* Arlene expected to drench much of central Mexico
* Storm path over Tamaulipas, oil refineries
* Likelihood of becoming hurricane decreasing
MEXICO CITY, June 30 (Reuters) - Large parts of central Mexico were facing torrential rains as Arlene, the first tropical storm of the Atlantic hurricane season, neared the coast early on Thursday.
Mexican emergency services advised communities in the storm's direct path evacuate and state oil monopoly Pemex [PEMEX.UL] was on alert for possible impact to refineries and other facilities in the storm's path.
However, the storm is seen unlikely to strengthen into a hurricane before making landfall and is not expected to hit major offshore oil fields directly, according to forecasts from the U.S. National Hurricane Center. But the agency said some strengthening could still occur.
Mexican authorities said the storm was expected to be 435 miles (700 km) wide and drench parts of central Mexico by Saturday, affecting areas as far away as the Pacific coast.
Arlene, moving west, had maximum sustained winds of 60 mph (95 kph) and was located 50 miles (80 km) east of the fishing town of Tuxpan in Veracruz state on Thursday morning, the NHC said.
Links to computer models:
Strong winds and rain of 4 to 8 inches (10 to 20 cm) are expected to batter the eastern Mexican states of Tamaulipas, Veracruz and eastern San Luis Potosi and reach Mexico City by Friday.
"These rains could cause life-threatening flash floods and mudslides," the center said. Local emergency services said about 200,000 people were exposed to the storm's worst impact.
"Everyone in an area which is at risk is urged to seek safe ground," Laura Gurza, a rescue official with the Interior Ministry, told a news conference on Wednesday.
Mexico's government extended its Gulf Coast hurricane watch from La Cruz southward to the beach area Barra de Nautla in Veracruz, which is about 200 miles (320 km) south of Tampico, where Pemex has its 190,000-barrel-per-day Madero refinery.