Typhoon Rai strengthened to the equivalent of a Category 4 hurricane on the Saffir-Simpson Scale over the South China Sea on Saturday morning. At 10:00 a.m. EST on Saturday the center of Typhoon Rai was located at latitude 12.2°N and longitude 113.0°E which put it about 450 miles (725 km) southeast of Da Nang, Vietnam. Rai was moving toward the west-northwest at 17 m.p.h. (28 km/h). The maximum sustained wind speed was 130 m.p.h. (210 km/h) and there were wind gusts to 160 m.p.h. (260 km/h). The minimum surface pressure was 942 mb.

Typhoon Rai strengthened to the equivalent of a Category 4 hurricane as it moved over the warm water in the South China Sea on Saturday morning. An eye with a diameter of 30 miles (50 km) was at the center of Rai’s circulation. The eye was surrounded by a ring of strong thunderstorms and the strongest winds were occurring in that ring of storms. Bands of showers and thunderstorms were revolving around the core of Typhoon Rai. Storms near the core generated upper level divergence that pumped mass away to the west and north of the typhoon.

Winds to typhoon force extended out 35 miles (55 km) from the center of Typhoon Rai. Winds to tropical storm force extended out 170 miles (275 km) from the center of circulation. The Hurricane Intensity Index (HII) for Typhoon Rai was 25.1. The Hurricane Size Index (HSI) was 14.3 and the Hurricane Wind intensity Size Index (HWISI) was 39.4. Rai was capable of causing regional severe damage.

Typhoon Rai will move through an environment that will become unfavorable for intensification during the next 48 hours. Rai will move over water where the Sea Surface Temperatures are near 28˚C. It will move under the southern side of an upper level ridge over the Western North Pacific Ocean. The ridge will produce southeasterly winds that will blow toward the top of Typhoon Rai’s circulation. A surface high pressure system over Asia will produce northeasterly winds in the lower levels over the atmosphere. A combination of northeasterly winds in the lower levels and southeasterly winds in the upper levels will cause the vertical wind shear to increase. The northeasterly winds in the lower levels will also transport drier air toward the northwestern part of Rai’s circulation. Stronger vertical wind shear and drier air will cause Typhoon Rai to weaken during the weekend.

Typhoon Rai will move around the western end of a surface high pressure system over the Western North Pacific Ocean during the next 36 hours. The high pressure system will steer Rai toward the north during the next 36 hours. On its anticipated track Typhoon Rai is forecast to turn toward the north before the center of Rai reaches the coast of Vietnam. Rainbands on the western side of Typhoon Rai could bring gusty winds and locally heavy rain to the central coast of Vietnam in 24 hours.