In February 2010, Michael Mann was scheduled to come from Penn State to deliver a guest lecture at the Department of Physics and Astronomy—invited by Mark Trodden, then chair of the department and today Dean of Penn Arts & Sciences. Instead, Mann spent his time in a hotel as a nor’easter blanketed Philadelphia in two-and-a-half feet of snow. “That was the second-highest snowfall event ever for the city,” says Mann, Presidential Distinguished Professor in the Department of Earth and Environmental Science and Vice Provost for Climate Science, Policy, and Action.

Though the weather prevented Mann from giving his talk 15 years ago, the experience left an indelible mark: “I vowed to eventually look into the impact that climate change might be having on these very powerful storms,” he says.

For several years now, that’s what he’s done, alongside PhD students Annabelle Horton and Mackenzie Weaver, who both begin their third year at Penn this fall. In Proceedings of the National Academy of Sciences, the team has published the results of that research, finding that although the number of nor’easters has decreased since 1940, the strongest of these storms are unquestionably getting stronger, with maximum windspeeds and precipitation rates increasing.

“Standard assessments of coastal risk account for the prediction that we’re likely to see more intense hurricanes, but here along the mid-Atlantic coast, the question of whether we’ll see more intense nor’easters may be as relevant—and the answer appears to be yes,” Mann explains. “Our findings have direct implications for managing coastal hazards.”

Those vulnerabilities aren’t negligible, notes Horton, who studies the history of nor’easters in the distant past using sedimentary evidence unique to these common winter storms. Beyond the significant threat to human life from the storms, which tend to drop massive amounts of rain and snow along the Atlantic coast of the U.S., there are real dollar costs, according to Horton.

“The 1962 Ash Wednesday nor’easter accounted for more than $150 million in damages along the New Jersey coast alone,” she says. “If you look more broadly at the past 40 years of storms, nor’easters can account for $5 to $10 billion of damages along our coastal communities. It’s really important for us to study these storms to understand how they’ll change in the future.”

Doing so isn’t as straightforward as it sounds, Mann says. To study nor’easters requires delineating a standard definition for this type of storm, as well as dividing the U.S. into sub-regions and applying a tracking algorithm to parse decades of daily winter weather data to pinpoint and follow each storm occurrence.

As an undergrad working in Mann’s lab, Kevin Chen, C’24, wanted to take on the challenge. He created a historical atlas of winter storms and sea-level pressure maps of nor’easters. He also developed an algorithm to track nor’easters using what’s known as re-analysis data, information about weather patterns dating back to 1940, along with data from a weather model that can fill in gaps. “These re-analysis datasets are widely used in climate studies because they provide a homogeneous daily archive of weather information that we can mine for all sorts of interesting purposes,” Mann says. “And that’s what we did.”

Standard assessments of coastal risk account for the prediction that we’re likely to see more intense hurricanes, but here along the mid-Atlantic coast, the question of whether we’ll see more intense nor’easters may be as relevant—and the answer appears to be yes.

The team defined a nor’easter as a winter storm that traveled a minimum distance of 1,000 kilometers (about 620 miles), lasted for at least 24 hours, reached minimum sea-level pressure of 980 hectopascals (the measurement for atmospheric pressure), and intersected with the U.S. East Coast at least once. Using those parameters, they identified 900 total nor’easters, some 10.6 per year, between 1940 and 2025.

With a definition in place, the researchers then determined which measures mattered to a storm’s impact, landing on maximum sustained windspeed and changes to precipitation. Examining these facets, they determined that over the past 85 years, the strongest nor’easters have intensified, likely driven by warmer ocean temperatures. The team also pinpointed a 6-percent increase in average windspeeds. “That might sound modest,” Mann says, “but that 6-percent increase actually translates to a 20-percent increase in terms of destructive potential.”

Horton notes it may seem counterintuitive that nor’easters—often associated with frigid temperatures and heavy snowfalls—would be getting worse in a warming world. “That’s where the results from this research are so interesting,” she says. “They show that these really intense, really destructive nor’easters will, in fact, only get more destructive and more powerful in a changing climate.”

This has real implications for people living along the East Coast, adds Weaver, who uses climate models to project potential future changes to such storms. “The fact that nor’easters are becoming more intense means that they will also be more damaging to the places that they impact, more dangerous to the people in their path,” she says.

Weaver adds that she hopes these findings can improve disaster preparedness and response, a sentiment Horton and Mann share. “They say forewarned is forearmed,” Mann says. “Hopefully, this research will help us communicate to the public and policymakers the full extent of the risks we face, providing a greater impetus for meaningful action. To me, it’s a wake-up call: If we don’t act, things will continue to get a whole lot worse.”