Hurricane Florence is bearing down on the US East Coast, bringing 125 mph winds, a life-threatening storm surge, and extremely heavy rain.
The storm’s center is expected to make landfall near the border between North and South Carolina. Its extreme winds and storm surge could be devastating, as a wall of water 13 feet high in some spots pushes its way into the shore.
But the worst impact of the storm may actually come from the rain.
The National Hurricane Center predicts that the storm will get stuck over North and South Carolina all weekend, stalling over portions of South Carolina, North Carolina, and Virginia until Sunday morning. Projections suggest Florence will inch its way west over the weekend, with its eye taking two full days to drift from the coast into central North Carolina.
During that time, up to 40 inches of rain could fall over parts of North Carolina’s coast, prompting immense floods. Even areas as far inland as Charlotte and Raleigh could get up to 10 inches of rain.
Several factors are combining to create this worrisome situation: a high-pressure ridge of calmer weather is developing over the Ohio Valley, and ocean and air temperatures are higher than average.
Why Florence will get stuck
High-pressure ridges form when warm air in the upper atmosphere descends toward the Earth, making for clear, cloudless skies. This provides a natural barrier for swirling storms like Florence.
“Hurricanes cannot plow through regions of high pressure, they prefer to go around them,” James Done, an atmospheric scientist from the National Center for Atmospheric Research, told Business Insider in an email on Tuesday. “In this case, there’s not an easy way around this high pressure.”
So this weather system coming in from the west will essentially block Hurricane Florence from moving and dispersing, holding the massive, 73,000 square mile-wide storm in place over the Carolinas and Virginia.
The National Weather Service is warning that “thousands of homes and businesses will be filled with water” in the storm-affected area. Roads on wet ground will likely erode from underneath.
It’s similar to what happened last year when Hurricane Harvey hit Texas. Harvey was downgraded to a tropical depression when it made landfall — with winds of less than 39 mph — but it lingered for days, hanging over low-lying Houston and causing devastating flooding.
Hurricanes are becoming wetter
The second reason that Florence could bring deadly rainfall and flooding is part of a larger trend in hurricane development: storms are becoming wetter and more powerful as the oceans and air warm up.
Peak rain rates, which arrive when the fierce core of a storm is overhead, have increased 30% over the past 60 years, according Andreas Prein, a project scientist with the University Corporation for Atmospheric Research.
This is due to greenhouse gases from fossil fuels, which trap more heat on the planet and make the air and oceans warmer as a result. Warmer air can hold more water vapor, and this allows storms to become stronger and produce more rain.
Hurricanes are also fed by warm ocean water, which pumps more heat and evaporating seawater up into a storm. The surface water in the ocean needs to be at least 80 degrees Fahrenheit in order for a hurricane to get swirling. (That’s why they don’t appear during the winter.) Currently, sea surface temperatures off the Carolinas are 3 or 4 degrees Fahrenheit higher than average for this time of year, giving Florence more fuel than previous storms that hit the region.
Because Florence is trapped by that high-pressure ridge and likely to stick around over warmer water for longer, that could give the storm even more strength.
More powerful hurricanes also bring higher storm-surge levels, a problem that’s exacerbated by sea-level rise. Hurricane Florence’s storm surge could be as high as 13 feet in places like Cape Fear and Cape Lookout on the coast of North Carolina.
Hurricanes typically lose strength as they move over land, but the warmer air and water means there’s more energy available to fuel their destructive paths. When storms like Florence have more moisture to draw from, they maintain more strength as they move inland and hold in moisture for longer.
“Storms are going to move deeper inland and affect more people,” climate scientist Cindy Bruyere, who studies storms at the University Corporation for Atmospheric Research, recently told Business Insider. The trend means places where people aren’t used to being in the flood path could be inundated.
Eastern North Carolina has a moderate to high risk of flash floods as the storm arrives, but flash flooding is possible up into Maryland and the southeast corner of West Virginia, per National Weather Service.
Storms are getting sluggish
Stalled storms like Harvey — and potentially Florence — might represent a new normal, according to recent research from the National Oceanic and Atmospheric Administration (NOAA).
The analysis, done by by NOAA researcher James Kossin, shows that hurricanes, typhoons, and tropical storms are moving more slowly over the Earth’s surface, especially over land. Storms slowed by an average of 10% from 1949 to 2016, that analysis found.
This cyclone slowdown is likely caused by changes in the circulation of Earth’s atmosphere due to climate change, Kossin said. Over the 67-year period he studied, the average global temperature rose by 0.5 degrees Celsius.
Stronger, wetter, slower storms are becoming a life-threatening consequence of living in a warmer world.