This weekend, NOAA’s National Weather Service forecasters are monitoring the possible formation of a significant low-pressure system that could lead to substantial flooding and strong winds across Southern and Mid-Atlantic coastal states. This situation arises shortly after massive surf from Hurricanes Humberto and Imelda resulted in the collapse of eight homes on North Carolina’s Outer Banks. As reported by the Washington Post, this developing system may meet the criteria for being classified as “subtropical,” potentially earning it the designation “Karen” from the National Hurricane Center.
While we continue to monitor this storm, particularly given the potential for limited governmental response during the shutdown, it is crucial to understand the distinctions between the various storm types affecting the US. What differentiates nor’easters from hurricanes? What qualifies a storm as “subtropical?” How do these storms develop? Additionally, how are they evolving as a result of climate change driven by fossil fuel consumption?
Storm System Categories: Extra-Tropical vs. Tropical
<p A storm system, commonly referred to as a low-pressure system or cyclone, can be categorized as either extra-tropical or tropical. Although both types of systems impact weather conditions across the US, they operate differently regarding their energy sources, structures, sizes, movements, and seasonal occurrences. Nevertheless, both types serve the same overarching purpose: to achieve equilibrium within Earth's climate system.
This diagram illustrates various types of low-pressure systems, or cyclones. Source: Union of Concerned Scientists.
What does this mean? As discussed in a previous blog post concerning the Atlantic Meridional Overturning Circulation (AMOC), I noted that the Earth’s climate system strives to maintain temperature equilibrium by mixing warmer air or water from the equator with cooler areas nearer to the poles. The AMOC exemplifies this process by transporting warm water from the Equator to the North Pole, blending it with cooler waters and vice versa, aiding in achieving thermal balance across the planet.
Extra-tropical and tropical storms represent essential mechanisms within this equilibrium-seeking toolbox, serving to interchange warm and cold air or water to attain temperature stability.
Tropical Cyclones
Tropical cyclones develop in the Tropics (between 30°S and 30°N) due to substantial temperature disparities between the ocean and the upper atmosphere. When ocean temperatures exceed 26°C (79°F), evaporation occurs, producing clouds and precipitation that transfer heat from the ocean into the atmosphere, facilitating temperature equilibrium.
As the storm picks up energy from the ocean, it begins to rotate because of Earth’s rotation, developing into a tropical depression, the initial stage of a tropical cyclone. Provided conditions are conducive, the storm will then strengthen into a tropical storm and, if located in the Atlantic or Eastern Pacific, further escalate into a hurricane. Once the storm reaches tropical storm status, it is named by the National Hurricane Center.
Since tropical cyclones derive their energy from warm sea surface temperatures, it follows that fossil fuel-induced climate change, which is causing rapid ocean warming, is influencing their development. As ocean temperatures rise, hurricanes gather more energy, enabling them to intensify more swiftly and more severely. Research shows that hurricanes in the Atlantic are rapidly intensifying as a result of increased ocean temperatures. Notably, an analysis by Climate Central indicated that all hurricanes that occurred during the 2024 hurricane season were strengthened by these increased ocean temperatures.
Climate change and elevated ocean temperatures are making hurricanes more intense compared to a scenario without climate change. Source: Climate Central.
Extra-Tropical Cyclones
Extra-tropical cyclones arise outside the Tropics and derive their energy from horizontal temperature contrasts between warm and cold air masses. The United States is ideally positioned to experience these systems, with cold Arctic air to the north and warm tropical air to the south. Once again, the climate system aims to equalize temperatures by mixing these differing air masses, and extra-tropical cyclones emerge as a means to achieve this.
In the US, four primary extra-tropical storm systems influence weather: the Alberta clipper, the Colorado low, the Pineapple Express, and the Nor’easter (or coastal low). Extra-tropical cyclones typically move from west to east, guided by the jet stream, a current of fast-moving air in the upper atmosphere that acts as a “highway” for these storm systems. One defining feature of extra-tropical cyclones is the presence of warm fronts and cold fronts—leading edges of differing air masses that advance into new regions as the cyclone progresses across the states.
These extra-tropical systems can vary in strength and size. A robust Pineapple Express, which is essentially an atmospheric river, can usher in heavy precipitation during winter. A strong Colorado low can generate supercell thunderstorms in spring, potentially leading to tornado formation. Meanwhile, a nor’easter tracking up the East Coast can deposit feet of snow in both inland and coastal locales during winter, compounded by high surf and winds.
In general, extra-tropical cyclones are adapting to climate change. As atmospheric temperatures rise, the air can retain more moisture (see the sponge analogy discussed in this blog), resulting in heavier precipitation across these systems. This effect is particularly notable in the atmospheric rivers associated with the Pineapple Express. Furthermore, a recent study revealed that nor’easters are gaining strength, partly attributable to increased temperature differences between cold air moving down from the North Pole and the significantly warmer ocean.
The four typical extra-tropical storm systems that affect the US. Source: Union of Concerned Scientists.
A Nor’easter with Subtropical Traits?
This weekend, we are observing a developing nor’easter or coastal cyclone anticipated to bring high surf and wind to southern coastal states extending up to the Northeast. By now, you understand that a nor’easter is classified as an extra-tropical cyclone. But what if this nor’easter develops subtropical characteristics?
A subtropical cyclone is intriguing: it is essentially a hybrid of extra-tropical and tropical cyclones. How so? A subtropical cyclone harnesses energy from both warm ocean waters (similar to a tropical cyclone) and horizontal temperature contrasts (like an extra-tropical cyclone). Often, subtropical cyclones are assigned names by the National Hurricane Center if they exhibit strength equivalent to that of a tropical storm.
Interestingly, when Hurricane Sandy made its landfall in New Jersey in October 2011, it was categorized as a subtropical storm. It drew energy not only from warm ocean waters but also from a stark temperature divide between frigid air from Canada and hot air from the southern US. This storm sought to mix these contrasting air masses, utilizing both warm ocean conditions and cooler upper atmosphere.
The Origin of the Term Nor’easter
To conclude on a lighter note, one might wonder why these storms are referred to as nor’easters (short for northeasters). The answer lies in wind direction. Meteorologists define wind based on its source rather than its direction of travel. For instance, a northerly wind originates in the north and travels southward. A nor’easter, moving up the coast, brings powerful northeast winds into New England. The term has been in use since at least the early 19th century in New England. Throughout history, several notable nor’easters have occurred, including the Portland Gale Nor’easter of 1898, which was so intense it altered the shape of Massachusetts’ coastline, and the 1993 Storm of the Century Nor’easter, which dumped more than 40 inches of snow in Upstate New York!
