The storm flooded roads, submerged homes, and destroyed property in parts of North Carolina, South Carolina, and Virginia. Air currents direct the movement of tropical storms and hurricanes. Fast currents move a storm along quickly, often around 15 miles per hour, but with weaker currents storms linger and drop more precipitation.
Why Hurricane Florence Caused So Much Flooding?
Published: February 26, 2019
Onscreen: Why did hurricane Florence cause so much flooding? The storm flooded roads, submerged homes, and destroyed property. In parts of North Carolina, South Carolina, and Virginia.
Joseph Pawlik: Just when you thought it was over, another band would come through with hurricane-level winds.
Onscreen: It set rainfall records in places like Wilmington, North Carolina.
Anantha Aiyyer: Some parts of North Carolina got over 30 inches of rain which, if you think about it is over 50% of the annual rainfall.
Onscreen: What made the rainfall so intense?
Aiyyer: The warmer the atmosphere is, the more water vapor we can evaporate into it.
Onscreen: Warming air and sea-surface temperatures are making storms wetter and more intense. But there is another factor.
Aiyyer: The biggest factor in tropical cyclone-related rainfall is how slow the storm is moving.
Onscreen: Air currents direct the movement of tropical storms and hurricanes. Strongly circulating currents move a storm along quickly, often around 15 miles an hour, with weaker currents storms become sluggish. Over the coast, Florence slowed to less than 5 miles and hour.
Aiyyer: So in this particular case, there wasn’t a particularly strong steering current. It was slow-moving and remained close to the ocean which is the primary source of its moisture and energy. That allowed the storm to meander slowly, collect all the moisture, and then rain it out in heavy amounts.
Pawlik: The eye essentially rotated over Wilmington, so the worst part of the storm had an extended period of time going over the city.
Onscreen: The same occurred with Harvey in 2017 in a matter of days. The storm flooded parts of Texas with a year’s worth of rainfall. Air currents are strongest when Earth’s poles are much colder than the equator. Recently, air at the poles has warmed.
Aiyyer: If the equator to pole temperature difference decreases, then the atmospheric circulation will decrease. That’ll lead to slow down of the hurricane motion.
Onscreen: Preliminary evidence suggests hurricanes are already traveling more slowly.
Aiyyer: Scientists calculated the speeds and reports that on average over the last 70 or so years, hurricane speeds have decreased by 10% globally. In the Atlantic, hurricane speeds have decreased by 20%. That slow down could’ve affected what we saw with Harvey and Florence.
Digital Producer: Brian Kantor
Special Thanks: Micah Marty, Michele Seidman, Nafisa Syed, Robin Kazmier, Samia Bouzid, Vincent Pham
Additional Footage: Brandi Thompson, Joseph Pawlik, Mark Huneycutt, NASA / Joshua Stevens, NASA / JAXA, Hal Pierce, NOAA / CIRCA, Shutterstock, Videoblocks
© WGBH Educational Foundation 2019