NEW YORK (BLOOMBERG) - Tropical Storm Cristobal will sweep ashore late on Sunday (June 7) in southern Louisiana, where more than more than 30 per cent of offshore energy production has halted, but the rains have already reached far inland across the US South.

With winds holding steady for the last day at 80 kilometres per hour, the South is bracing for wind, a storm surge and flooding rains as Cristobal comes ashore and eventually moves up the Mississippi River Valley to the Midwest and Canada, according to the National Hurricane Centre.

Stretched out by larger weather patterns, Cristobal's core doesn't have the fury of a better-organised storm and some of its worst impact could come far from the centre of its forecast track.

"The impacts to the east and north-east of its centre are really extending hundreds of miles away," said Mr Dan Pydnynowski, a meteorologist with AccuWeather Inc.

"Rain is definitely the main threat here. There is some wind to deal with, minimal surge, but by far the rain is going to be the main impact."

On its current track, Cristobal will pass through the eastern side of US offshore energy installations, where some companies have already evacuated non-essential workers.

The storm will then likely slam into Louisiana some time after 6pm Sunday, Mr Pydynowski said.

Parts of extreme southern Louisiana have ordered mandatory evacuations.

Its disorganised shape will rob it of much of its power, keeping damages to about US$200 million (S$279 million) at the high end, said Mr Chuck Watson, a disaster modeller with Enki Research in Savannah, Georgia.

Coastal areas and rivers will flood, trees will be toppled and power lines brought down.

"Once it moves inland, the winds will die down quickly," Mr Pydynowski said. "Later tomorrow when it gets to Arkansas and eventually Missouri, it will accelerate northward and then northeastward."

Offshore platforms account for 16 per cent of U.S. crude oil production and 2.4 per cent of natural gas output, according to the Energy Department.

Additionally, more than 45 per cent of US refining capacity and 51 per cent of gas-processing capacity is located along the Gulf coast.

Across the Gulf, 174 platforms have been evacuated, shutting down 33 per cent of crude output and 31.5 per cent of natural gas production there, according to the US Bureau of Safety and Environmental Enforcement.

The storm's trip through Mexico's Bay of Campeche in the southern Gulf caused national energy company Petroleos Mexicanos, or Pemex, to suspend some work for at least 10 days, according to the news site Reforma.

Energy prices are reacting in the US, with Houston benchmark gasoline prices moving higher as traders secure supply. Cristobal could push the average retail price of a gallon of gasoline over US$2, according to AAA.

RAIN SPREADS NORTH

The US crude complex has also come under pressure, with the storm potentially affecting logistics in the Gulf and causing a backup of supplies, according to market participants.

After moving north through Louisiana, Cristobal will transition to a more typical continental storm and drag a tail of heavy rain through the central US. By the middle of next week, it will probably be over Chicago.

From eight to 13 centimetres of rain could fall across a large swath of the Mississippi Valley in the next few days, and parts of Canada could get 5cm or more, according to the US Weather Prediction Centre.

As it moves north, the winds will die down and it will pick up speed, eventually merging with another storm system coming out of the west, Mr Pydynowski said. The two will then move north-east out of the Midwest and could become a potent system as it crosses Lake Superior, but it won't be a tropical storm by then.

Cristobal is the third storm to form in the Atlantic this year, marking the fastest start to hurricane season on record.