MEXICO CITY, Oct 23 (Reuters) - Tropical Storm Rick strengthened into a hurricane on Saturday off Mexico"s Pacific coast and is expected to gain force throughout the day, the U.S. National Hurricane Center (NHC) said. Rick was moving north-northwest at about 7 miles (11 km) per hour with maximum sustained winds near 80 miles (130 km) per hour and strong gusts. The storm was located 205 miles (335 km) south of the beach city of Zihuatanejo in Guerrero state. "Rapid strengthening is forecast during the next day or so, and Rick is expected to become a major hurricane on Sunday," the NHC said. The center also said that there was some uncertainty in the storm"s track forecast. "The arrival time of hazardous conditions could change significantly with future forecasts," it said. It added that Rick was expected to pour rain across coastal stretches of Guerrero and Michoacan states from Saturday night through Tuesday, with rainfall in some places amounting to 20 inches, likely causing flooding and mudslides. Mexican authorities warned that rains could also pummel the southern state of Oaxacan as well as several central and western regions of the country. Storm Pamela hit Mexico in mid-October as a hurricane, leaving downed trees, damaged businesses and flooded streets. read more