(CNN) A storm system strengthened into Tropical Storm Olga on Friday afternoon, with forecasters saying it will bring 2-4 inches of rain to the central Gulf Coast and into the Mississippi Valley through Saturday morning.

Olga earlier had been classified as a tropical depression.

A tropical depression has wind speeds of less than 39 mph, while a tropical storm can reach wind speeds of up to 73 mph. Olga had winds of 40 mph.

Olga will move into Louisiana late Friday as it merges with a cold front. That combination should bring rainfall. Isolated amounts over 4 inches are possible.

Newly-formed Tropical Depression 17 may briefly become a tropical storm before being absorbed by a cold front. Regardless, heavy rainfall in excess of 2 inches is likely along the Central Gulf Coast along with Gale-force winds and coastal flooding.https://t.co/VyWINDk3xP pic.twitter.com/qCZrt4YtTx — National Weather Service (@NWS) October 25, 2019

"The main threat from the system will be heavy rain," CNN senior meteorologist Dave Hennen says. "Over 8 million are under flash flood watches in the South. Cities like New Orleans, Jackson and Memphis could see flooding later today into tomorrow."

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