Early on August 11, a mesoscale convective system flared up in southern Louisiana around a weak area of low pressure that was situated next to an outflow boundary. It remained nearly stationary, and as a result, torrential downpours occurred in the areas surrounding Baton Rouge and Lafayette. Rainfall rates of up to 2–3 inches (5.1–7.6 cm) an hour were reported in the most deluged areas. Totals exceeded nearly 2 feet (61 cm) in some areas as a result of the system remaining stationary. Accumulations peaked at 31.39 inches (797 mm) in Watson, just northeast of Baton Rouge.
The Washington Post noted that the "no-name storm" dumped three times as much rain on Louisiana as Hurricane Katrina. It dropped the equivalent of 7.1 trillion gallons of water or enough to fill Lake Pontchartrain about four times. Hurricane Katrina, by comparison, dumped about 2.3 trillion gallons of rainwater in the state (though more in other states). The flood also dumped more water than Hurricane Isaac. According to the National Weather Service Hydrometeorological Design Studies Center, the amount of rainfall in the hardest-hit locations had a less than 0.1 percent chance of happening or was a (less than) 1-in-1,000-year event