Tropical Storm Debby was moving so slowly that the Olympians could overtake it as it passes through the Southeast in early August 2024. That gave its rain time to flood cities and farms across large parts of Florida, Georgia, and the Carolinas. More than a foot of rain fell in some areas early on August 7, 2024, and more rainy days are forecast there and in the Northeast.
Mathew Barlow, a climate scientist at UMass Lowell, explains how storms like Debby pack so much moisture, what might cause them to slow down or stall, and what climate change has to do with it.
What causes hurricanes to stop?
Hurricanes are driven by the weather systems they interact with, including other storms that move across the United States and the Bermuda High over the Atlantic Ocean.
The reason a hurricane is slow-moving may be that there is no weather system close enough to pull it along, or there may be a high pressure system to the north of the hurricane that is blocking the hurricane’s forward motion.
In this case, the high pressure system over the western United States was slowing Debby’s progress, and the Bermuda High Pressure (a large clockwise wind circulation that generally blows along the East Coast) was not close enough to be a factor.
This is one of the best-known examples of a stalled hurricane, similar to what happened with the remnants of Hurricane Harvey in 2017. High pressure over the U.S. blocked its forward motion, allowing it to drop more than 50 inches of rain on parts of Texas.
Slow-moving storms need to dump rain on the same area for longer periods of time, which can significantly increase the risk of flooding like what the Southeast experienced with Debby.
How does Debby stand all this rain?
Surface temperatures in the Atlantic Ocean and the Gulf of Mexico have been well above average over the past year. There are now unusually warm sea surface temperatures there across a very wide area, meaning a lot of evaporation.
The weather is also warmer than average, and warm air can hold much more moisture.
So, more evaporation from a warmer ocean pumps more moisture into the warmer air, which can hold more. This more-more-more has created a lot of moisture in the air across this broad area from the Gulf of Mexico all the way down the East Coast, and that’s where Debby is moving. The storm is pulling in extraordinarily high humidity from all directions – the perfect environment to produce very heavy rainfall.
Warm, humid air is extending up the East Coast, so heavy rainfall and flooding are expected across the Mid-Atlantic and possibly the Northeast as the storm moves north and interacts with other weather systems.
Debby is also moving closer to the coast. With 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 more strongly, the storm would have died down faster.
How does climate change affect tropical cyclones?
Human-caused climate change is warming the ocean and atmosphere. This warming is largely due to the burning of fossil fuels, which emit greenhouse gases that trap heat near the Earth’s surface.
The combination of a warmer ocean and warmer atmosphere can lead to higher precipitation rates and faster-intensifying storms.
There is also evidence that the rate of progression of hurricanes affecting the United States is slowing and that this may be linked to climate change, although this link is not yet fully understood.
Without rapid reductions in fossil fuel emissions, these storms and their rainfall will continue to intensify. The good news is that humanity knows how to reduce emissions, and efforts are beginning to move the world in that direction.
What’s in store for the Northeast from Debby?
Tropical Storm Debby is expected to continue to move slowly over the southern or offshore United States for several days, creating the risk of heavy rainfall and catastrophic flash flooding, particularly in some areas of the Carolinas.
Significant rainfall is also expected across parts of the Mid-Atlantic and Northeast. The area from Philadelphia to New York City is already under flood warnings ahead of the storm, in part because of a frontal boundary that is allowing tropical moisture to accumulate there.
Debby is expected to move faster as it interacts with other systems and head toward Atlantic Canada, leaving widespread flood damage in its wake.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. By Mathew Barlow UMass Lowell
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Mathew Barlow received funding from the US National Science Foundation to study extreme rainfall.