Tropical Storm Bertha strengthened in the Gulf of Mexico on Tuesday but was expected to weaken this week as it chugs closer to the US, potentially bringing coastal flooding and heavy rain from the Florida Panhandle to Louisiana.

Those areas along the northern Gulf coast were under a tropical storm warning through Wednesday and could see rain and flooding into Thursday morning, the Miami-based National Hurricane Center said. New Orleans also was under a tropical storm warning.

The storm was about 120 miles (190km) south-west of Panama City, Florida, and 105 miles (170km) south-east of Mobile, Alabama, the Miami-based National Hurricane Center (NHC) said on Tuesday evening. It was moving at 6 mph (9 km/h) to the west with maximum sustained winds near 60 mph (95 km/h), the hurricane center said. Gradual weakening was forecast Wednesday through Friday.

Bertha was forecast to make landfall Wednesday night somewhere in the coastal and lightly populated part of Louisiana south-east of New Orleans, said Michael Buchanan, a meteorologist with the National Weather Service in Louisiana.

Wind will be the biggest threat there, he said.

Bertha is the second tropical storm of the Atlantic hurricane season after Tropical Storm Arthur brought heavy rainfall to the south-eastern US in June.

The storm was expected to stay near or along the northern Gulf coast, dropping heavy rain on coastal areas of Florida, Alabama, Mississippi and Louisiana while pushing west toward Texas.

Potentially life-threatening storm surges of up to 4ft (1.2 meters) are possible in portions of Louisiana, Alabama and Florida, the NHC said, warning Bertha could spin off an isolated tornado or two over the Florida Big Bend region.

Areas of flash flooding also were possible through Friday along the coast from western Florida to middle Texas, forecasters said, adding that rainfall could total 3in to 5in (7cm to 12cm) along the Louisiana coastline, with higher amounts in some scattered areas.

Red flags on the beach at Orange Beach, Alabama, cautioned swimmers about high surf and currents. In Escambia county, Florida, emergency officials offered sand for people living in flood-prone areas to use for sandbags.

Meanwhile, in the eastern Pacific Ocean, Hurricane Fausto churned far from Mexico’s Baja California peninsula and posed no threat to land, the NHC said.

Fausto gained hurricane status on Monday night. It was about 780 miles (1,260 km) south-west of the southern tip of the Baja California peninsula, had top sustained winds of 75mph (120 km/h) and was moving north-west at 12mph (19 km/h).

The storm was expected to strengthen on Tuesday and Wednesday. No coastal watches or warnings were in effect, though ocean swells generated by the storm could cause life-threatening surf and rip tide conditions on the Baja California coast, the NHC said.

Guardian staff contributed reporting