El Niño could become the strongest on record this fall, with the chance of a 'super' El Niño, now more than 90 percent likely, National Weather Service forecasters warned Thursday.
The National Oceanic and Atmospheric Administration's Climate Prediction Center issued an El Niño Advisory on Thursday as the climate pattern rapidly strengthens in the tropical Pacific.
Forecasters say there is a greater than 95 percent chance El Niño will become very strong this fall and winter, and a 97 percent chance it will last into the spring.
More strikingly, they put the odds at 69 percent that it will reach historic strength between October and December — a development that could increase the risk of powerful storms, heavy rain, flooding, and other winter impacts in California.
The forecast marks a sharp escalation from earlier this year. In May, federal forecasters said El Niño was likely to emerge but cautioned that considerable uncertainty surrounded its eventual strength.
At that point, no strength category carried odds higher than 37 percent, according to the national Climate Prediction Center. Pacific Ocean measurements now show a much stronger signal, the agency warned.
Sea surface temperature anomalies have surpassed 2 degrees Celsius in the eastern equatorial Pacific, while temperatures below the ocean surface have climbed as much as 10 degrees Celsius above average at depth, according to the advisory.
Changes in Pacific winds, rainfall, and atmospheric pressure also show that the ocean and atmosphere are reinforcing each other, a hallmark of a strengthening El Niño.
Climate Prediction Center scientists expect the event to continue intensifying through the end of 2026.
Forecasters used a three-month Relative Oceanic Niño Index, or RONI, that they said would reach at least 2.5 degrees Celsius above the threshold used to measure the climate pattern.
Such a reading would exceed previous El Niño events in records dating to 1950, according to the Climate Prediction Center.
A stronger El Niño increases the chances of weather patterns typically associated with the phenomenon, but it does not guarantee that any particular region will experience heavy rain, flooding, or other specific impacts.
For California, however, the strengthening forecast comes as climate scientists and emergency officials have already urged residents to prepare for the possibility of a consequential winter.
During a July 15 "Super El Niño Town Hall" hosted by California Sen. Mike McGuire, experts warned that elevated sea levels, powerful storms, large waves, coastal flooding, and heavier rainfall could combine to cause significant impacts across the state.
UCLA climate scientist Daniel Swain said during the event that exceptionally warm waters in the eastern tropical Pacific could help the developing El Niño rival the strongest events of the modern era, although California's eventual rainfall totals remain uncertain.
Scripps Institution of Oceanography Director Mark Merrifield warned that El Niño could push California sea levels above normal this fall and winter. Higher seas combined with king tides, large waves, and heavy rainfall could increase coastal flooding, erosion, saltwater intrusion, and threats to shoreline infrastructure.
Experts said California could face its greatest risks later in the rainy season, particularly from late December through March. Strong El Niño events can steer repeated Pacific storms toward the state, creating the potential for consecutive storms and saturated watersheds.
Warmer storms could also raise Sierra Nevada snow levels, sending more precipitation down as rain instead of snow and increasing runoff and flood risks.
California emergency officials have urged residents to use the months before winter to prepare by signing up for local emergency alerts, assembling emergency supplies, securing important documents, making evacuation plans, and keeping vehicles fueled or charged.
The strengthening El Niño also comes against the backdrop of long-term ocean warming. Scientists have stressed that El Niño is a naturally occurring climate cycle, while human-caused warming can amplify some of the conditions surrounding extreme weather.
NOAA recently adopted the RONI index as its primary standard for official advisories and classifications. The index measures tropical Pacific temperature departures relative to temperatures across the broader tropical oceans, allowing forecasters to separate the El Niño signal more effectively from broader ocean warming trends.
The Climate Prediction Center is scheduled to issue its next diagnostic discussion on Sept. 10.
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