An exceptionally powerful atmospheric river fueled the deadly early April flooding disaster in the central and southern United States — a historic flood event and a poignant example of how the phenomenon acts differently in the East than it does in the West.
It could signal added danger in a region less accustomed to hearing about them, particularly with another, less-potent one expected in the coming days.
Atmospheric rivers are long, narrow streams of very moist air that act like a firehose to send gushing rain or snow where they point. They happen frequently around the globe, but are most well-known in the West, where they are responsible for a huge chunk of the region’s water supply and flood risk.
But early April’s was special, a Category 5.
“This was an exceptional atmospheric river, regardless of where you are in the United States,” said Jay Cordeira, an atmospheric scientist with the Center for Western Weather and Water Extremes (CW3E). “It was as strong as they come.”
Scientists say atmospheric rivers are becoming more frequent and transporting more moisture as the world warms due to planet-heating fossil fuel pollution.
The Category 5 was one of multiple atmospheric rivers that flowed into parts of the South, Midwest and Appalachians in quick succession and provided “significant moisture which supported heavy rainfall” from April 1 to 7, according to a recent report from Cordeira and his colleagues at CW3E.
Six to 10 inches of rain fell over more than 750 miles from northeastern Texas to the Kentucky-Ohio border, while more than a foot of rain fell in parts of Arkansas, Tennessee, Missouri and Kentucky in just three-to-four days, causing widespread and deadly flooding.
Flash flood emergencies — the most severe flash flood warning — were issued in parts of five states. Hundreds of roads were flooded in Kentucky and the state ran out of road-closing barricades. More than 500 homes were damaged by floodwater in Frankfort, Kentucky, alone.
West vs. East
Atmospheric rivers already occur much more frequently in the eastern half of the US than many people realize, according to Cordeira.
“It’s kind of a misconception that they don’t happen on the East Coast, but they do happen with some regularity,” Cordeira explained. “The issue is that they don’t usually cause as many impacts as the ones on the West Coast.”
Except for when they do, like earlier this month, and potentially in the days to come.
New rounds of storms fueled by another atmospheric river will dump heavy rain from the southern Plains through parts of the Midwest this weekend, and “numerous” instances of flash flooding are likely in eastern Oklahoma, northern Arkansas and central Missouri, according to the Weather Prediction Center.
A few important differences between West Coast atmospheric rivers and those that hit the eastern half of the US explain why their flood disasters are more common in the West but can have added danger in the East.
The biggest difference is where each atmospheric river pulls its moisture from. Those in the West tap into the Pacific Ocean; those in the East tap into the Caribbean Sea and what the US government now calls the Gulf of America, but is still known outside the US as the Gulf of Mexico.
Pacific atmospheric rivers pull together plenty of moisture, but the amount generated pales in comparison to the Gulf and Caribbean, which are basically the MVPs of warm, moist air. Plus, both are much warmer than normal, which could be supercharging how much moisture is on offer.
More moisture generally leads to more intense atmospheric rivers, multiple studies have shown. The amount of moisture that flowed into the US in early April was remarkable in its own right.
“There’s only been one or two storms that have hit the West Coast in the last 60 years that have had that value of water vapor transport,” Cordeira said of the East’s Category 5 atmospheric river.
If eastern atmospheric rivers are generally juicier, why aren’t historic flooding events happening constantly? It has to do with the way each type wrings moisture out of the atmosphere.
The West’s mountainous terrain is mainly what sends rain pouring to the ground below, according to Cordeira. Moist air forced up the tall terrain will eventually condense due to changes in temperature and rain itself out.
The eastern half of the US also has mountains, but they’re not as tall and plentiful as those out West. Instead, eastern atmospheric rivers need to slam into a front to send rain gushing down, Cordeira explained.
A front draped from Arkansas through Kentucky was stuck in place for days in early April. That front acted like the West’s mountains, forcing storms to form and dump torrential rain. But fronts like this aren’t always in place, so many atmospheric river events go by with their potential untapped.
Atmospheric rivers out West also tend to dump their heaviest rain over the open water in the Pacific, because the storms they’re associated with typically peak in strength there. That’s not the case in the East. Eastern atmospheric rivers reach peak strength as they rush over land, putting many more people at risk of the most intense rain and flooding.
Why Kentucky is at the epicenter
Kentucky and surrounding states have been a magnet for disaster this year. Disastrous flooding struck the Bluegrass State in mid-February, killing 12 people.
At the time, Kentucky Gov. Andy Beshear called the flooding “one of the most significant natural disasters we’ve seen in our lifetime.”
That event was also fueled by an atmospheric river, according to Cordeira — albeit one that wasn’t quite as strong and didn’t stick around for nearly as long as April’s.
The answer to why this keeps happening is found out over the Atlantic Ocean, between Florida and Bermuda. High pressure meanders in this area year-round and helps steer moisture from the Gulf and Caribbean into the US. Exactly where the bulk of it goes depends on the season.
Moisture usually flows right through the central Gulf Coast and into Tennessee and Kentucky and then parts of the Midwest in the spring and fall. It shifts a bit east in the winter and a bit west in the summer.
“Kentucky and Tennessee just end up being this prime geographic battleground where you can get the right ingredients of moisture to the south and some type of (front) to the north that overlap,” Cordeira explained.
And it could get worse as the world warms.
The tropics are a “key source of moisture for atmospheric rivers” and as they warm the “moisture is intensifying the precipitation associated with these systems across the western and eastern United States,” the Fifth National Climate Assessment found.
Atmospheric river studies have mainly focused on the West because of their more frequent impacts there, but eastern events have garnered more attention from researchers in recent years. A 2022 study found atmospheric rivers originating in the Gulf of Mexico are becoming more intense and larger with time as sea surface temperatures rise.
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