Coastal Storms 101

Coastal storms are more than just dramatic weather events — they shape our coastlines, impact ecosystems, and influence the lives of millions. Understanding what drives them helps us better prepare for their effects, especially as climate change makes these storms more unpredictable. Dive in to explore the forces behind the storms, their unique impacts on the Gulf of Maine, and how communities are working to adapt.

The Science of Storms

Storms are disturbances in the atmosphere caused by differences in temperature, pressure, and moisture content. Imbalances in these factors drive air movement, creating strong winds and precipitation. While all storms share these fundamental characteristics, they are distinguished by their energy sources, location, and wind patterns.

The most destructive types of coastal storms are tropical cyclones and extratropical cyclones. Tropical cyclones are called hurricanes in the Atlantic, and extratropical cyclones are often called nor’easters. Understanding the science behind these storms helps to explain their differences and predict their potential impacts, which is important in regions where coastal flooding, high winds, and heavy precipitation pose serious threats.

The Science Behind Tropical Cyclones

Tropical cyclones are powered by heat from the sea surface and form over warm ocean waters, typically in tropical (i.e., from the equator to 23.5° latitude) and subtropical (i.e., 23.5° to ~35° latitude) regions. As warm, moist air rises from the ocean, it cools and condenses, releasing energy that fuels the storm. This process forms a “warm core,” meaning the storm has a low-pressure center that causes air to constantly flow inward. These winds are turned relative to Earth’s surface due to the planet’s rotation, forming the familiar spiraling wind pattern you see in the news. How we refer to tropical cyclones changes based on where they form — they are called hurricanes if they form in the Atlantic Ocean and typhoons if they form in the Pacific Ocean.

Relative to extratropical cyclones, tropical cyclones have a small geographic footprint, travel faster, and have higher maximum wind speeds (sometimes upwards of 175 miles per hour). Since they rely on warm water for energy, cold Gulf of Maine water typically causes tropical storms to fizzle out as they head north past Cape Cod.

"Despite the potential for tropical cyclones to be incredibly destructive, we don’t see them causing as much damage in the Gulf of Maine as extratropical cyclones because they lose their energy crossing Cape Cod and traveling over colder water," says coastal scientist Dr. Hannah Baranes. "In the coastal Northeast, you can imagine Cape Cod as a dividing line where north of Cape Cod in the Gulf of Maine, extratropical storms are the main driver of coastal flooding, and south of that imaginary line, tropical storms tend to be the main culprit."

In the Gulf of Maine, the difference between low tide and high tide is usually far greater than the height of a typical storm surge, making coastal flooding from storm surge only possible during high tide. This means storms (currently) only cause coastal flooding if they intersect with high tides. Even on the rare occasions when tropical storms reach the Gulf of Maine, their small size and fast forward movement lowers their likelihood of coinciding with high tide and causing flooding on the Gulf of Maine coast. Hurricane Sandy exemplified this; while Sandy drove extreme coastal flooding south of Cape Cod, the storm did not cause flooding in the Gulf of Maine because peak storm surge occurred during low tide.

Although damaging tropical cyclones are less frequent than extratropical cyclones — and are extremely rare in the Gulf of Maine — their high wind speeds can churn up large waves, knock down trees, and cause damage to coastal infrastructure. These cyclones typically occur in the Northeast US between August and October and can threaten life and produce billions of dollars of damage.

The Science Behind Extratropical Cyclones

Extratropical cyclones develop in mid-latitudes (areas between approximately 35°–55° latitude), where cold air masses meet warm air masses. They are “cold core” storms, gathering their energy from rapid changes in temperature over short distances that drive high winds. Nor’easters, named for their winds that blow from the northeast, are an example of an extratropical storm that causes severe winter weather along the U.S. East Coast. They develop when cold Arctic air transported south by the jet stream across the North American continent encounters warm air heated by the Gulf Stream (the warm ocean current flowing along the US East Coast that brings warm water from the tropics to the North Atlantic). These temperature differences lead to the development of “fronts”, or boundaries between these air masses. Once the cyclone is formed, it moves to the east/northeast, and up the U.S. East Coast.

Extratropical cyclones occur in the colder months across the Northeast US, from November through April. Relative to tropical cyclones, they are larger, move more slowly, occur much more frequently, and have lower wind speeds, maxing out at around 75 miles per hour. While typically less intense than tropical cyclones, these storms have their own set of hazards. Their slower movement and large footprints can result in sustained periods of heavy rain or snow, high winds, and storm surges that cause disruption over a large area. These longer-duration storm surges are more likely to intersect with high tide, and the high frequency of extratropical storms makes it more likely that a storm surge will intersect with an extreme high tide (such as a perigean spring tide, commonly known as a “king tide”).

As Coastal Dynamics Research Associate Katie Giannakopoulos explains, "Extratropical storm surges are generally lower than tropical storm surges. In extreme cases, tropical cyclones can turn up 20-foot storm surges in some places — extratropical cyclones in North America generally don’t do this. However, the slower, more persistent nature of extratropical cyclones can cause significant coastal impacts, especially in the northeast US.”

Storm Transitions and Hybrid Events

Interestingly, storms are not always confined to one category. Tropical cyclones can transform into extratropical storms as they move into colder waters. For example, when a hurricane moves northward into regions with cooler ocean temperatures, it may lose its warm core and transition into a cold core extratropical cyclone. While reducing wind speeds, this transformation can still result in significant impacts. This was demonstrated when Hurricane Ida (2021) transitioned to an extratropical cyclone as it moved into the Northeastern US, leading to record rainfall in places across Pennsylvania, New York, and New Jersey.

The Role of Sea Level Rise and Ocean & Atmospheric Warming

Human activities such as fossil fuel combustion and deforestation drive the accumulation of greenhouse gases in our atmosphere, warming the air, land, and water of the planet. A warmer Earth causes ice stored on land (e.g., glaciers and polar ice caps) to melt and ocean water to expand. When combined, these factors are driving rates of sea level rise that are faster than at any time in the past 2,500 years. Sea level rise will continue to exacerbate the impacts of coastal storms globally as higher sea level increases both the severity and frequency of coastal flooding.

For example, in Portland, Maine, minor high tide flooding has occurred an average of 12 days per year since 2010. The expected 1-foot increase in sea level between the late-2010s and the early-2050s would increase flood frequency to upwards of 84 days per year, or 7 to 9 flooding days per month. (source: 2024 Maine Climate Council Scientific and Technical Subcommittee Report).

"It can be surprising that sea level rise, which we measure in inches, can drive such a rapid increase in flooding — particularly when storm surges and tides can exceed 10 feet," says Dr. Baranes. "I think this is a helpful metaphor: it’s likely that only the very tallest people you know currently hit their heads when they walk through doorways. However, if we put everyone on 6-inch stilts, even though 6 inches is small relative to human height, suddenly more average-sized people would be bonking their heads. That’s a lot more people with their heads above the tops of doorways, the same way that’s a lot more high tides crossing flood thresholds.”

However, warming has different — and nuanced — effects on tropical versus extratropical storms. "Saying that 'climate change is causing stronger and more frequent storms' is too broad of a statement, and not scientifically accurate," explains Climate Center Director Dr. Dave Reidmiller. "For tropical storms, the most extreme storms are getting more intense and these more intense storms are becoming more frequent, but there is not a change in the frequency of storms overall. So while we are experiencing a higher number of extreme tropical storms as a result of climate change, the total number of storms generally, both tropical and extratropical, has not really changed. There is not yet clear attribution showing that climate change is making extratropical storms more intense. However, the way we experience these storms on land — the impacts they’re causing — is changing as a result of rising seas."

Warming-driven changes to extratropical cyclones are more uncertain. While there is disagreement about whether a warmer climate will influence the intensity of extratropical cyclones, it is understood that the most extreme extratropical cyclones are projected to become more frequent due to increased moisture in the atmosphere, leading to more extreme precipitation events. That said, most scientists expect the number of extratropical cyclones overall will decrease due to a smaller temperature difference between the Arctic and tropical regions.

Our Role & Impact

Here in the Northeast US, the Gulf of Maine is warming at a rate nearly three times faster than the rest of the world’s oceans. Being on the frontlines means we are some of the first to develop solutions to these climate change impacts. We work closely with communities to develop climate resilience and adaptation strategies that can serve as a blueprint for climate action globally. Our Coastal Dynamics Lab works on developing localized present and future flood risk information, and on translating this emerging science into technical guidance, decision-support tools, and trainings that support adaptation and policy. Our Municipal and Business Climate Action Programs draw on research from our Coastal Dynamics Lab to develop climate vulnerability assessments with coastal towns and businesses, collaboratively developing the knowledge, skills, and tools to improve their resilience. Finally, our Coastal Flooding Community Science program gets the public involved by providing guidance on how to contribute observations that will help pinpoint high-risk flooding areas in your community.

As the world faces a rapidly changing climate, understanding the drivers and impacts of coastal storms is critical. Whether it’s through cutting-edge research, community-driven solutions, or innovative adaptation programs, we are dedicated to helping coastal communities stay resilient. Learn more, get involved, and join us in building a stronger, more prepared future.