It’s tough to think about on a beautiful summer day, but remember how freezing we were last winter? Well, don’t worry: Philadelphia could see a warmer-than-average winter this year, thanks to an El Niño that’s forming in the Pacific.

What’s that? Oh. It’s a complicated system that involves trade winds and the temperature of the Pacific Ocean, and it can throw off weather patterns across the globe in a variety of ways — including making the northeastern and mid-Atlantic states susceptible to warmer, wetter winters.

And meteorologists with the National Weather Service have early models that show there is a 90 percent chance that an El Niño will continue through the Northern Hemisphere during the winter, and around an 80 percent chance it will last into early spring of next year.

Mitchell Gaines, a NWS meteorologist based in Mt. Holly, NJ, said there’s still a lot of uncertainty for what the winter could look like (literally, when we called, the person who answered said “It’s July”). But if a strong El Niño does form, it could cause warmer winter temperatures into our region. It’d also cause warmer weather in the Great Lakes and Ohio Valley, cooler temperatures in the southeastern states and potential storminess in the southern states.

Here’s the short version of how El Niño works:

Under normal conditions, trade winds travel from east to west in the Pacific, bringing along ocean water heated by the sun on the surface and warmed neutrally. But under El Niño conditions, the trade winds become disrupted and the cold ocean water near South America isn’t rising up as usual. The waters start to warm.

As more water warms up the Pacific, the more the trade winds get frisky. Precipitation follows the warm water, and once sea surface temps in the Pacific rise more than 0.5 degrees Celsius over what they normally are during a three-month span, then meteorologists will declare an El Niño.

All of this changes how jet streams flow, and this is the effect is has on the world:


But other factors could come into play in our area, Gaines said. For instance, this year Philadelphia was in an avenue of cold air traveling south from Canada and the North Pole — remember the Polar Vortex? — which caused frigid temperatures in February and one of the coldest eight-week stretches on record. That *could* happen again this winter.

Theoretically, a strong El Niño would override that and still cause warmer air around here. But that’s if, and only if, the El Niño develops like meteorologists think it will. History shows that doesn’t always happen.

Experts predicted a strong El Niño last year around this time, would that they said would have been similar to one in 1997-98 that totally threw off weather patterns around the world. It can cause crazy flooding in South America, but can also help send needed rain to areas in drought.

But last year’s El Niño never came like meteorologists expected. A weak El Niño did form in the Pacific and surface temperatures rose, but it didn’t have the dramatic worldwide effects some expected. Models this year show one much stronger seems to be forming.

Moral of the story? There’s still a lot we don’t know about these types of weather patterns.

“It’s still July, so we have a lot of uncertainty for the winter,” Gaines said. “It does look like it will be a pretty strong El Niño. But there will also be a lot of smaller keys to the picture out here.”

Anna Orso was a reporter/curator at Billy Penn from 2014 to 2017.