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El Niño winter forecast: a region-by-region guide to wetter, drier and milder odds

This El Niño winter forecast guide breaks down how the Pacific warming pattern tends to shift rain, snow and temperatures across U.S. and Canadian regions

A shift of half a degree Celsius in the surface waters of the tropical Pacific sounds too small to matter in Denver, Houston or Seattle. It is enough to reroute the jet stream over North America. That shift can steer storms toward one part of the continent and away from another for an entire season.

That pattern is El Niño. It describes a warming of surface waters in the central and eastern tropical Pacific, and it returns every two to seven years. It usually reaches its peak in late fall or winter. That timing puts its influence on the months when households plan heating budgets, ski areas count on snow and water managers watch reservoirs.

The effects split roughly along a north-south line. Across much of the southern U.S., from California to Texas to Florida, El Niño winters tend to run wetter than normal. In many of those places they also run cooler. Across the northern tier, from the Pacific Northwest to the Great Lakes, the lean goes the other way, toward milder temperatures and less precipitation. Alaska and western Canada also tend to run mild. In the Northeast and Mid-Atlantic, the signal is weaker and harder to read.

None of this is a promise. An El Niño winter forecast describes shifted odds, not a fixed outcome. Some El Niño winters in California have been near average or dry. Cold outbreaks still reach the northern Plains. Other climate patterns, including the Arctic Oscillation and the North Atlantic Oscillation, can blur or override El Niño's influence in any given season. Strong events, such as those in 1982-83, 1997-98 and 2015-16, have tended to produce the clearest regional patterns.

The 10 slides below start with what El Niño is and how NOAA's Climate Prediction Center decides that one is underway. They then walk through what the pattern tends to mean for rain, snow and temperatures, region by region. The final slide explains why seasonal outlooks are written as probabilities rather than guarantees. Each regional slide stands on its own. Readers can jump straight to the part of the map where they live and see what an El Niño winter has tended to bring there.

What El Niño is and how NOAA declares it

El Niño is a warming of surface waters across the central and eastern tropical Pacific Ocean. NOAA declares one when a key ocean region runs at least 0.5 degrees Celsius above average, forecasters expect that warmth to last and the atmosphere shows it is responding.

The name is Spanish and refers to the Christ child. Fishermen off the coast of Peru used it for warm currents that showed up around Christmas. Scientists later linked those coastal warm spells to a much larger pattern that stretches across the equatorial Pacific.

NOAA's Climate Prediction Center monitors that pattern through the Niño-3.4 index. The index tracks sea surface temperatures in a box of ocean straddling the equator. The box runs from five degrees north to five degrees south latitude. It spans 170 degrees west to 120 degrees west longitude, in the open Pacific between the International Date Line and South America.

Warm water alone does not settle the question. The Climate Prediction Center looks for the anomaly to hold rather than flicker for a few weeks. It also looks for signs in the air above the ocean. During El Niño, the trade winds that normally blow from east to west along the equator weaken. Rain clouds and thunderstorms that usually cluster over the western Pacific near Indonesia shift east toward the central Pacific.

That coupling between ocean and atmosphere gives El Niño its reach. The shift in tropical rainfall changes the flow of air high in the atmosphere. That includes the jet streams that steer storms over North America. Scientists call the full cycle the El Niño-Southern Oscillation, or ENSO. Its opposite phase, La Niña, brings cooler-than-average water to the same part of the Pacific.

El Niño does not arrive on a fixed schedule. It returns every two to seven years. An event usually reaches its peak in late fall or winter. That timing puts El Niño at the center of winter outlooks for the Northern Hemisphere.

Strength varies from one event to the next. The 1982-83, 1997-98 and 2015-16 events were all strong El Niños. Forecasters often compare a new event with those three as it begins to build in the Pacific.

California and the Southwest tilt wetter in El Niño winters

El Niño raises the odds of a wetter-than-normal winter across southern California, Arizona and New Mexico. The pattern makes a soggy season more likely, but it does not lock one in.

The main driver is the subtropical jet stream. This band of fast-moving air high in the atmosphere steers storm systems. During El Niño, it tends to grow stronger and sit farther south than usual.

That southern position changes where storms come ashore. Systems that might otherwise head toward the Pacific Northwest are more likely to aim at California. Some carry their moisture inland across the deserts of Arizona and New Mexico.

The tilt is strongest toward the south. Northern California sits between the wet southern lean and the drier lean farther north. Its odds are less clear-cut.

Timing gives the signal extra weight. Southern California has a Mediterranean-type climate, with dry summers........

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