1991 Pacific typhoon season

Westerly bursts, a monsoon gyre and a developing El Niño

The Joint Typhoon Warning Center issued 835 warnings on 32 western North Pacific cyclones in 1991, 41 more than its record the year before. It classed them as 5 super typhoons, 15 other typhoons, 10 tropical storms and 2 depressions, and for 18 days it tracked three storms at once. Other season lists count 30 tropical storms and 17 typhoons, mixing JTWC and Japan Meteorological Agency ten-minute winds. The site's 28 tropical storms and 19 typhoons come from US-agency one-minute winds during each storm's tropical stage within the calendar year, and its accumulated cyclone energy of 403.2 reflects the long run of intense storms.

JTWC tied the year's rhythm to shifts in the tropical circulation. In March, as the Southern Oscillation Index turned negative, bursts of equatorial westerlies east of New Guinea spun up paired storms north and south of the equator, including Sharon and Tim, and a strong burst in early May produced Typhoon Walt alongside a Southern Hemisphere twin. In August a huge monsoon gyre replaced the usual trough and spawned six systems in three weeks. From October a moderate El Niño took shape in the central Pacific and pushed formation eastward.

Typhoon Walt, then Yunya over an erupting volcano

Tropical Storm Sharon crossed the central Philippines in early March, and Typhoon Tim became the year's first typhoon on 23 March. Typhoon Walt formed below 5°N in early May and reached 140 kt and 915 hPa in the archive on 12 May. It developed an unusually symmetrical ring of storms around its eye, went through a four-hour eyewall replacement that left an eye about 65 km wide, and stayed at sea until it merged with a midlatitude low northeast of Japan.

Typhoon Yunya, a midget storm, peaked at 105 kt in the archive on 14 June and weakened before reaching central Luzon on 15 June, the day Mount Pinatubo erupted. JTWC recorded one death from Yunya's own floods. Its circulation dragged the volcano's ash cloud back over land, and the rain-soaked ash that fell on Clark Air Base and the Subic Bay naval complex brought down power lines and collapsed most flat-roofed buildings.

July straight-runners into China and lahars on Luzon

From early July JTWC stayed in almost continuous warning until December. Typhoon Zeke, Typhoon Amy and Typhoon Brendan ran west-northwest in succession. Zeke made three landfalls and, by later counts, killed at least 23 people on Hainan. Amy peaked at 125 kt in the archive; the freighter Blue River sank near the Pescadores with all 31 aboard, and JTWC reported 99 people killed, at least 5,000 injured and over 15,000 homes destroyed when Amy came ashore in southeastern China on 19 July.

Brendan's rain loosed lahars up to 15 feet high in the valleys around Pinatubo. Typhoon Caitlin, passing far to the east later in July, strengthened the monsoon over Luzon; JTWC recorded 16 deaths and more than 20,000 evacuated as mudflows ran again. Caitlin then relieved Okinawa's drought, with one death there, and press reports cited by JTWC gave 2 deaths and over $4 million in damage in Korea.

August's gyre storms: Fred and Gladys

Typhoon Fred formed outside the gyre, east of the Philippines, and crossed the South China Sea to Hainan and northern Vietnam. The barge DB29 sank south of Hong Kong with 195 aboard, and 22 of them died; the storm also killed 16 people on Hainan and 5 in Vietnam.

Typhoon Gladys, the largest of the gyre's storms, reached only 65 kt in the archive but grew a vast wind field as it tracked south of Korea. South Korea's Disaster Relief Center reported at least 90 people killed or missing and 40,000 homeless, with property losses near $45 million. Pusan took 610 mm of rain in 20 hours, and in central Japan landslides from rain of up to 710 mm killed 10 people west of Tokyo.

Mireille and a September siege of Japan

The monsoon trough returned in September and sent a series of storms toward Japan. Typhoon Kinna crossed Okinawa directly, the first typhoon to do so since 1986, then passed over Sasebo and Nagasaki; later totals give nine deaths across Japan and Okinawa. Tropical Storm Luke's rain killed at least 8 people with 10 missing along the Pacific coast of Japan.

Typhoon Mireille began as a midget storm that passed 20 km north of Saipan, damaging 70 to 80 percent of the island's crops. It then grew and peaked at 130 kt on 22 and 23 September, recurved southwest of Okinawa, and cut across western Kyushu and Honshu before turning into a powerful extratropical storm over the Sea of Japan. JTWC called it the worst storm to strike Japan in three decades.

Japanese press reports cited by JTWC linked 52 deaths to Mireille, including all ten crew of a South Korean freighter that capsized in port at Hakata, along with 777 injuries and power cuts to nearly 6 million homes. Crop losses reached an estimated $3 billion, with apples hit hardest. Mireille stood as the costliest typhoon on record until 2023. JTWC retired the name.

Typhoon Nat ran for 17 days alongside Mireille, changing course four times and making two landfalls, in southern Taiwan and southeastern China. JTWC wrote 61 warnings on it, 18 short of the record. Typhoon Orchid then stalled about 370 km off Japan in October as it interacted with Typhoon Pat, flooding Tokyo, and surf from the pair killed two people on Guam.

Tropical Storm Thelma and the Ormoc disaster

Typhoon Ruth reached 145 kt and 895 hPa on 24 October, the archive's lowest pressure of the year, which it shares with Yuri. JTWC's model guidance forecast recurvature two days too early, and Ruth struck northern Luzon on 27 October well below its peak. It killed 12 people and left at least 76,000 homeless there, and the freighter Tung Lung later sank west of Taiwan with all 18 aboard.

Tropical Storm Thelma never exceeded 45 kt in the archive, yet it caused the year's greatest loss of life in the basin. It crossed the Visayas in early November and dropped about 150 mm of rain in 24 hours. News accounts cited by JTWC put the toll near 6,000 dead and 20,000 homeless from landslides, flash floods and a dam failure. The worst losses came at Ormoc on Leyte, below hills that logging had stripped bare. JTWC retired Thelma, and the Philippine agency retired its local name, Uring.

A crowded November and Typhoon Yuri near Guam

JTWC counted six storms of at least typhoon strength in November, the busiest November since 1964. Typhoon Seth reached 130 kt in the archive before weakening to a tropical storm over the northern Philippines. Tropical Storm Wilda, another midget, passed about 75 km south of Manila on 17 November, two weeks after Thelma, and caused little damage there.

Typhoon Yuri strengthened steadily east of Pohnpei, where it caused about $3 million in damage and toppled the island's only AM radio tower. It reached 150 kt in the archive on 27 November, the season's highest wind, as it passed south of Guam. JTWC called it the most intense cyclone to pass within 110 km of Guam since 1962. Waves above 12 m hit the southeast coast at high tide, and damage on Guam came to about $33 million. Yuri then grew to more than 1,600 km across.