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

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.

Illustration: Quartz

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.

Illustration: Quartz

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 and most of its yearly rain arriving in the cooler months. A wetter winter there can shape the whole water year. Arizona and New Mexico also get a summer monsoon. Winter storms are one part of their yearly moisture rather than nearly all of it.

In desert cities such as Phoenix and Albuquerque, winter rain totals are normally small. A handful of extra storms can make a noticeable difference there.

The 1997-98 El Niño ranks among the strongest events of recent decades. It is a common reference point for this pattern in California, where it brought heavy rain and flooding.

The link is not automatic. Some El Niño winters in California have come in near average, and some have been dry. Even a strong event shifts the odds toward extra rain rather than guaranteeing it.

The wet lean also carries risk. Heavy rain on steep hillsides and recently burned slopes can raise the danger of floods and mudslides. Urban storm drains can also be overwhelmed when rain falls quickly.

Illustration: Quartz

The Pacific Northwest tends to run warmer and drier than normal during El Niño winters. That combination often leaves mountain snowpack lighter than usual across Washington state, Oregon and Idaho.

Temperature drives much of the snowpack loss. Precipitation that would fall as snow in a colder winter arrives as rain when the air runs warmer. In an El Niño winter, that shift reaches more elevations on the slopes of the Cascades and the Olympic Mountains.

Elevation shapes how much this matters. Many ski areas in the Cascades sit lower than resorts in the Rocky Mountains. Their slopes are closer to the line where rain turns to snow. A modest rise in winter temperatures can push that line uphill and leave lower runs with rain or thin cover. Rain falling on existing snow can also melt and compact it. That erodes a base that built up earlier in the season.

Lower precipitation compounds the problem. With fewer storms reaching the region, less moisture is available to build snow, even at higher elevations where temperatures stay cold enough.

The effects extend beyond ski season. Mountain snowpack works as a natural reservoir in the Pacific Northwest, storing winter moisture and releasing it as meltwater in spring and early summer. Summers in the region are typically dry. Rivers depend on that melt during the months when rain is scarce. A light snowpack can mean lower streamflows later in the year for farms, cities, fish and hydropower dams on rivers such as the Columbia.

The 2015-16 winter, a strong El Niño event, is a commonly cited example of this warm pattern in the region. Temperatures across the Pacific Northwest ran above normal that season.

The lean describes a tendency, not a forecast for any single storm. Individual El Niño winters can still deliver heavy mountain snow during cold, wet stretches.

La Niña, the cool counterpart to El Niño, tends to push the region the other way. Its winters lean cooler and wetter, conditions that favor deeper snowpack.

Illustration: Quartz

Winters across the northern Rockies and northern Plains tend to run warmer and drier than normal during El Niño. The pattern tilts the odds toward milder temperatures and fewer storms in states such as Montana, Wyoming, Idaho, North Dakota and South Dakota.

The storm track explains the dry side. During El Niño, the jet stream tends to steer storms along a more southerly route across North America. That leaves the northern tier of the U.S. outside the main path. Fewer storm systems reach the region, and winter precipitation leans below normal.

Temperatures follow a related pattern. El Niño tends to strengthen the Pacific jet stream and stretch it farther east. That flow carries relatively mild Pacific air inland across the northern U.S. It also makes it harder for Arctic air to settle over the region for long stretches.

In the mountains, a drier winter can mean a thinner snowpack. The effects reach well beyond ski season. Snow in the northern Rockies feeds the headwaters of major river systems, including the Missouri and the Columbia. A light snow year can leave less meltwater flowing downstream in spring and summer.

On the Plains, below-normal precipitation can mean patchier snow cover through the winter. Snow reflects much of the sunlight that reaches it. Bare ground absorbs more of that energy and warms more readily during the day.

Snow cover also insulates winter wheat, a major crop in Montana and South Dakota. When the ground is bare, the dormant crop is more exposed if a hard freeze arrives.

That freeze can still come. Arctic outbreaks can sweep south from Canada during an El Niño winter. A single cold snap can bring subzero readings across the northern Plains. The warm lean describes the season as a whole, not every week within it.

For people in the region, the signal points to a milder, drier winter as a more likely outcome than in a typical year. It does not mean a winter without serious cold or heavy snow.

Illustration: Quartz

Texas and the rest of the southern Plains typically get a wetter and cooler winter than normal when El Niño is in place. The cooler readings come mainly from extra cloud cover and rain.

The southern Plains take in Oklahoma, much of Texas and neighboring areas. In an El Niño winter, the southern branch of the jet stream tends to strengthen and run farther south than usual. That band of fast-moving air steers a steadier series of storm systems across northern Mexico and Texas. Each system brings clouds and a chance of rain.

The cooling is mostly a matter of sunshine. Thick cloud decks block sunlight during the day, which holds afternoon highs down. Wet ground also takes longer to warm. The result is a season that averages cooler than normal even without a long run of hard freezes.

More moisture brings its own hazards. Where heavy rain piles up over several days, flood risk can climb. Low-lying neighborhoods and fast-rising creeks and rivers face the greatest exposure. Winter-storm risk can rise as well. A storm that carries extra moisture into the region just as cold air pushes south can produce sleet, freezing rain or snow.

The pattern runs roughly opposite to La Niña, the cool phase of the same Pacific cycle. La Niña winters tend to leave the southern tier of the U.S. warmer and drier than normal.

Winter moisture also matters for agriculture. Texas and Oklahoma both grow large amounts of winter wheat, a crop planted in fall that stays in the field through the cold months. Winter rain and snow add to the soil moisture that crop draws on when it resumes growth in spring.

A wetter season still includes dry spells and mild stretches. The lean toward rain and cooler weather shows up across the winter as a whole, not in every week.

Illustration: Quartz

The Gulf Coast and Southeast lean wetter and cooler than normal during El Niño winters. Heavy rain and the flooding it can cause are the main concerns for the region.

This part of the country runs along the northern Gulf of Mexico and across the southeastern U.S. It includes Louisiana, Mississippi, Alabama, Georgia and the Carolinas. Florida sits inside the usual wet signal, from the Panhandle down through the peninsula.

Flood risk climbs where the land is low and flat, as it is across much of the Gulf Coast. Rain that lands on ground already soaked by earlier storms runs off instead of sinking in. A string of wet systems over several weeks can push rivers, bayous and city drainage systems beyond what a typical winter brings.

In Florida, the pattern works against the normal calendar. Most of the peninsula gets the bulk of its yearly rain in summer. Winter is usually the dry season. An El Niño winter can deliver repeated rounds of rain during months that are normally quieter.

The cooler lean describes the season as a whole. It does not mean a run of hard freezes. Arctic air can still push south in an El Niño winter. A single cold snap can arrive regardless of the seasonal tilt.

Flood insurance is one practical issue an El Niño winter raises for households in low-lying areas. Standard U.S. homeowners policies generally exclude flood damage. Flood protection is usually bought separately, often through the federal National Flood Insurance Program. Those policies typically carry a 30-day waiting period before coverage starts. A policy bought after a wet forecast arrives may not be in effect for the first storms.

The wet lean is a shift in the odds for the season, not a forecast for any single storm. The amount of extra rain also differs from one El Niño to the next.

Illustration: Quartz

El Niño winters tend to run milder than normal across the Great Lakes and Ohio Valley, with somewhat less precipitation than usual. That pairing often adds up to a lighter snow season. It does not stop cold snaps or heavy lake-effect snow.

The pattern traces back to the storm track. During El Niño, more Pacific storms ride a southern path across the U.S., while the northern branch of the jet stream tends to stay farther north. The Great Lakes, the Midwest and the Ohio Valley sit between those two paths and miss some of the activity from each.

Snowfall reflects both effects. Fewer storm systems reach the region, which leaves less moisture to work with. Warmer air also means some storms that do arrive deliver rain or a wintry mix rather than snow. In cities such as Chicago, Detroit, Cleveland, Indianapolis, Cincinnati and Pittsburgh, the result can be a winter with fewer major snow events than usual.

Lake-effect snow works through a separate mechanism. It forms when frigid air moves across the relatively warm, open water of the Great Lakes. The air picks up heat and moisture. It then drops that moisture as narrow, intense snow bands on the downwind shore. The snowbelts east and south of the lakes depend on this process. They include the Buffalo area at the eastern end of Lake Erie and the Tug Hill Plateau east of Lake Ontario.

That process still runs in El Niño winters whenever a cold air mass arrives. A milder season can leave more of the lakes free of ice. Open water is what lake-effect bands feed on. A single Arctic outbreak can therefore produce heavy local snowfall even in a winter that runs warm overall.

The same caution applies to temperature. El Niño shifts the seasonal average, not every day within it. A mild December and January can still include a week of bitter cold and dangerous wind chills.

For households across the region, a milder winter generally means lower heating demand. Furnaces and boilers run less often when temperatures stay above normal for long stretches of the season.

Illustration: Quartz

El Niño has a weak and inconsistent influence on winter in the Northeast and Mid-Atlantic. The pattern shows up clearly in other parts of the U.S. but often blurs along the East Coast. Results here vary from one El Niño winter to the next.

Some seasonal outlooks lean toward a milder-than-normal winter in the northern Northeast during El Niño years. That lean is modest. It is not as dependable as the warm tilt El Niño tends to bring to regions farther west.

The Mid-Atlantic sits closer to the southern jet stream, which strengthens during El Niño. That position can bring the region more storms and more rain in some winters. The jet stream does not decide whether a given storm falls as rain or snow. That depends on how much cold air is in place when the storm arrives.

The North Atlantic Oscillation is a major reason the picture is so mixed. It is a separate climate pattern over the North Atlantic Ocean. In this part of the country, it often matters as much as El Niño, or more. When the two patterns point in different directions, the Atlantic pattern can shape the season more than the Pacific one.

For residents of the Northeast and Mid-Atlantic, an El Niño declaration says less about the coming winter than it does in regions with a stronger signal. The milder lean in the north and the stormier lean in the Mid-Atlantic are both weak tendencies. Neither shows up as consistently from one El Niño winter to the next as the patterns farther south and west.

Illustration: Quartz

Alaska and much of western and central Canada generally run milder than normal during El Niño winters. Eastern Canada has a weaker and less consistent signal. The warm lean fades toward the Atlantic side of the country.

The pattern starts over the North Pacific. During El Niño, the Aleutian Low tends to deepen and shift east. That low is the semi-permanent area of low pressure that sits near Alaska's Aleutian Islands in winter. Its counterclockwise winds pull mild Pacific air north along its eastern edge, toward the Alaska coast and western Canada.

A second feature forms downstream. El Niño winters tend to favor the positive phase of the Pacific-North American pattern. That pattern is a recurring arrangement of high and low pressure across the continent. In its positive phase, a ridge of high pressure builds over western Canada and the northwestern U.S. The ridge bends the polar jet stream northward. Arctic air then has a harder time pushing south into the western interior.

For Alaska, the combination tilts the odds toward a warmer-than-normal winter across much of the state. Alaska is part of the Climate Prediction Center's seasonal outlooks. Residents there can read the same temperature and precipitation probability maps that cover the lower 48 states.

In Canada, the mild lean is strongest in the west and extends into the central part of the country. The western ridge sits closest to these regions. They feel its effects most directly. Canadians looking for an official outlook can turn to Environment and Climate Change Canada. That federal department runs the Meteorological Service of Canada and issues the country's seasonal forecasts.

Eastern Canada sits far from the western ridge. Its winters draw on Arctic air masses and on weather systems from the Atlantic. El Niño does less to decide which of those will dominate in a given season. Forecasts for the east therefore carry a weaker El Niño signal than those for the west.

A milder lean is not a promise of a mild winter. Alaska and western Canada can still see deep cold spells in an El Niño year. The warm tilt shows up in seasonal averages rather than in every week of the season.