A record-shattering 20.73 inches of rain fell in 24 hours in Humphreys County, triggering extreme floods that killed 20 people on Aug. 21, officials said. 
The National Weather Service Nashville confirmed the total on Monday.
Early estimates topped out rainfall at 17.03 inches in Humphreys County that day, but further investigation into a measurement at a wastewater treatment plant in McEwen revealed the new record-high total, NWS meteorologist Krissy Hurley said. 
She also said it is the highest 24-hour rainfall record for any state not touching an ocean. 
“To have that much rain without it being a tropical system is just mind-blowing,” Hurley said Monday.
The new record far exceeds the previous state high of 13.6 inches of rain that fell in on Sept. 13, 1982 in Milan, Tennessee. The 17.03 inch measurement, captured by a Tennessee Valley Authority rain gauge in McEwen, now sits as the second-highest rainfall total for a single day, officials said.
 The final determination was made after a careful review of the equipment involved in the measurement, in coordination with the State Climate Extremes Committee and the Tennessee State Climate Office. 
Middle Tennessee, on the whole, is around 10 inches above average for rainfall this year, according to NWS Nashville meteorologist Scott Unger. 
“This is just another reminder of how wet it’s been,” he said. 
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Meteorologists have said the massive rainfall was extremely rare, with one calling it a once-in-a-thousand-years event. Entire homes were swept away, and the floods left widespread damage across several counties.  
NWS issued four cell-phone emergency warnings to Humphreys, Houston, Hickman and Dickson counties in three hours starting at 6:09 a.m. that day, NWS meteorologist Krissy Hurley told The Tennessean in August.
Flash flood warnings usually mean that people should stay off the roads to avoid having cars swept away by flood waters. They typically don’t mean houses are knocked off their foundations.
“The problem was a rush of water came into your home in less than five minutes,” Hurley said. “To get rainfall like that with just thunderstorms is something that will be researched by the meteorological and climatology communities for years to come,” 
Brad Schmitt contributed to this story.
Find reporter Rachel Wegner at rawegner@tennessean.com or on Twitter @rachelannwegner.