Tropical Storm Cristobal is gathering strength as it moves north toward U.S. oilfields in the Gulf of Mexico and the coast of Louisiana, where flooding rains will probably bring the worst of the damage.The storm's threatened fury has been enough to shut more than than 30% of offshore gas and oil production as of Saturday afternoon.

Cristobal's winds have strengthened to 50 mph, and its center was about 310 miles southwest of the mouth of the Mississippi River, the U.S. National Hurricane Center said in an advisory at 1 p.m. EDT. Its tropical-storm strength winds reach out about 240 miles from its center with the worst of them on the eastern side.

As it moves back over water, away from Mexico's Yucatan Peninsula, the storm is expected to bulk up even more, with winds peaking at around 60 mph. Tropical storm and storm surge warnings have been issued for coastal areas from Louisiana to Florida.

"Overall, the main threat from the storm is going to be the flooding rainfall," said Rob Miller, a meteorologist with AccuWeather Inc. "It is still a few hundred miles away, but the rain shield is already spreading into Florida and along the Gulf Coast. We are already seeing heavy rain sneaking up the west coast of Florida."

On its current track, Cristobal will pass through the eastern side of U.S. offshore energy installations, where some companies have already evacuated non-essential workers. The storm will then likely slam into Louisiana some time after 6 p.m. Sunday, Miller said.

Parts of extreme southern Louisiana have ordered manadatory evacuations.

Cristobal has been stretched out by larger weather patterns, so its worst winds are actually far from its center. The public shouldn't fixate on the track, which is based on its core, Jack Beven, a senior hurricane specialist at the National Hurricane Center, wrote in an analysis.

Its disorganized shape will rob it of much of its power, keeping damages to about $200 million at the high end, said Chuck Watson, a disaster modeller with Enki Research, in Savannah, Georgia. Coastal areas, rivers will flood, trees will be toppled and power lines brought down.

Offshore platforms account for 16% of U.S. crude oil production and 2.4% of natural gas output, according to the Energy Department. Additionally, more than 45% of U.S. refining capacity and 51% of gas-processing capacity is located along the Gulf coast. A

Across the Gulf, 174 platforms have been evacuated, shutting down 33% of crude output and 31.5% of natural gas production there, according to the U.S. 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 U.S., with Houston benchmark gasoline prices moving higher as traders secure supply. Cristobal could push the average retail price of a gallon of gasoline over $2, according to AAA.

The U.S. 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 U.S. By the middle of next week it will probably be over Chicago.

From 5 to 7 inches of rain could fall across a large swath of the Mississippi Valley in the next few days, and parts of Canada could get 2 inches or more, according to the U.S. Weather Prediction Center.

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

This article was written by Brian K. Sullivan a reporter for Bloomberg News.