The multi-day rainfall totals, shown both in the map above and in the list below, are stunning — many in the 20-30 inch range.
Some areas of Louisiana saw more rain from this event in three days than Los Angeles has in the last several years.
What makes the rainfall output most remarkable is that it didn’t originate from tropical storm or depression, but just a weak area of low pressure that tapped into a unusually deep tropical moisture stream — fueled by warmer-than-normal ocean waters.
Ryan Maue, a meteorologist for WeatherBell Analytics, computed that the no-name storm deposited the equivalent of 7.1 trillion gallons of water on Louisiana. “Enough water fell that it could fill Lake Pontchartrain about four times,” he said.
Hurricane Katrina, by comparison, only left behind about 2.3 trillion gallons of rainwater in the state. (It produced a lot more rain in Mississippi, about 4.26 trillion gallons). The flooding in Louisiana from Katrina was mostly caused by storm surge, the tsunami-like swell from the Gulf of Mexico, which caused levees and flood walls to fail, not freshwater rain.
This no-name storm also generated more rainwater (in Louisiana) than 2012’s Hurricane Isaac, which produced the equivalent of 5.31 trillion gallons, Maue said.
The exceptional nature of this event is best described by how statistically unlikely it was determined to be. 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.
This great Louisiana flood reinforces the point that weak or unnamed tropical weather systems, frequently embedded within weak steering currents, produce some of the most devastating rain events. They deserve comparable media and public attention as many hurricanes as they are frequently just as threatening and ultimately damaging.