A study in npj Climate and Atmospheric Science has found that warmer North Atlantic conditions are linked to wetter tropical cyclones, broader heavy-rain areas and slower storm movement in some warm low-latitude regions. The research, led by Newcastle University scientists, analyzed North Atlantic storms from 2001 to 2024 to examine how rainfall intensity, storm size and forward speed change as the ocean and lower atmosphere warm.
The findings point to a troubling combination for coastal and inland flood risk. A storm that rains harder can already strain drainage systems and river basins. A storm that rains harder over a larger area, then lingers, can raise the danger much more quickly.
The study focuses on North Atlantic tropical cyclones during their tropical and post-tropical phases. That distinction matters because storms can behave very differently after they begin to transform while moving into higher latitudes.
Satellite Data Tracks Storms From 2001 to 2024
The research team used satellite observations and modern atmospheric data to follow tropical cyclones across more than two decades. The period covered storms from 2001 through 2024, giving the researchers a recent view of how storm rainfall has behaved in a warming ocean basin.
Rather than treating every storm as a simple circle, the team examined several moving pieces at once. They looked at the intensity of rainfall, the area covered by heavy precipitation, storm size and translation speed. Translation speed is the rate at which a cyclone moves across the map.
This approach is important because flood risk depends on the whole storm. A compact hurricane can produce extreme rain near its center. A broader system can spread heavy rain across a much larger region. A slow storm can keep feeding the same rivers and neighborhoods for many hours.
In the North Atlantic, tropical cyclones are already a major part of warm-season rainfall. During peak hurricane months, especially August through October, they can contribute a large share of total precipitation in affected areas. Changes in these storms can therefore have outsized effects on flood planning.
Hurricane Rainfall Rises Sharply With Moisture
The clearest signal in the study is the rise in rainfall intensity as the local air becomes warmer and more humid. The researchers used dewpoint temperature as a key measure because it is closely tied to how much water vapor the air contains.
Warmer air can hold more moisture. For tropical cyclones, that extra moisture can become fuel for heavier rain. The study found that storm precipitation increased at a median rate of about 21% per 1°C rise in local dewpoint temperature.
The paper’s abstract describes the response directly, stating that “precipitation intensity rises sharply with all temperatures.” This is a compact summary of one of the study’s central results.
That result fits the basic physics of a warming atmosphere. As humidity rises, storms have more water available to condense into rain. The new analysis shows how strongly this relationship appears in real North Atlantic storms observed during the satellite era.
For communities in a storm’s path, the practical meaning is clear. A wetter cyclone can produce larger rainfall totals in less time. That can increase flash flooding and overwhelm small streams before larger river systems have time to respond.
Heavy Rain Covers More Ground
The study also found that the footprint of heavy rain expands as conditions warm. That means the problem extends beyond intense rain near the storm center. More places can end up under rainfall rates high enough to create dangerous flooding.
According to the research, the area experiencing heavy precipitation expands by about 12.5% per degree of warming in the relevant temperature measure. In other words, a warmer storm environment can make the zone of heavy rain wider as well as stronger.
This matters for emergency managers because flood impacts often depend on coverage. Heavy rain over one small drainage basin can cause severe local damage. Heavy rain across several basins can produce a broader emergency, with more roads, neighborhoods and reservoirs affected at once.
The combination of heavier rain and a larger rain shield also makes forecasts more consequential. Small shifts in the storm track can change which watershed receives the worst rainfall. When the heavy-rain area grows, the number of places facing serious runoff can grow with it.
Very Warm Waters Can Change Storm Size
Storm size showed a more complicated response. On average, the study indicates that tropical cyclones may shrink slightly with warming. Yet that relationship weakens when sea surface temperatures become extremely high.
In the warmest waters, especially in parts of the Caribbean, storms can become larger. That is a key regional detail because the Caribbean is both oceanographically warm and highly exposed to hurricanes. The finding suggests that local ocean conditions can shape storm structure in important ways.
The researchers also found links between very warm waters and slow-moving storms in low-latitude regions. These storms can remain in favorable tropical environments while producing persistent rain near their cores.
For the public, storm size can be confusing because it differs from storm category. A hurricane’s category is based on wind speed. Its rainfall hazard depends on moisture, structure, speed and where the rain bands set up.
That is why a storm’s flood threat can extend well beyond the area of strongest winds. The rain field may expand, shift, or reorganize as the cyclone interacts with its surrounding atmosphere.
Slow-Moving Storms Raise Flood Risk
Forward speed is one of the most important factors in a hurricane flood disaster. A storm that moves quickly may still bring intense rain, but it spends less time over any one location. A slow storm can keep dropping rain over the same place until soils, creeks and rivers are overwhelmed.
The Newcastle-led study found that some storms in warmer low-latitude regions may move more slowly. The abstract states, “Slower motion in these warmer, low-latitude regions prolongs precipitation.” That prolonged rainfall can raise flood totals even when the storm’s peak rain rate is only part of the danger.
Haider Ali, a senior research associate at Newcastle University, connected the findings to flood risk in the North Atlantic. “This trend will likely continue with increased warming,” Ali said.
The concern is especially sharp for coastal areas and river basins downstream from landfall. If the storm’s rain core stalls over a vulnerable watershed, the effects can build hour after hour. Flooding can then spread from streets and small streams into larger river networks.
Recent hurricane disasters have shown how damaging this pattern can be. The study’s source material points to storms such as Harvey and Helene as examples of persistent extreme rainfall events that can cause catastrophic flooding.
Tropical and Post-Tropical Storms Behave Differently
The study separates tropical cyclones from their post-tropical counterparts because the two phases respond differently to warming. This distinction is especially important in the North Atlantic, where storms can move north or northeast and begin changing as they approach cooler waters and different weather systems.
During the tropical phase, rainfall tends to be more tightly linked to temperature and moisture. The most intense rain often remains concentrated near the storm’s core. When tropical storms slow down in warm regions, that concentrated rain can linger over a smaller area for longer.
Post-tropical storms behave differently. They often expand in size as they interact with mid-latitude weather systems. Their rainfall can spread over a wider area and the heaviest rain may shift to the northeast side of the storm center.
This does not make post-tropical systems harmless. They can still cause serious disruption across broad regions. Their flood patterns simply depend on different atmospheric controls than those shaping a fully tropical hurricane.
That difference has practical value for forecasting. A tropical cyclone nearing landfall may pose its greatest rainfall risk near the core and nearby rain bands. A post-tropical system may spread rain and wind impacts across a wider swath.
A Dynamic Measure of Storm Size
A major feature of the study is its use of a dynamic measure of storm size. Earlier work often relied on fixed-radius methods because they were easier to apply across many storms. The Newcastle-led team used a wind-based radius designed to change as each storm evolved.
This is a more realistic way to follow a cyclone. Storms grow, shrink, reorganize and transition during their lifetimes. Their rain fields shift along with those changes.
The researchers used ERA5 near-surface winds to help define storm size. ERA5 is a widely used atmospheric reanalysis dataset. It combines observations with weather modeling to provide a consistent picture of the atmosphere through time.
By linking storm size with rainfall and movement, the team could examine the storm structure behind flood risk. That creates a fuller picture than rain rates alone. It also helps explain why two storms with similar peak rainfall can create different impacts.
This method is especially useful for North Atlantic storms because many of them change character as they move. A cyclone can begin as a compact tropical system, then expand as it becomes post-tropical. A fixed storm outline would miss part of that evolution.
Why the Worst Floods Depend on More Than Rainfall
The study reinforces an important point about flood disasters. The heaviest rain total does not automatically produce the worst river flooding. Flood outcomes depend on where the rain falls, how long it lasts and how water moves through the landscape.
Soil moisture matters. A basin that is already wet before a storm arrives can respond quickly to new rainfall. Urban surfaces can also speed runoff because water has fewer places to soak into the ground.
River basin shape plays a role as well. Some watersheds route water downstream quickly. Others spread the flow over time. The same storm can therefore produce very different flood peaks depending on terrain and drainage patterns.
Study co-author Hayley Fowler tied the findings to the growing damage from persistent rainfall. “Tropical cyclones appear to be causing increasing damages from widespread damaging floods from persistent extreme rainfall events, such as in Hurricane Helene,” Fowler said.
The researchers identify a clear next step. They want to connect storm structure and rainfall to river flow using hydrological models. That could help identify which storms are most likely to cause damaging floods, rather than only which storms are likely to produce heavy rain.
The broader message is that warming oceans can increase hurricane rainfall hazards in several connected ways. Rain rates can intensify. Heavy-rain areas can expand. Some storms can slow down in warm low-latitude regions. Together, those changes can raise the odds of persistent rainfall over vulnerable places.
