Tropical Storm Alex, the first named storm of the hurricane season, formed in the Atlantic on Sunday morning on a track to come ashore in southern Florida with heavy rain and gusty winds.

National Hurricane Centre forecasters said in an advisory at 5am local time that Alex had sustained winds of 50mph (80kph) and was located about 270 miles (435km) north-east of Fort Pierce, Florida.

Parts of South Florida were experiencing flooding from heavy rain and wind on Saturday. Officials in Miami warned drivers about road conditions as many cars were stuck in flooded streets.

Tropical Weather Florida
Young boys paddle an inflatable kayak in a flooded Miami street (Marta Lavandier/AP)

“This is a dangerous and life-threatening situation. Travelling during these conditions is not recommended. It’s better to wait. Turn around, don’t drown,” the city of Miami tweeted.

The city was towing stranded vehicles from flooded roads.

Miami Beach Mayor Dan Gelber said the storm tested the system of drainage pumps the city has recently installed as climate change has increasingly made flooding an issue in the low-lying area.

“We moved the water off pretty quickly, but in some areas, obviously, it was really challenging,” he said.

“There were some problems getting through on some streets – one of the main arteries was unpassable – but by and large water is dissipating.”

Tropical Weather
An elderly man uses a stick in an attempt to unblock a drain in the Little Havana neighbourhood of Miami (Sam Navarro/Miami Herald/AP)

Alex is a new version of the storm that was called Hurricane Agatha when it slammed into Mexico’s Pacific Coast earlier this week, killing at least nine people and leaving 20 missing. It got a new name once the storm crossed Mexico into the Atlantic basin.

A tropical storm warning was in effect for Bermuda.

In Cuba, the storm killed three people, damaged dozens of homes in Havana and cut off electricity in some areas, according to authorities. Heavy rain continued on Saturday, but was diminishing as the weather system moved away from the island.

Miami-Dade County Mayor Daniella Levine Cava said most government services, such as bus routes and trains, planned to operate as normal over the weekend. Canal levels in South Florida have been lowered to minimise flooding from heavy rain.

The Atlantic hurricane season officially began on Tuesday. This is an unusually early start to the storm season but not unprecedented for Florida.

The National Hurricane Centre predicted up to 10in (25cm) of rain is possible in South Florida, including the Florida Keys.

The storm is not expected to produce huge winds or a major storm surge nut local flooding is likely.