TAMPA, Fla. (WFLA) — On Tuesday, nine inches of rain fell in St Louis, MO in 12 hours.
Two days later, on Thursday, up to 10 inches of rain fell in 12 hours near Hazard, Kentucky.
While this much rain is not uncommon in Florida, our porous ground allows the rain to seep in, limiting flooding. But in most other areas the ground can not accept rain that falls too hard and too fast.
The built environment, like infrastructure and pavement, is becoming more widespread, also making it harder for water to seep into the ground. So the water runs off and causes devastating flooding. In the case of eastern Kentucky, hills exacerbate the flooding as well.
As a result of the raging water, roads and highways became rivers, homes were washed away, and several people died.
Both of these extreme rainfall events are considered extremely rare using NOAA’s historical climate records.
As seen below in the purple shading, in a 20th century climate we should expect to see rainfall that extreme in St Louis once every 200+ years.
For Hazard, Kentucky the event was even more rare. The data below illustrates the event should only happen once every 1000 years or more.
The rain fell so rapidly that the North Fork River at Whitesburg, KY reached 20.91 feet, well above the previous record of 14.7 feet. That’s 42 percent above the previous record.
So what happened to make these events so extreme? Climate change does not cause heavy rain. A hot weather pattern and stalled frontal boundary laid the groundwork and then a warmer climate amplified it.
In both cases, rounds of heavy downpours formed along the stalled front, fueled by the hot weather. The storm cells moved parallel to the front, over the same towns time and time again. Meteorologists call that training.
That’s where climate change comes in. The heaviest rainfall events are becoming heavier, pushing severe flood events into the historic category.
That’s because a warmer atmosphere holds more moisture and dumps more rain. There is a simple math equation for this. For every 1 degree Fahrenheit rise in air temperature, the air holds 4 percent more moisture.
Since 1900 the U.S. has warmed 2 to 3 degrees Fahrenheit, so the average increase in atmospheric moisture is about 10 percent.
So conservatively one can assume these rain events are 10 percent wetter. But that baseline is too simplistic. The extra heat and moisture also help to make the parent storms more dynamic, spiking rain rates and pulling in even more moisture. The net result is that rainfall rates in the most extreme events increase significantly more than the baseline.
The map below illustrates that rainfall in the most extreme events has increased by around 50 percent in the Great Lakes and northeast, and 25 percent in the southeast.
Rain rates are steadily increasing as the atmosphere continues to warm. That is true for both areas that were struck with historic flooding this week.
In the future, as the planet continues to warm, heavy rainfall events will continue to get wetter and floods will continue to defy the odds. The image below, from the 2021 UN IPCC Climate Report illustrates this for various potential warming scenarios.
Just how extreme the weather becomes will be determined by how warm humans allow the climate to get and how quickly humanity reels in climate change.