Why the flooding in the South, Midwest was so severe

The multi-day outbreak of tornadoes, torrential rain and flooding that killed at least two dozen people across the Midwest and South was caused by a storm system that stalled and hovered over the same area for long periods of time, according to the National Weather Service.

The system caused widespread extreme flooding in the region, with 26 river gauges in major flood stage as of Wednesday.

At least 24 people in Kentucky, Tennessee, Missouri, Georgia, Arkansas, Mississippi and Indiana have died as a result of the storms, according to officials.

Here's why the flooding was so severe:

The system stalled over the same area for days

As the storm system began to sweep across the country last week, it triggered widespread severe thunderstorms across the Great Plains. By the end of the week, the system continued to march eastward, leading to an outbreak of severe weather across much of the Midwest and South.

A strengthening high-pressure system over the Atlantic caused the system to stall and remain nearly stationary over parts of the Midwest and South for several days.

It was further held in place by a second area of high pressure over the Rocky Mountains and Plains, which created a weather pattern “roadblock” over the eastern part of the country.

Over a four-day period starting April 2, more than a foot of rain was recorded across portions of Arkansas, Kentucky and Tennessee, including the city of Memphis, records show. The biggest rainfall total from this event came from Benton, Kentucky, which recorded 15.68 inches of rain -- the most rain on record for a four-day period in western Kentucky.

On Saturday, 5.47 inches of rain fell at Memphis International Airport, marking the wettest April day on record for the city dating back to 1872. Jonesboro, Arkansas, also experienced its wettest April day on record on Saturday, with 5.06 inches of rain recorded.

It wasn’t until the high-pressure system over the Atlantic weakened on Sunday and Monday that the system was finally able to resume eastern movement and clear out of the hard-hit regions.

These systems frequently continue to trek east and then move off the East Coast and out to sea. But this particular setup allowed for a persistent surge of tropical moisture to come up from the Gulf. The extra moisture enhanced the torrential, flooding rain and caused water levels across the Mississippi and Ohio River valleys to rise significantly.

On Monday morning, the Kentucky River in Frankfort, Kentucky, crested at near-record levels and reached its highest level in decades. The Ohio River in Louisville, Kentucky, crested on Wednesday; the last time the Ohio River was this high in the Louisville area was February 2018.

Portions of the region were slammed with several rounds of heavy rain in the weeks leading up to the multi-day precipitation event, which contributed to the high water levels in the river valleys. Central Tennessee and Kentucky, southern Indiana and southwestern Ohio recorded above average precipitation during the month of March. This included cities like Nashville, Louisville and Cincinnati.

The preceding heavy rain events left the soil saturated with excess water in the region’s rivers and streams, increasing the risk of flash flooding and river flooding.

Early spring river flood events across the Mississippi and Ohio River Valleys typically involve a combination of melting snowpack and heavy rain events. This year, however, snowpack was well below average over the Northern Plains, Midwest and Ohio Valley as the spring season got underway, according to the National Oceanic and Atmospheric Administration. The widespread major flooding was driven by the relentless, multi-day stretch of extreme rainfall.

But the flood threat is not over yet. Major river flooding is expected to remain in the region through the rest of the week.

Climate change may have played a role in the deadly flooding

While the stalling system and overall weather pattern were the primary drivers behind the extreme rainfall and flooding event, human-amplified climate change is causing extreme rainfall events to become more frequent and more intense across the U.S., according to the federal government's Fifth National Climate Assessment, released in November 2023.

More intense extreme rainfall events increase the frequency and scale of flash flooding. In the Midwest, extreme precipitation events have increased by about 45% in recent decades -- the second-largest regional increase in the U.S., according to the Fifth National Climate Assessment. Extreme precipitation events, defined as the top 1% of daily extreme precipitation events, are very rare.

In the Southeast, extreme precipitation events have increased by about 37% in recent decades, according to the Fifth National Climate Assessment.

ABC News' Kenton Gewecke, Max Golembo and Dan Manzo contributed to this report.