"The reason for the more pessimistic outlook, said climate scientist Michelle L'Heureux, is that warmer-than-usual surface temperatures in the equatorial Pacific measured this spring have since cooled.
The same underwater swell that pushed heat to the surface, known as a Kelvin Wave, is having its normal counter-effect, but that effect has been much stronger than usual and has moved more cold water up farther than expected, L'Heureux said.
"We're still banking on seeing a reinvigoration of El Niño," she said. "But with that said, we wanted to lower our projections because there are structural weaknesses that have made this El Niño less likely."
The federal forecast calls for the El Niño to be weak or moderate. The consensus earlier this year was that the event would be at least of moderate strength.
San Francisco's wettest rainy season in the last century came during the strong El Niño of 1997-98. Weak and moderate El Niños, however, have sometimes been associated with abnormally dry years in Northern California, even while bringing more rain to Southern California.
The absence of a strong El Niño doesn't sentence Northern California to a dry winter. Warm equatorial surface waters are just one harbinger of wet weather, and any number of factors could trigger rain in the months ahead."
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