A ‘strong’ El Nino is likely to affect U.S. weather through winter. Philly is ‘on the bubble’ for snow.

The leaves have barely begun changing into their autumn finery, but meteorologists are saying that the atmosphere already is signaling that Philadelphia and other places in the East may be in for a very strange and stormy winter.

A “strong” El Niño has ripened in the tropical Pacific, with sea surface temperatures in an area critical to the course of U.S. winter weather averaging 2.5 to 3 degrees above normal, according to the government’s Climate Prediction Center.

And the probability that it will remain strong through the winter is more than 70%, climate center physical scientist Michelle L’Heureux said last week.

In the short term, El Niño’s intensity may bode well for residents of the Atlantic and Gulf Coasts in what has been an active hurricane season. Historically, El Niño warming has generated upper-air west-to-east winds that have sheared apart incipient tropical storms.

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“There are small signs of El Niño beginning to impact the circulation over the Atlantic,” said Matt Rosencrans, the climate center’s chief hurricane forecaster.

And in the months ahead, El Niño’s influence on the atmosphere over North America will only grow, said L’Heureux.

In fact, the climate center said the Pacific warming played a “key role” in the updated outlook it posted Thursday for the December-through-February period. That forecast had quite a wet look for the region and noted that “an East Coast storm track is generally favored during El Niño winters.”

It is impossible to know what may eventually fall from the skies, but based on several other winters during strong El Niños, for snow around here it may well be an all-or-nothing situation, offering hope for the snow lovers and loathers. The record, L’Heureux said, suggests Philly would be “right on the bubble.”

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The periodic warming, which occurs every two to seven years, is part of a natural cycle, the El Niño-Southern Oscillation, or ENSO. When the waters are cooler, as they were for the previous three years, the Pacific is said to be in the state of La Niña.

In the warm phase, the overlying atmosphere is heated, resulting in rising air and storminess that, in turn, affects the west-to-east winds that deliver weather to the United States.

The atmosphere has an insatiable appetite for chaos, but the ocean is slow to change. (One reason the earth’s temperature changes gradually is the fact that the planet is about 70% ocean-covered.) Thus, having a strong El Niño in the fall is a decent indicator that it will persist through the winter, said L’Heureux, who is the climate center’s ENSO team leader.

No two of these things behave identically, nor does the atmosphere respond in precisely the same way, but the winds incited by the Pacific energy tend to drive the weather-producing subtropical jet stream winds across the Gulf of Mexico.

They stir up storms that can make a hard left up the East Coast and become major snow- and rainmakers. The climate center has the odds favoring wet all along the Gulf and Atlantic Coasts.

A hallmark of El Niño winters is persistence — as in, what happens, keeps happening.

Snowfall would depend on just what routes those storms would take and, of course, the presence or absence of cold air, which has been in mighty short supply lately.

If Atlantic sea-surface temperatures remained above-normal, coast-hugging tracks likely would mean rain since they would draw in warm air off the ocean. Farther offshore, a storm’s winds from the northeast could import colder air.

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Four of the seven winters coinciding with strong El Niños were snowy, producing historic snowfalls in the Philly region, with 20-inch plus storms in 2009-10 and 2015-16. In three others, blizzards didn’t have a snowball’s chance. In one of those, 1972-73, no measurable snow fell upon Philadelphia.

The region is often on the border between rain and snow — and between snow and nothing. That’s especially true during El Niño, said the climate center’s L’Heureux. “You’re in that little no-man’s-land of snow.”

L’Heureux, who works out of the climate center headquarters in the Washington area and is the keeper of the government’s “ENSO Blog,” acknowledges that she wouldn’t mind a 2009-10 encore. That was Philadelphia’s snowiest winter on record, with 78.7 inches. Washington’s total of 56.1 inches was nearly quadruple the normal.

“I’m not going to be upset if we see a snowier winter materialize,” she said. She’s also not counting on it.

“Unfortunately, our climate trends are toward warmer winters.”

A lot of folks are not complaining.

Four of the seven “strong” El Niño winters coincided with some of the region’s biggest snowfalls:

1957-58: Up to 50 inches reported in Chester County, March 19-21; 11. 4 in the city.

1982-83: 21.3 inches in a Feb. 11-12 blizzard.

2009-10: 23.2 inches, Dec. 9-10; 28.5 inches, Feb. 5-6.

2015-16: 22.4 inches, Jan. 23-24.

The other three results in three of the most snow-starved seasons.

1972-73: Trace

1997-98: 0.8 inch

1991-92: 4.7 inches

Sources: Climate Prediction Center, National Weather Service