winters driven by powerful Pacific storm systems. If a very strong El Niño helps steer more storms into the state, flood hazards could become one of the biggest risks.

Without severe drought to mitigate, the bigger concern comes to flooding, Swain said.

The risk becomes even larger if more precipitation falls as rain rather than snow. Rain reaches rivers and reservoirs much faster, increasing runoff and raising the likelihood of flooding, particularly during atmospheric river events.

As a result, California enters this winter in an unusual position. A wetter-than-average season could further bolster water supplies, but it could also force reservoir operators to release water, elevate flood dangers and highlight the growing challenge posed by declining snowpack in a warming climate.

What Happens Next

Although NOAA has officially declared El Niño is underway, the full extent of it remains unclear, as most El Niño impacts occur during winter. For now, forecasters stress that El Niño affects probabilities rather than guarantees. Yet with NOAA projecting a high likelihood of a very strong event, California officials will be watching closely. After years in which drought dominated water conversations, the state’s biggest challenge this winter may be managing too much water rather than too little.

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This story was originally published July 9, 2026 at 4:49 PM.