What a 'super El Niño' could mean for winter in 7 U.S. regions

A patch of unusually warm water in the tropical Pacific can shift the odds for a soggy winter in Los Angeles. The same patch can tilt Minneapolis toward a thin snow season, thousands of miles from where the warming starts. That reach is why seasonal forecasters watch the Pacific so closely.

El Niño is a warming of surface water in the central and eastern tropical Pacific. It is the warm phase of the El Niño–Southern Oscillation, or ENSO. NOAA's Climate Prediction Center declares El Niño conditions when sea surface temperatures in a stretch of ocean known as the Niño-3.4 region run at least 0.5°C above average. The agency must also expect that warmth to persist.

A 'super El Niño' is a stronger version of the same event. The phrase is an informal media label for an unusually strong El Niño, and NOAA has no official category by that name. Usage varies, so the term has no single agreed threshold. The strong El Niño winters most often cited as benchmarks are 1982–83, 1997–98 and 2015–16.

A strong El Niño tends to push the Pacific jet stream farther south and make it stronger across North America. In a super El Niño winter, that shift typically leaves the southern U.S. wetter than average and the northern tier warmer than average. That broad split hides real regional differences. Heavier rain and flood risk in California, cooler and wetter weather along the Gulf Coast and strain on snowpack in the Pacific Northwest all trace back to the same shift.

The slides that follow group states into seven regions, from California and the Southwest to the Northeast and mid-Atlantic. Three more slides explain what the term means, how the jet stream moves and why the pattern can fail to show up. The list does not cover every state individually. Each slide describes what strong El Niño winters have tended to bring, not a forecast for any particular year.

Seasonal outlooks shift the odds for an entire season. They do not predict individual storms. Any single winter can also break from the typical pattern. Other large-scale atmospheric patterns can override the El Niño signal in a given year.

Illustration: Quartz

A super El Niño is an informal label for an unusually strong El Niño, the periodic warming of surface water in the central and eastern tropical Pacific. NOAA has no official "super" category. The term describes intensity rather than a separate kind of event.

El Niño is the warm phase of a natural climate cycle called the El Niño–Southern Oscillation, or ENSO. The cycle swings between that warm phase, a cool phase known as La Niña and a neutral state in between. Episodes recur irregularly, typically every two to seven years.

The name has two parts. El Niño refers to the ocean side. Peruvian fishermen used the Spanish phrase for "the little boy," a reference to the Christ child, for warm coastal water that often arrived around Christmas. The Southern Oscillation refers to the atmosphere, a seesaw in air pressure between the eastern and western tropical Pacific.

During El Niño, the trade winds that normally push warm water west across the Pacific weaken. Warm water spreads east, and the zone of heavy tropical rainfall shifts with it. That change in where heat and moisture enter the atmosphere is how ocean temperatures near the equator end up affecting weather far from the Pacific.

NOAA's Climate Prediction Center measures events with the Oceanic Niño Index. The index is a three-month running average of sea surface temperature departures from normal in the Niño-3.4 region. A typical El Niño clears the agency's minimum warming threshold. Super El Niño is the phrase news outlets and some forecasters use when the warming runs far beyond that minimum.

Usage of the label varies from one outlet to the next. No single official temperature cutoff separates a super event from a strong one. NOAA does not publish a dividing line between the two.

Timing explains why the strength of an event gets so much attention ahead of winter. ENSO events typically reach peak strength in the Northern Hemisphere's late fall or winter. The strongest past events serve as reference points. Each of those winters played out differently across the U.S., and none sets a formula for the next one.

Illustration: Quartz

A strong El Niño tends to pull the Pacific jet stream farther south and make it stronger as it crosses North America. That shift steers more storms into the southern U.S. and leaves the northern tier warmer than average.

The jet stream is a narrow band of fast wind high in the atmosphere, near the cruising altitude of commercial airliners. It flows from west to east. Winter storms tend to form along it and follow its path. Where the jet sits helps decide which regions get rain and snow.

During El Niño, unusually warm water in the central and eastern tropical Pacific heats the air above it. That warm, moist air rises and builds towering clusters of thunderstorms farther east than usual. The heat released by those storms high in the atmosphere alters wind patterns far beyond the tropics.

Over North America, the result is an extended, energized Pacific jet that often stretches across the southern U.S. Storm systems ride that jet from the Pacific toward the southern states. This is the main reason the southern tier of the U.S. leans wetter than average in a strong El Niño winter.

The northern tier sits on the other side of the pattern. With the main storm track displaced to the south, the northern states see fewer of the Pacific systems that would otherwise reach them. Those areas tend to finish the season warmer than average.

The effect is strongest in winter. El Niño's influence on U.S. weather is much weaker in summer, when the jet stream sits farther north and loses strength.

Seasonal outlooks built on this pattern deal in probabilities for an entire season. NOAA's Climate Prediction Center expresses its winter outlooks as the odds of above-normal, near-normal or below-normal temperature and precipitation. A tilt toward a wet winter in one region does not mean any particular week or storm will arrive on schedule. A wetter season can still include long dry spells. A mild season can still include a cold snap.

Illustration: Quartz

California tends to get heavier winter rain during a strong El Niño, and that raises the risk of flooding and mudslides across much of the state. The winter of 1997–98 is the standard example of how that pattern can play out.

California depends on a short wet season. Most of the state's yearly precipitation falls between late fall and early spring, and summers across much of California are dry. A winter with more storms than usual can refill reservoirs and build snowpack in the Sierra Nevada. The same storms can also overwhelm rivers, storm drains and hillsides that are already soaked.

Flooding is only part of the hazard. Steep slopes in the coastal ranges and foothills can give way once the soil is saturated. Land recently burned by wildfire is more vulnerable still. Fire strips away the vegetation that holds soil in place. Scorched ground can also shed water instead of absorbing it. Heavy rain on a burn scar can send mud, rock and debris downhill with little warning.

Winter 1997–98 brought repeated storms to California, along with flooding and mudslides in many parts of the state. It is the El Niño winter most often cited as a guide to what a strong event can deliver there.

A strong El Niño improves the odds of a wet California winter, but it does not guarantee one. Even a benchmark strong event can leave parts of the state short of rain. The northern and southern halves of California do not always respond the same way. A wet season in Southern California does not ensure one in the north.

Insurance is one concrete consequence for property owners. Standard homeowners policies in the U.S. generally do not cover flood damage. That coverage is sold separately, often through the federal National Flood Insurance Program. New policies under that program typically carry a 30-day waiting period before coverage takes effect. Buying one after the storms arrive can leave a gap.

Illustration: Quartz

Arizona, New Mexico and southern Colorado lean toward wetter-than-average winters during a strong El Niño. More Pacific storms tend to reach the desert Southwest and the mountains just to its north and east.

The region lies beneath the southern storm track that a strong El Niño tends to reinforce. Systems that might otherwise pass farther north get more chances to cross it. In Colorado, the wet lean applies most clearly to the southern part of the state.

Winter is one of the region's two wet seasons. Much of Arizona and New Mexico gets a large share of its yearly rain from the North American monsoon, which brings summer thunderstorms from roughly July through September. Winter precipitation comes by a different route, carried inland by storms off the Pacific. Those winter storms are the ones a strong El Niño affects.

At lower elevations, around cities such as Phoenix, Tucson and Albuquerque, the El Niño tilt shows up mostly as rain. Higher terrain turns the same storms into snow. In Arizona, that includes the high country around Flagstaff, which sits at nearly 7,000 feet. It also includes the Mogollon Rim, the escarpment that marks the southern edge of the Colorado Plateau. In New Mexico and southern Colorado, it includes the Sangre de Cristo Mountains, which run across the state line between the two.

Southern Colorado's San Juan Mountains carry particular weight downstream. The Rio Grande begins there and flows south through New Mexico. The San Juan River also rises in these mountains before crossing into New Mexico and joining the Colorado River at Lake Powell in Utah.

Snow that builds up in these ranges over the winter melts in spring and summer. That runoff feeds the rivers and reservoirs that farms and cities across the region draw on. A wetter-than-average El Niño winter can add to that snowpack and to the runoff that follows.

Illustration: Quartz

Texas, Louisiana, Mississippi and Alabama tend to see cooler and wetter winters than average when a strong El Niño is in place.

The wetter side of the pattern comes from where storms travel. In a strong El Niño winter, Pacific storm systems tend to move along the southern tier of the U.S. instead of tracking farther north. That route carries them over Texas and then east along the Gulf Coast.

As those systems reach the coast, they sit next to the Gulf of Mexico, a large source of warm, moist air. Storms that can draw on that moisture are capable of producing heavy rain. For Gulf Coast states, a busier southern storm track means more chances for soaking rain events across the winter months.

The cooler side is linked to the wetter side. More frequent storms bring more cloudy and rainy days. Clouds block sunlight during the day, which holds afternoon temperatures down. The result is a season that averages cooler than normal, even if it does not bring unusual cold snaps.

Cooler than average is a statement about the seasonal mean. It is measured against the region's own normal winter, which is already mild by national standards. A cooler El Niño winter in Houston or Mobile is still far milder than a typical winter in the northern U.S.

In Texas, extra winter rain can matter for water supply. Much of the state relies on reservoirs and on rain-fed rangeland and cropland. A wetter winter can leave soils and reservoirs in better shape heading into spring.

The same rain carries a different risk farther east. Louisiana, Mississippi and Alabama already receive plenty of rain in a normal year. Additional storms falling on saturated ground raise the chance of flooding in low-lying areas and along rivers.

For this region, a strong El Niño winter typically means more gray, damp days and more frequent heavy rain, not a season defined by snow or ice.

Illustration: Quartz

Florida and much of the Southeast tend to have cooler and wetter winters than average during a strong El Niño. In some strong events, Florida also faces a higher risk of severe weather, including damaging thunderstorms and tornadoes.

The pattern follows the winter storm track. In a strong El Niño, low-pressure systems often cross the Gulf of Mexico and move over the Florida peninsula and southern Georgia. Each system brings another round of clouds and rain. In a more typical winter, many of those storms would track farther north.

The cooler lean comes mostly from cloud cover and rain, not from bitter cold. Overcast, damp days keep afternoon highs below normal. That differs from the classic Florida cold snap, when Arctic air pours in behind a front under clear skies. A strong El Niño does not, on its own, make those hard freezes more likely.

Winter is normally Florida's dry season. In a strong El Niño, a powerful jet stream can sit overhead while warm, humid air flows north from the Gulf. That combination can support organized lines of thunderstorms and tornadoes.

The 1997–98 winter offers an example of that risk. In February 1998, a deadly tornado outbreak struck central Florida, including the Kissimmee area. Outbreaks of that kind do not occur in every El Niño winter.

The wet tilt extends beyond Florida. Southern Georgia and coastal South Carolina also tend to see more winter rain than usual when El Niño is strong. Repeated soakings on flat, low-lying terrain can leave fields and roads waterlogged for long stretches.

The effects reach the region's farms. Florida grows strawberries and many vegetables during the winter months. Most of the state's strawberry crop comes from the Plant City area east of Tampa, where the harvest runs through the winter. A persistent wet, cloudy pattern in that season is a concern for growers. Rain during what is normally the dry season can also keep brush damp that might otherwise feed wildfires.

Illustration: Quartz

A strong El Niño winter tends to run milder and drier than average in Washington state, Oregon, Idaho and Montana. That combination can leave mountain snowpack below normal, and the effects reach well beyond the ski season.

The pattern starts with the storm track. In a strong El Niño, the Pacific jet stream tends to sit farther south than usual. Storms that would otherwise move into the Northwest are steered toward California and the southern U.S. instead. Fewer storms mean less precipitation reaching the Cascades and the northern Rockies.

Warmth adds a second problem. Snow needs cold air to build up. When winter temperatures run above average, more of the precipitation that does arrive can fall as rain at lower and middle elevations. Rain falling on an existing snowpack can also melt it earlier than usual.

Snowpack works as a natural reservoir across the region. Snow that piles up in winter melts in spring and early summer. That meltwater feeds rivers and streams after the wet season ends. In the Columbia River Basin, the runoff supports hydropower dams, irrigation for farms and river flows for salmon. A thin snowpack can leave less water available in the drier months that follow.

Idaho and Montana depend on mountain snow in a similar way. Farms on Idaho's Snake River Plain draw heavily on water that begins as snowmelt. In Montana, the Missouri and Clark Fork rivers start in high country and carry that melt to towns and farmland downstream.

Winter recreation is the most visible exposure. Ski areas in the Cascades, including those on Mount Hood in Oregon and Mount Baker in Washington state, rely on natural snowfall. A milder, drier winter can shorten the season, with lower-elevation slopes most exposed to rain and thin cover.

Water managers track the snowpack as the season unfolds. The Natural Resources Conservation Service, part of the U.S. Department of Agriculture, runs a network of automated SNOTEL stations. Those stations measure snow depth and water content across the mountain West. Their readings show whether the milder, drier lean is playing out in a given winter.

Illustration: Quartz

Strong El Niño winters tend to run warmer than average across the northern Plains, the Upper Midwest and the Great Lakes, with below-normal snowfall. North Dakota, Minnesota and Michigan sit inside that zone of milder, less snowy odds.

The pattern traces back to the jet stream. In a strong El Niño, the Pacific jet dips south and steers storms along the southern U.S. The northern tier sees fewer of those systems. The polar jet also tends to stay farther north over Canada. That keeps the coldest Arctic air locked away from the region more often than in a typical winter.

The 2015–16 strong El Niño shows how that can play out. That season opened with unusual warmth. NOAA ranked December 2015 as the warmest December on record for the contiguous U.S.

Less snow does not mean no snow. Milder air means more storms arrive with temperatures hovering near freezing. Precipitation can then fall as rain, sleet or a wintry mix instead of snow. Snow cover across Minnesota and North Dakota may come and go during the season rather than building a deep, lasting base.

The Great Lakes add a wrinkle for Michigan. Warmer air slows the growth of ice on the lakes. Open water that lasts deeper into the season can fuel lake-effect snow whenever a cold air mass does sweep across it. Western Lower Michigan, downwind of Lake Michigan, and the Upper Peninsula, downwind of Lake Superior, are the areas most exposed to those bursts.

The milder lean has practical effects. Inland lakes in Minnesota and North Dakota may freeze later or form thinner ice. That matters for ice fishing and snowmobiling, which depend on safe ice and steady snow cover. Warmer winters also cut demand for home heating in a region where heating season runs long.

The tilt applies to the season as a whole. A warm-leaning winter can still deliver sharp cold snaps and individual snowstorms in all three states.

Illustration: Quartz

In the Northeast and mid-Atlantic, a strong El Niño sends a weaker and less dependable winter signal than it does elsewhere in the U.S. Mild stretches are common, but a single coastal storm can still bury the region in snow.

That mix makes this corner of the country one of the hardest to call in an El Niño year. Cities from Washington, D.C., to Boston sit between the zones where the pattern is most consistent. They often get a share of both the southern storminess and the northern warmth.

The mild side of the pattern can show up as long runs of above-average temperatures. When that happens, precipitation that might have fallen as snow arrives as rain instead. Ski areas in New England and the mid-Atlantic depend on how many of those stretches a winter delivers. So do municipal snow-removal budgets.

The storm side comes from the coast. An El Niño winter tends to carry an active storm track across the southern U.S. Systems moving along that track can turn north and strengthen near the Atlantic seaboard. Those coastal lows, known as nor'easters, pull moisture off the ocean. If enough cold air is in place when one arrives, the result can be heavy snow along the Interstate 95 corridor.

January 2016 showed how that can play out. During one of the strongest El Niño events on record, a major blizzard struck the mid-Atlantic and Northeast. It shut down travel across large parts of the region, including Washington, D.C., and New York City.

The same winter also had a warm start. December 2015 ran well above average across much of the Northeast before the January storm arrived. One season produced both outcomes the region associates with El Niño.

A strong El Niño does not settle whether a winter here will be snowy or nearly snowless. It can favor milder temperatures overall while leaving room for one or two high-impact storms. Total seasonal snowfall in a given city can hinge on the timing of a handful of events. It also depends on whether cold air is present when the moisture shows up.