“Total rainfall of 6 to 8 inches is possible”, according to the NWS Weather Prediction Center (WPC).
Five million people in parts of southern Louisiana, Mississippi and Alabama are under flood surveillance as of Thursday.
Life-threatening floods can occur from Louisiana, through southern Mississippi, to lower Alabama, in the next 48 hours. Rain rates approaching 1.5 inches per hour have already dropped with some of the storms.
The main ingredient for the potential for heavy rain is the humidity of the Gulf of Mexico. The old adage “It is not heat; it is humidity” applies here. Relative humidity is usually shown only as a number in a daily weather forecast. When the number is 100%, you know the air is stifling – there may even be fog.
That number you see is a measurement only on the Earth’s surface, but when the relative humidity is 100% up to 40,000 feet above the ground – which will occur along the Gulf Coast today through Thursday – the potential for significant flooding increases from the intense rains with the available humidity.
This very humid air pattern will remain in place for the next 48 hours in Louisiana, Mississippi and Alabama.
Urban and small stream flooding will be significant with the strongest storms. Even larger rivers can experience major flooding.
Given the recent heavy rains that have already happened, some rivers are now inundated and may continue to rise, said Michael Mugrage of the NWS in Mobile.