El Niño 2026–27

How El Niño and La Niña Work

ENSO

El Niño–Southern Oscillation: the swing of the tropical Pacific between El Niño, neutral and La Niña, every two to seven years.

Trade winds

Steady winds that blow east to west along the equator, pushing warm surface water toward Asia.

Thermocline

The boundary between the warm surface layer and the cold water below it.

Upwelling

Cold, deep water rising to the surface, mostly off South America and California.

Niño-3.4

The patch of the central Pacific NOAA watches. Since February 2026 NOAA measures it against the whole tropics (the RONI index).

Three states of the Pacific · in the tropics, and over the North Pacific

1. 1

   Neutral

   The usual Pacific

   In the tropics

   Over the North Pacific

   Trade winds pile warm water up near Asia, where it rises as storms. Off South America cold water wells up. The storm track over the North Pacific aims mostly at the Pacific Northwest.

   Trade winds steadyWarm pool westBay Area winter average
2. 2

   El Niño

   Trade winds weaken

   In the tropics

   Over the North Pacific

   The trade winds weaken and the warm water slides east. Storms follow it to the central and eastern Pacific, and a stronger jet stream runs straight at California, often farther south.

   Trade winds weakWarm pool eastBay Area winter wetter odds
3. 3

   La Niña

   Trade winds strengthen

   In the tropics

   Over the North Pacific

   The opposite: stronger trade winds push the warm water farther west and more cold water wells up in the east. The storm track lifts north, and California winters lean dry, especially in the south.

   Trade winds strongWarm pool far westBay Area winter drier odds

Schematic cross-sections along the equator, west to east, and storm-track maps; not to scale [3].

This Year’s Event

• NOAA: El Niño Advisory. A strong to very strong El Niño is likely through January–March 2027 (greater than 83% chance) [1].
• El Niño tilts the odds toward a wet California winter, and the tilt is strongest in Southern California. It does not decide any one storm.
• NOAA updates its outlook monthly; the next is November 12, 2026.

What Strong El Niños Did Here

Higher Seas: Kelvin Waves

1. 1

   The winds let go

   Western Pacific

   When the trade winds weaken or reverse, the warm water they held against Asia is released. It starts to move east as a bulge of warm water tens of feet thick, called a Kelvin wave.

2. 2

   Across the Pacific

   Along the equator

   The wave travels east along the equator for about two to three months, deepening the warm layer as it goes. NOAA tracks each one; several can follow in a strong event.

3. 3

   Up the coast

   South America to California

   At South America the wave splits. Part of it turns north and runs up the coast of Central America, Mexico and California as a coastal Kelvin wave, carrying the warm, raised water with it.

4. 4

   Higher water at the shore

   The Bay Area

   Warm water takes up more room than cold, upwelling weakens, and southerly storm winds push water toward shore. Together they hold the ocean several inches above normal for months.

Schematic, not to scale. Sea-level figures [6][7][8][9].

None of these floods a street on its own. It is the stack, on the same day, that reaches shoreline streets, beaches and bluffs.

Where it shows up

• Ocean Beach, where winter waves erode the bluffs south of Sloat Boulevard.
• The Embarcadero and the Bay shoreline at King Tides.
• Low ground near Mission Creek and Islais Creek.
• Pacifica's bluffs and beaches, and low stretches of the Marin shore.

Lake Tahoe: Heavy Snow

• Under the strongest El Niños, the Reno–Tahoe region has averaged about 20% more winter precipitation than normal; weaker events have run drier [14].
• Whether it falls as snow or rain depends on elevation. Warm, wet storms can bring rain onto the snowpack [14].
• 1982–83 brought 671 inches to the Central Sierra Snow Lab near Donner Summit [11].

For Tahoe homeowners

• Know your roof's design snow load (the county building department has it) and have a contractor clear heavy, wet snow before it builds up.
• Watch for ice dams at the eaves; keep gutters clear and the attic well insulated and ventilated.
• Keep gas meters, propane tanks and furnace and water-heater vents clear of snow.
• Decks, balconies and stairs: have snow cleared, and check posts and connections after a big season.
• Vacant homes: keep the heat on, know where the water shut-off is, and arrange regular checks.
• Line up snow removal and plowing before the first storm; carry chains for I-80 and US-50 controls.

Napa and Sonoma Valleys

• Winter rain mostly falls while the vines are dormant; the risks are to land, roads and houses more than to the vines.
• 1997–98: about 60 inches of rain in Napa from November to February, with widespread mildew and botrytis and a much smaller crop [15].
• 1997–98: the Rio Nido slide on the Russian River forced 140 homes to evacuate [12].
• Wet years refill reservoirs and groundwater.

For wine-country property owners

• Clear culverts, ditches and drains on the property; keep driveways and private roads draining.
• Hillside vineyards and yards: erosion control (cover crops, straw wattles) in place before the rain.
• Below recent burn scars, rain can bring debris flows; know the county's evacuation zone.
• Wells: if floodwater reaches the wellhead, have the water tested before drinking it.
• Septic: saturated ground can back up a system; keep heavy water use down during long storms.
• Along the Russian, Napa and Petaluma rivers: check the county flood map and the river forecasts.

Protect Your Home

Buying and Selling

• San Francisco sellers and landlords must disclose whether a property is in SFPUC's 100-Year Storm Flood Risk Zone. The map shows flooding from heavy rain only, not from the Bay or the ocean [17].
• California's Natural Hazard Disclosure covers FEMA flood zones, dam inundation, fire and seismic zones.
• Ask the seller about past water in the basement or garage, sewer backups, drainage work and roof leaks.
• Inspections to consider: roof, drainage and grading, sewer lateral (camera), and a geotechnical review on a slope or bluff.