TALLAHASSEE, Fla. — Tropical Storm Colin was gaining speed and strength Monday on its way to strike Florida, threatening rains forecasters said could cause some serious flooding along much of the state’s Gulf coast.

A large portion of Florida’s western and Panhandle coast was already under a tropical storm warning when the National Hurricane Center announced that a quickly moving depression had become a named storm. The center said it is the earliest that a third named storm has ever formed in the Atlantic basin.

Colin’s maximum sustained winds Monday morning had increased to near 50 mph (85 kph) with some slow strengthening possible during the next two days. The storm was centered about 360 miles (580 kilometers) west-southwest of Tampa and moving north-northeast near 14 mph (22 kph).