Tropical Storm Debby roams the Carolinas, bringing days of heavy rain and flooding – a climate scientist explains why

Tropical Storm Debby was moving so slowly, Olympic athletes could have overtaken it as it moved through the Southeast in early August 2024. That gave the rains time to flood cities and farms in large parts of Florida, Georgia and the Carolinas. More than a foot of rain fell in some areas early on August 7, 2024, with more rainy days forecast for there and the Northeast.

Matthew Barlowclimate scientist at UMass Lowell, explains how storms like Debby’s absorb so much moisture, what can cause them to slow down or stop, and what climate change has to do with it.

What causes hurricanes to stop?

Hurricanes are controlled by the weather systems they interact with, including other storms that move across the U.S. and the High shorts over the Atlantic Ocean.

A hurricane may be moving slowly because there are no weather systems close enough to pull it, or there may be a high pressure system to the north of the hurricane that blocks its forward movement.

In this case, a high pressure system over the western US it was slowing Debby’s progress, and the Bermuda High — which is a large circulation of clockwise winds that generally moves up the East Coast — wasn’t close enough to be a factor.

This is similar to what happened to the remnants of Hurricane Harvey in 2017, one of the best-known examples of a stalled hurricane. High pressure on the USA blocked his forward movement, allowing him to fall more than 50 centimeters of rain in parts of Texas.

Slower storms take longer to rain in the same area, and this can drastically increase the risk of flooding, like the Southeast is experiencing with Debby.

How is Debby carrying so much rain?

The surface temperatures of the Atlantic Ocean and the Gulf of Mexico have been much warmer than average for the past year. At this moment, there is a very wide area of ​​exceptionally hot climate sea ​​surface temperatures there, which means a lot of evaporation.

The air is also warmer than average and warm air can retains much more moisture.

So more evaporation from a warmer ocean is pumping more moisture into warmer air, which can hold more. This more-more-more has resulted in a lot of moisture in the air in this large area, from the Gulf of Mexico to the East Coast, and it is this area that Debby is moving through. The storm is attracting exceptionally high humidity from all directions – it’s the perfect environment to produce very heavy rainfall.

This warm, moist air also extends up the East Coast, which is why the Mid-Atlantic and potentially the Northeast are forecast to see heavy rain and flooding as the storm moves north and interacts with other weather systems.

Debbie too moving close to the coast. Since there is more water in the air over the ocean, this means the storm can stay stronger for longer because it has a reservoir of fuel. If Debby had moved inland with more force, the storm would have died down more quickly.

How is climate change affecting tropical cyclones?

Human-caused climate change is warming the ocean and atmosphere. This warming comes largely from the burning fossil fuelswhich emit greenhouse gases that trap heat near the Earth’s surface.

The combination of warmer ocean and warmer atmosphere may result in higher precipitation rates and storms that intensify more quickly.

There is also evidence that hurricanes that affect the US are slowing down in its forward movement and that this may be linked to climate change, but this link is not yet well understood.

Without a rapid reduction in fossil fuel emissions, these storms and their associated rainfall will continue to intensify. The good news is that humanity Know how to reduce emissionsand efforts are starting to move the world in that direction.

What does Debby have in store for the Northeast?

Tropical storm Debby is expected continue moving slowly over the southern US or just offshore for a few days, with a high risk of excessive rainfall and the potential for catastrophic flooding in areas of the Carolinas in particular.

Substantial rainfall is also expected across parts of the Mid-Atlantic and Northeast. The area between Philadelphia and New York is already under flood warnings before the storm, in part because of a frontal boundary that allows tropical moisture to accumulate there.

Once Debby passes through and interacts with other systems, it is predicted to move faster and head towards Atlantic Canada, leaving vast areas of flood damage in its wake.

This article was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you understand our complex world. It was written by: Matthew Barlow, UMass Lowell

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Mathew Barlow received funding from the US National Science Foundation to study extreme precipitation.