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Wetter weather sets up across the South
01:54 - Source: CNN
CNN  — 

Significant flash flooding is possible over the Gulf Coast today through Thursday as heavy rain is forecast to drench the area.

“Rainfall totals of 6 to 8 inches are possible,” according to the NWS Weather Prediction Center (WPC).

Five million people across parts of southern Louisiana, Mississippi and Alabama are under flash flood watches until Thursday.

Life-threatening floods could occur from Louisiana across southern Mississippi to lower Alabama over the next 48 hours. Rainfall rates approaching 1.5 inches per hour have already been falling with some of the storms.

A level 2 of 4 risk of excessive rainfall is forecast by the WPC, along portions of the central Gulf Coast today and Wednesday.

forecast rain accum gulf coast

The main ingredient for the heavy rain potential is the Gulf of Mexico moisture. The old saying “It’s not the heat; it’s the humidity” applies here. The relative humidity is usually just shown as a number on a daily weather forecast. When the number is 100%, you know the air is muggy – there might even be fog.

That number you see is a measurement only at the Earth’s surface, but when the relative humidity is 100% all the way to 40,000 feet above the ground – which will occur along the Gulf Coast today through Thursday – significant flooding potential increases from the intense rainfall from that available moisture.

This very humid air pattern will stay in place for the next 48 hours in Louisiana, Mississippi and Alabama.

Urban and small stream flooding will be significant with the stronger storms. Even larger rivers could see major flooding.

Given the recent heavy rainfall that has already happened, some rivers are in flood stage now and may continue to rise, said Michael Mugrage from the NWS in Mobile.

Along with heavy rain, storms could produce severe weather, particularly later in the week when a potential outbreak of severe storms, including strong tornadoes, exists Thursday.