Of all the regional El Niño impacts forecasters talk about, California's is the most consistent and best-studied. The mechanism is straightforward: El Niño shifts the Pacific jet stream south and east, aiming the main storm track directly at the California coast instead of the Pacific Northwest.
The Historical Pattern
Strong and very strong El Niño winters — 1982-83, 1997-98, 2015-16 — have each brought above-normal precipitation to Southern and Central California, with the signal weaker and less consistent in Northern California. The 1997-98 winter in particular produced major flooding across the state; the 2015-16 "Godzilla El Niño" was more of a mixed bag, delivering a wet Southern California but a relatively disappointing Northern California season, which is a useful reminder that El Niño raises the odds of a wet winter — it doesn't guarantee one everywhere in the state.
What This Means Practically
Given the current trajectory — a likely strong-to-very-strong event peaking around November–January — Southern California should be the more reliable beneficiary of increased winter precipitation, with elevated flood and mudslide risk in burn-scar areas as a real concern by January-February. Northern California's signal is historically noisier; expect normal-to-above-normal precipitation there, but with more year-to-year variability than the south.
One additional wrinkle worth tracking: West Coast waters have already been running unusually warm this spring and summer due to a separate marine heatwave (driven by the Pacific Meridional Mode), distinct from El Niño itself. That warmth is expected to interact with — and likely amplify — moisture transport once El Niño-driven storms start arriving, which is worth watching as the season develops.