Nor'easters rivaling hurricanes as flood threat to the Mid-Atlantic

Residents on the Mid-Atlantic coast face a dual threat when it comes to coastal flooding, which is one of the most costly, devastating and pervasive natural hazards in the region. 

Not only has the Mid-Atlantic experienced increased rates of sea level rise, the area also gets hit with large tropical weather systems, such as hurricanes, as well as battered with non-tropical weather events — midlatitude weather systems such as nor’easters like the one that hit the Delaware coast in mid-May. 

These large weather events exacerbate coastal flooding, and when combined with the higher rates of sea level rise, they pose a threat to human life, damage natural and human-built critical infrastructure, erode beaches, and disrupt important ecosystems found along the coast. 

John Callahan, climatologist and visiting assistant professor in the University of Delaware’s College of Earth, Ocean and Environment, was the lead author on three papers published in the past year that focused on these large-scale weather events to see just how much coastal areas — particularly the Chesapeake and Delaware Bays — are inundated by tropical and non-tropical weather events. Dan Leathers, professor and Delaware State Climatologist, was a co-author on all of the papers, and Christina Callahan, scientist for the Center for Environmental Monitoring and Analysis (CEMA), was a co-author on two of the papers.

The most recent paper was published in the Journal of Applied Meteorology and Climatology and compared extreme coastal flooding events from tropical cyclones and mid-latitude weather systems in the Delaware and Chesapeake Bays from 1980-2019.

Callahan looked at the past 40 years of measurements from several National Oceanic and Atmospheric Administration (NOAA) tide gauges in the Delaware and Chesapeake Bays. This helped him to quantify the storm surge — the rising sea as the result of atmospheric pressure and winds associated with a storm — from these large weather events. 

While coastal flooding from tropical weather events tend to get a lot of media attention — and actually have a higher average surge level — Callahan said that midlatitude weather events can produce flood levels just as severe and occur much more frequently in the Mid-Atlantic.

“About 85 to 90% of our coastal flooding events here in the Mid-Atlantic come from the midlatitude events; they don’t come from the tropical cyclones and the hurricanes,” said Callahan. “You can get strong nor’easters that have just as high coastal inundation levels and cause just as much — if not more — damage than tropical cyclones.”

One of the reasons that the midlatitude events can cause so much damage is that, unlike the tropical systems that commonly impact coastal areas in the southeastern United States before hitting the Mid-Atlantic, the intensity and size of midlatitude events are most difficult to forecast and can strengthen quickly without much warning. Also, while tropical systems usually peak and are well-formed storms before reaching the Mid-Atlantic, a nor’easter can strengthen quickly right on or just off-shore of the region. Additionally, mid-latitude systems are often bigger in size, move slower, and remain over our region for longer periods of time.

Because they happen frequently in the cold season — from November to March — not much attention is paid to how nor’easters cause coastal flooding. Instead, more attention is paid to the amount of ice and snow and wind that the nor’easters bring and not as much focus is on the coast.

“Our attention is diverted between these other impacts or factors of these storms in the winter and spring, but this is where most of our coastal flooding comes into play,” said Callahan.

The most recent paper was published in the Journal of Applied Meteorology and Climatology and compared extreme coastal flooding events from tropical cyclones and mid-latitude weather systems in the Delaware and Chesapeake Bays from 1980-2019.

Callahan looked at the past 40 years of measurements from several National Oceanic and Atmospheric Administration (NOAA) tide gauges in the Delaware and Chesapeake Bays. This helped him to quantify the storm surge — the rising sea as the result of atmospheric pressure and winds associated with a storm — from these large weather events. 

While coastal flooding from tropical weather events tend to get a lot of media attention — and actually have a higher average surge level — Callahan said that midlatitude weather events can produce flood levels just as severe and occur much more frequently in the Mid-Atlantic.

“About 85 to 90% of our coastal flooding events here in the Mid-Atlantic come from the midlatitude events; they don’t come from the tropical cyclones and the hurricanes,” said Callahan. “You can get strong nor’easters that have just as high coastal inundation levels and cause just as much — if not more — damage than tropical cyclones.”

One of the reasons that the midlatitude events can cause so much damage is that, unlike the tropical systems that commonly impact coastal areas in the southeastern United States before hitting the Mid-Atlantic, the intensity and size of midlatitude events are most difficult to forecast and can strengthen quickly without much warning. Also, while tropical systems usually peak and are well-formed storms before reaching the Mid-Atlantic, a nor’easter can strengthen quickly right on or just off-shore of the region. Additionally, mid-latitude systems are often bigger in size, move slower, and remain over our region for longer periods of time.

Because they happen frequently in the cold season — from November to March — not much attention is paid to how nor’easters cause coastal flooding. Instead, more attention is paid to the amount of ice and snow and wind that the nor’easters bring and not as much focus is on the coast.

“Our attention is diverted between these other impacts or factors of these storms in the winter and spring, but this is where most of our coastal flooding comes into play,” said Callahan.