(CNN)The first tropical system to slam the US this year lashed parts of the Deep South with heavy rains Thursday morning as it spins through the Gulf of Mexico.

More than 4 million people are under flash flood watches from south-central Louisiana to the western Florida panhandle as the system moves over the warm waters, CNN meteorologist Haley Brink said.

If it reaches winds of 39 mph, it will officially become Tropical Storm Barry and could intensify into a Category 1 hurricane by late Friday. The storm is expected to make landfall along the Louisiana or upper Texas coast on Saturday.

But even before it reaches tropical storm status, it'll lash the Gulf Coast region with heavy rains, leading to flash flooding that could cause the Mississippi River to rise to dangerous levels.