Early on September 12, the NHC began to monitor a tropical wave over the central tropical Atlantic for gradual development, though environmental conditions for development were assessed as only marginally favorable.[109] Even so, shower and thunderstorm activity within the disturbance began to become more concentrated later that same day,[110] then increased and became better organized during the next day.[111] The circulation associated with the system became more defined and persisted overnight and into the morning of September 14, attaining sufficient organization to designated as Tropical Depression Seven later that day.[112] Despite the continued effects of moderate westerly shear and dry mid-level air flow,[113] new satellite imagery indicated the depression had strengthened, thus at 01:45 UTC on September 15, it became Tropical Storm Fiona.[114] The storm moved over Guadeloupe as a 50 mph (85 km/h) tropical storm around 00:00 UTC on September 16 and then entered the eastern Caribbean.[115] Early on September 18, the storm strengthened into a hurricane as it approached Puerto Rico,[116] before making landfall there that afternoon about 15 mi (25 km) south-southeast of Mayaguez, with maximum sustained winds of 85 mph (140 km/h).[117] It then emerged over the Mona Passage, and strengthened slightly further before making landfall near Boca de Yuma, Dominican Republic, with maximum sustained winds of 90 mph (150 km/h).[118] Fiona weakened slightly as it moved overland, but began to rapidly strengthen once back over water, becoming a category 2 hurricane by 21:00 UTC on September 19,[119] and then a Category 3 major hurricane early the next morning near Grand Turk Island.[12] Further intensification resulted in it reaching Category 4 strength with maximum sustained winds of 130 mph (210 km/h) at 06:00 UTC on September 21,[120] while moving northward across very warm waters with surface temperatures of 84–86 °F (29–30 °C).[121][120]