HOUSTON, Sept 12 (Reuters) - Forecasters predicted heavy rains and flooding for the Houston area as Tropical Storm Humberto formed on Wednesday in the northern U.S. Gulf.

Hurricane trackers were also keeping an eye on a second storm system making its way west in the central Atlantic after being declared Tropical Depression 8 on Wednesday.

The U.S. National Hurricane Center said that a system offshore of Galveston, Texas, had become Tropical Storm Humberto.

At 1 p.m. CDT (1800 GMT), Humberto was located 70 miles (115 km) south-southwest of Galveston and moving north at 6 mph (9 kph) with maximum sustained winds of 45 mph (75 kph).

"Additional strengthening is possible prior to landfall," a hurricane center advisory said. "Rainfall amounts of 5 to 10 inches (13 to 25 cm) are expected along the middle and upper Texas Coast and in extreme southwestern Louisiana."

Humberto already was affecting the Houston Ship Channel, which serves one of the United States' busiest ports and primary oil-refining centers.

Pilots, who guide ships through the 50-mile (80-km) waterway, halted boarding ships at the channel entrance at midday on Wednesday due to rough seas.

The U.S. Coast Guard said pilots planned not to resume guiding ships up the channel until after Humberto passes.

Texas Division of Emergency Management positioned response and rescue teams to prepare for flooding in Houston and Beaumont-Port Arthur, officials said.

"These type storms get very intense quickly, and the threat right now is it's slow-moving," said Jack Colley, chief of the agency.

"The big concern is heavy rains in areas that have already been saturated with rain for some months now," he said.

Hurricane trackers warned that Tropical Depression 8, which is farther out in the Atlantic, could eventually threaten the United States.

At 11 a.m. EDT (1500 GMT), it was moving west-northwest at near 12 mph (19 kph) about 1,130 miles (1,815 km) east of the Lesser Antilles islands.

In the U.S. Gulf, forecasts called for Humberto to make landfall around midnight CDT on Wednesday (0500 GMT on Thursday) with heavy rain and tides as high as 2.5 feet (0.75 metre) above normal.

Forecasters said it appeared the storm would come ashore just west of Galveston, track northward on Thursday over Houston and then eastward across Beaumont and Port Arthur.

The area is a major oil-producing and gasoline-refining center, but industry officials said they were not expecting significant impact on operations.

"We're watching it," said Valero Energy Corp. spokesman Bill Day. "We're not expecting it to affect operations at our refineries."

Valero owns refineries in Houston, Texas City and Port Arthur, Texas.

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