2023-11-06 19:16:25
PlayA strong El Niño is expected to remain in place through spring.The last two strong El Niño winters were very different. That’s because El Niño is not the only driver of weather patterns. Preliminary indications suggest this El Niño winter has some similarities to one of those past events.
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.
What 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.
Since 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.
Sea surface temperature departures from average (in degrees Celsius) during the last week of Oct. 2023. The El Niño is highlighted by the red arrows.
(NOAA/PSL)
Why 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, the 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.
Across much of the northern U.S., a stronger El Niño tends to produce a warmer winter.
Further beef up your forecast with our detailed, hour-by-hour breakdown for the next 8 days – only available on our Premium Pro experience.
Typical impacts during a stronger El Niño from December through February in North America.
(NOAA)
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.
2015-16
This 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.
2015-16 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.
Temperature percentiles for December 2015 through February 2016, relative to other December-February periods since 1895. Areas in darkest red were record warm in 2015-16.
(NOAA)
But 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.
Winter 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.
That’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.
Morning commuters pass plowed snow on Wall Street in front of Federal Hall in New York’s Financial District, Monday, Jan. 25, 2016. East Coast residents who made the most of a paralyzing weekend blizzard face fresh challenges as the workweek begins: slippery roads, spotty transit service and mounds of snow that buried cars and blocked sidewalk entrances.
(AP Photo/Richard Drew)
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.
This 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.
Locations in the contiguous U.S. which set either a record wet or second wettest December through February period in 2015-2016. (Note: Only locations with at least 60 years of data were included.)
(Data: NWS, SERCC)
2009-2010
This El Niño didn’t develop nearly as early as 2015-16 or the current one, and just barely nudged into strong territory by winter.
It was the nation’s coldest winter this century. Considering what we laid out for 2015-2016, this seems shocking, right? Much of the Lower 48 states were persistently cold from December 2009 through February 2010, despite the late-developing, strong El Niño.
Almost every southern state had one of its top 10 coldest wintersfrom 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.
But there were still some warm pockets, including parts of the Great Lakes, Northeast and Northwest. Maine had its third warmest winterat the time.
Temperature percentiles for December 2009 through February 2010, relative to other December-February periods since 1895.
(NOAA/NCEI)
The mid-Atlantic was buried in historic snow. Winter 2009-2010 rewrote snowfall record books in parts of the East.
It 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).
This 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.
Left: A high-resolution visible satellite image shows the extensive snow cover over the mid-Atlantic on Feb. 8, 2010, following the first “Snowmageddon” snowstorm. Right: A worker of the Architect of the Capitol removes snow with a front-end loader on the ground of the U.S. Capitol, Feb. 11, 2010, in Washington, D.C.
(Left: NASA Worldview; Right: Alex Wong/Getty Images)
Why 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.
When 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.
The area of blocking high pressure near Greenland forces a southward plunge in the jet stream across the eastern states when the North Atlantic Oscillation is in its negative phase. This leads to persistent cold temperatures and the potential for East Coast snowstorms.
One 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.
This shows the key difference between the two El Niño winters best.
In winter 2009-2010, Greenland blocking was dominant, with a sharply negative NAO as the graph below shows.
Winter 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.
The North Atlantic Oscillation (NAO) index from 2009 through summer 2016. Times of more Greenland blocking (negative NAO) are shown by red bars. Less Greenland blocking (positive NAO) is shown by blue bars. The timeframes of winter 2009-2010 and 2015-2016 are annotated.
(NOAA/NCEI)
What’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.
There are also other wildcards such as a weakening of the polar vortex that might also lead to a colder, blockier pattern for a time this winter.
And 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.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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