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MIAMI, Sept 25 (Reuters) - A tropical depression formed on Tuesday in the southwest Gulf of Mexico, but it did not appear to pose a threat to U.S. and Mexican oil and natural gas installations.

The weather system, which computer models indicated would not grow into a hurricane with winds above 74 miles per hour (119 km per hour), was located 190 miles (305 km) east of Tampico in Mexico at around 6 p.m. EDT (2200 GMT), the U.S. National Hurricane Center said.

Most computer models used to forecast tropical storm tracks and intensities took the system into central Mexico after drifting aimlessly for a couple of days.

The depression's top sustained winds had reached 30 mph (45 kph), the hurricane center said. If the system's top sustained winds reached tropical storm strength of at least 39 mph (63 kph), it would be named Lorenzo.

That could happen on Wednesday, the Miami-based hurricane center said, adding a tropical storm watch could be required for parts of the Mexican Gulf Coast Tuesday evening. A watch means tropical storm conditions can be expected within 36 hours.

Forecasters also were watching Tropical Storm Karen, which swirled in the Atlantic Ocean about 1,430 miles (2,305 km) east of the Windward Islands of the Caribbean but was not expected to threaten land.

The 11th named storm of the 2007 Atlantic hurricane season, Karen was expected to turn to the north before reaching the Lesser Antilles.

(Additional reporting by Michael Christie in Miami)

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