Roads have been flooded and people forced onto rooftops as Storm Barry dumped heavy rains on Louisiana.

Barry had been deemed a Category 1 hurricane but weakened to a tropical storm as it made landfall at Intracoastal City, about 160 miles west of New Orleans.

There were fears the rainfall would test the levees and pumps that were bolstered after Hurricane Katrina devastated New Orleans in 2005.

HeraldScotland:

"This storm still has a long way to go before it leaves this state," Louisiana Gov. John Bel Edwards said on Saturday night.

"Don't let your guard down."

By early Saturday evening, New Orleans had been spared the worst effects, receiving only light showers and gusty winds.

A National Weather Service forecaster said the city may escape with only five to 10 centimetres of rain.

But officials warned that Barry could still cause disastrous flooding across a wide stretch of the Gulf Coast and drop up to 50 centimetres into Sunday across other parts of Louisiana.

HeraldScotland:

Forecasts showed the storm on a path towards Chicago that would swell the Mississippi River basin with water that must eventually flow south again.

The Coast Guard rescued a dozen people from flooded areas of Terrebonne Parish, south of New Orleans, some of them from rooftops, a spokeswoman said.

The people included a 77-year-old man who called for help because he had about 1.2 metres of water in his home.

Nearly all businesses in Morgan City, about 85 miles west of New Orleans, were shuttered with the exception of Meche's Donuts Shop.

Owner Todd Hoffpauir did a brisk business despite the pounding winds and pulsating rain.

While making doughnuts, Mr Hoffpauir said he heard an explosion and a ripping sound and later saw that the wind had peeled off layers of the roof at an adjacent apartment complex.

In some places, residents continued to build defences against rising water.

At the edge of the town of Jean Lafitte just outside New Orleans, volunteers helped several town employees sandbag a 600-foot stretch of the two-lane state highway.

The street was already lined with one-ton sandbags, and 14-kilogram bags were being used to strengthen them.

"I'm here for my family, trying to save their stuff," volunteer Vinnie Tortorich said. "My cousin's house is already under."