Four to eight inches of rain are likely from Friday night through Sunday over a region that has been raked with torrential downpours over the past couple of days. The ground is already saturated, which will prevent this weekend’s rain from being absorbed. Instead it will run into rivers and streams, which will lead to flooding and flash flooding from Oklahoma to the Ohio Valley.
Southern Missouri looks to be the hardest hit by excessive rainfall, a portion of which is still under a flood warning from previous storms.
Perhaps the best way to illustrate how serious the flooding could be is through these seven graphics, from flash flood warnings to record-breaking air moisture.
How do you get East record heat, Midwest floods, Texas severe wx, and Rockies snow on the same day? With an upper-level pattern like this. pic.twitter.com/R6vIAItSrg
— MDA Weather Services (@MDA_Weather) April 28, 2017
The big-picture weather pattern responsible for this weekend’s flooding is an area of low pressure in the South, with two areas of high pressure acting as bookends on each side.
You can easily see by animation how efficient this pattern is at maximizing moisture transport with a strong high off the southeast coast. pic.twitter.com/b1QO8nCh2k
— Taylor Trogdon (@TTrogdon) April 28, 2017
Low pressure spins counterclockwise, and high pressure spins clockwise. On the left side of the low, you get strong winds from the north, and on the eastern side you get strong winds from the south. The southerly flow is going to originate over the Gulf of Mexico and transport the moisture to the Midwest.
We measure the amount of moisture in the air using “precipitable water.” It’s how much rain would fall if all of the moisture in the air condensed and was wrung out over a certain location.
The graphic above is precipitable water data for Little Rock. It shows the average through the year, as well as the daily records. Jon Erdman at Weather.com added notes to this graphic that shows the highest precipitable water ever recorded in the month of April in Little Rock, along with where it’s expected to be this weekend.
If this forecast is correct, Little Rock will set a record for the amount of moisture in the air during the month of April.
With the high air moisture in mind, the National Weather Service is calling for as much as eight inches of rain through Sunday in the Ohio and central Mississippi River valleys, with a bull’s eye on southern Missouri and Illinois.
— Stu Ostro (@StuOstro) April 28, 2017
Four to eight inches of rain is not insignificant, but it isn’t necessarily enough to cause flooding on its own. What’s going to make this weekend so flood-tastic is how wet the ground already is. On Thursday night, long-lived thunderstorms dumped as much as two inches of rain on parts of Illinois and Missouri. The soil in these locations will not be able to hold much more rain.
Given the rain that’s already fallen, plus the rain expected to start Friday night, the National Weather Service’s Storm Prediction Center says there’s a high risk for flash flooding on Saturday. It’s the highest risk on the scale, used only for particularly excessive rainfall events.
“An impressive influx of moist and unstable air will lead to a broad zone of heavy convective rainfall and embedded swaths of very intense rainfall,” forecasters at the National Weather Service wrote. “Flooding and flash flooding impacts are likely … beginning Friday night from Oklahoma to Southern Illinois.”
Ten states are under a flash flood watch for the weekend. It covers 15.6 million people, including the major metro areas of Cincinnati, Louisville, St. Louis and Little Rock.