Most Recent Strong El Niños Show Variety Of Outcomes Possible In Winter 2023-24

A​ strengthening El Niño is in place, but the last two U.S. winters with strong El Niños were different in several key aspects, adding uncertainty to this season's outlook.

W​hat is El Niño? It’s a periodic warming of the equatorial eastern and central Pacific Ocean. This year’s El Niño began developing in June, becoming the first in more than four years.

S​ince the end of August, its warm temperature anomalies have pushed above the threshold of a strong El Niño, at least 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, warmer than average. It's the first time that's happened in more than seven years.

W​hy does this strip of warm ocean water matter? El Niño and its cool counterpart La Niña can affect weather patterns thousands of miles away in the United States and around the world. Since most El Niños peak in late fall or winter, they can have their strongest influence in the colder months of the year.

In general, t​he classic El Niño winter tends to be wetter than average through much of the southern U.S., from parts of California to the Carolinas, due in part to a stronger, more southern jet stream track.

A​cross much of the northern U.S., a stronger El Niño tends to produce a warmer winter.

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Not all El Niños are exactly the same. Even a stronger El Niño doesn't necessarily guarantee strong impacts on the weather pattern like we discussed above. It's not the only factor influencing winter weather. The two most recent strong El Niño winters bear this out.

2​015-16

T​his El Niño was already strong by late summer and eventually topped out as one of the strongest on record by that winter.

It was, and still is, the warmest winter on record in the U.S. In general, most stronger El Niños generate a mild winter over much of the country.

2​015-16 did that in spades. It was the nation's warmest winter in records dating to 1895, according to NOAA. It was record warm in every New England state, as well as New York City, despite a short-lived Northeast cold outbreak around Valentine's Day. It even soared into the 70s in late February in North Dakota.

B​ut it delivered a whopper East Coast storm. Despite all this warmth, just enough cold air was in place ahead of a vigorous jet-stream plunge to wring out a massive snowstorm for parts of Northeast in late January 2016.

W​inter Storm Jonas was New York City's all time heaviest snowstorm (27.5 inches), as it also was for Baltimore, Allentown and Harrisburg, Pennsylvania. NOAA estimated 21 million in population picked up at least 20 inches of snow from Jonas.

Eleven states, and the District of Columbia, declared states of emergency, according to NOAA. Travel bans were issued in both New York City and Newark, and more than 13,000 flights were cancelled.

T​hat's despite New York City and Philadelphia waiting until January for their first accumulating snow. Even a record mild winter can generate one or more major winter storms.

Rainy areas did fit the model. 2015-16 was the wettest winter on record in parts of the Southeast, including Atlanta and Miami, in Iowa, and also in parts of the Pacific Northwest, including Seattle. This fit the typical stronger El Niño picture of a wetter Gulf Coast and Florida.

T​his soggy South scenario continued into the spring, leading to major flooding along the Sabine River in Texas and Louisiana in March, a Tax Day flood in Houston, then another Houston metro flood in May.

2​009-2010

T​his El Niño didn't develop nearly as early as 2015-16 or the current one, and just barely nudged into strong territory by winter.

A​lmost every southern state had one of its top 10 coldest winters, from South Carolina to Florida to Texas, according to NOAA. Overall, it was the nation's coldest winter since 1984-1985 and still ranks among the top 25 coldest on record.

B​ut there were still some warm pockets, including parts of the Great Lakes, Northeast and Northwest. Maine had its third warmest winter, at the time.

The mid-Atlantic was buried in historic snow. Winter 2009-2010 rewrote snowfall record books in parts of the East.

I​t was the snowiest season (fall through spring) on record in Baltimore (77 inches), Philadelphia (78.7 inches) and Washington, D.C. (56.1 inches). February 2010 was the snowiest single month since 1869 in New York City (36.9 inches) and Pittsburgh (48.7 inches).

T​his was primarily due to several massive winter storms. One happened just before Christmas. Two others happened within a week in early February. Dubbed "Snowmageddon" and "Snowmageddon II", they dumped a combined 44 inches of snow on both Baltimore and Philadelphia, and almost 29 inches in Washington, D.C. Hundreds of thousands were without power and some roofs collapsed.

W​hy was that strong El Niño winter so different? While winter 2009-2010 had a few small thumbprints of a stronger El Niño winter, one other factor overwhelmed El Niño's influence: the Greenland block.

This is an area of high pressure in the upper levels of the atmosphere near Greenland. When that forms, it blocks the west-to-east flow of the jet stream, forcing it to take a sharp southward plunge into the eastern U.S.

W​hen that happens in winter, this pattern delivers cold air from Canada into the central, southern and eastern U.S. It can also open the door to one or more East Coast snowstorms.

O​ne way meteorologists keep track of this blocking pattern is from an index called the North Atlantic Oscillation (NAO). A more negative NAO means more Greenland blocking, while a more positive NAO means there's little blocking near Greenland.

T​his shows the key difference between the two El Niño winters best.

I​n winter 2009-2010, Greenland blocking was dominant, with a sharply negative NAO as the graph below shows.

W​inter 2015-2016, however, had mainly positive NAO values, indicating little Greenland blocking. In that case, colder air tended to drain more often eastward across eastern Canada, rather than plunge into the U.S.

W​hat's the takeaway for this winter? How often this Greenland block pattern develops in any winter season is difficult to forecast months ahead of time.

T​here are also other wildcards such as a weakening of the polar vortex that could also lead to a colder, blockier pattern for a time this winter.

A​nd there are preliminary indications the El Niño's influence on the large-scale weather pattern in the Pacific is starting out more like 2009-2010 than 2015-2016.

This stronger El Niño gives us a bit more confidence it will have some influence during the colder months ahead. Time will tell how much these other factors will muddy up that picture.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He studied physics at the University of Wisconsin-Madison, then completed his Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads and Facebook.

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