Here’s how La Niña and climate change are shaping Pittsburgh’s winter forecast

With winter just around the corner in Pittsburgh, the unseasonably warm weather this fall has left many wondering what the season will bring. 

Pittsburgh’s weather remains uncertain due to the influence of a developing La Niña, which typically brings cooler conditions but may lead to more variability this year. Local meteorologists say while La Niña can drive extreme weather patterns worldwide, the effects of climate change are also becoming more pronounced, resulting in a lower predicted snowfall this year despite colder temperatures. 

Mark Abbott, a professor in the department of geology and environmental science, said “It looks like a La Niña phase is picking up.”

La Niña, which is an extreme of the El Niño/Southern Oscillation (ENSO) cycle, is the cold phase of the ENSO cycle. 

“It occurs when trade winds blow from east to west across the Tropical Pacific,” Abbott said. “El Niño and La Niña alter back and forth about every three to seven years, but it’s not necessarily regular.”

The impact of La Niña on different regions varies, according to Abbott. It’s a tropical Pacific phenomenon, but the tropics “really drive the heated engine of the planet.” 

“With warm water building up on the coast of South America, you may see heavy rainfall and flooding in places like Peru and Ecuador, and you may get droughts in Australia and Indonesia during a La Niña,” Abbott said. 

Matt Brudy, a Pittsburgh-based meteorologist at the National Weather Service, said La Niña is looking “weaker” this year in comparison to other years. 

“When it’s weaker, there’s a little bit more variability,” Brudy said. “We’re still going to have cold enough temperatures that we can still get snow. But, we may experience some warmer periods that could raise the average temperature slightly above normal during the winter months.” 

Mary Ours, a meteorologist at KDKA, said Pittsburgh could see a “wetter and cooler winter.” 

“This year it’s 74% chance that La Niña conditions will develop and stick around through spring,” Ours said. 

Even though Pittsburgh could see cooler weather this winter, snowfall is predicted to be “below average.”

“Our KDKA First Alert Weather team is predicting around 28” this winter,” Ours said. “The pattern we had for 2022 was a ‘triple dip’ La Niña, meaning it was the third we had. That winter we only had 17.6 inches of snow. In 2023, we had an El Niño pattern and only had 16.3” of snow. This is well below average.”

During an El Niño, which is the opposite phase of La Niña, the atmosphere tends to be warmer, since there is a “pulling of warm water along the coast of South America.” 

“During La Niña, you’re possibly putting more heat into the ocean and turning up the amount of upwelling on the South American side of the Pacific. This tends to cool the planet,” Abbott said. 

While the ENSO cycle affects temperatures worldwide, climate change is a key factor in the warming planet. 

“If you look at temperature increases over the last century, we’re up over a degree,” Abbott said. “We’re almost a degree and a half Celsius above what it would have been without additional greenhouse forcing, which is greater than anything you’re going to see within El Niño typically.”

Brudy said the effects of climate change can be felt in Pittsburgh, and that “Transition seasons are becoming shorter.” 

“There are trends that the first freeze in fall is becoming later within the last 10 years,” Brudy said. “We’re seeing trends of spring and fall becoming shorter at the expense of longer winters and summers. We’re getting more extremes as opposed to the transitions between the two.”

The effects of shorter transitional seasons are still unknown. While colder winters are a possibility, Brudy said it’s “less likely with an overall warming climate.” 

“It’s important to look at long-term averages when discussing what could happen, not just last year’s temperatures,” Brudy said. “We could see extreme heat and extreme cold, but there’s not a lot we can speak on for certain.”

Brudy also said the amount of ice covering the Great Lakes can affect winter weather in Pittsburgh. 

“Since the turn of the century, the number of years with less than 80% ice cover on Lake Erie has increased by nearly 170%,” Brudy said. “The less ice there is on the lake, the more lake effect snow we can get because open water allows for more evaporation, which fuels snowstorms. Once the lake freezes over, the lake effect shuts down.”

However, Abbott said that as winters get warmer, we’ll see “less snow and more rain.” 

“In the western United States, that’s more of an issue because it’s so arid to begin with,” Abbott said. “Snow is beneficial to recharging aquifers because it melts slowly and trickles into the ground. It’s more effective at recharging aquifers than rainfall, since rain comes fast and runs back to the rivers. We’ll see these effects in Pittsburgh, too.” 

Abbott said the difference between the Eastern and Western U.S. is the last century of data on precipitation. 

“Today, the western U.S. is drier than it was on average during the 19th and 20th centuries, while the eastern U.S. is actually slightly wetter,” Abbott said. “This is heating the Gulf of Mexico, hence the strong hurricanes we’ve been seeing. More water is evaporating from the gulf and raining that water out across the eastern U.S. The West isn’t seeing that, which is why it’s drier.” 

With the warming climate, Abbott said that the El Niño pattern provides “a glimpse into the future.”

“In the future, we could see the climate resemble an El Niño pattern, and in decades to come that could become the average condition,” Abbott said.