2011 Pacific typhoon season

A quieter basin with erratic tracks

By the archive's count, 2011 fell well short of a typical western Pacific year. The Joint Typhoon Warning Center's long-term averages for 1959 to 2011 are close to 27 tropical storms and 17 typhoons a year, against 18 and 10 this season, and the season's accumulated cyclone energy of 188.5 sat far below the average of about 295 used by the Tropical Storm Risk consortium, whose forecasts had called for a near-average year. The warning center described 2011 as a continuation of below-average activity that had begun in 2005, with 27 cyclones warned on compared with a long-term average of 31.

La Niña conditions shaped where the storms formed. The warning center noted that the formation region was displaced to the north and west, a pattern common in La Niña years, and that several of the early and mid-season systems followed S-shaped, looping or otherwise erratic tracks, with repeated passages near or over Okinawa.

How the season unfolded

The year opened with two short-lived tropical depressions in early April, one off southern Vietnam and one southwest of Yap, and neither reached tropical storm strength. Tropical Storm Aere became the first named storm in early May, crossing the eastern Philippines as Bebeng, and Typhoon Songda followed later that month, curving past the Philippines and Okinawa toward Japan. June brought weaker systems: Tropical Storm Haima moved through southern China into northern Vietnam, and Tropical Storm Meari ran north past Okinawa into the Yellow Sea and toward the Korean Peninsula.

Activity picked up from mid-July. Typhoon Ma-On formed near Minamitorishima, absorbed the weaker Tokage and wandered for close to three weeks before crossing western Japan, making it the longest-tracked storm in the archive at 486 recorded hours. Typhoon Nock-Ten crossed northern Luzon, Hainan and northern Vietnam in late July, while Typhoon Muifa began a long, looping path that lasted into August. Typhoon Merbok stayed over open water east of Japan, and late August brought Nanmadol and the slow-moving Talas at the same time.

September was the season's busiest month, with eight tropical storms active at some point: Talas, Noru, Kulap, Roke, Sonca, Nesat, Haitang and the late-month Nalgae all developed or persisted in a crowded few weeks. After Banyan crossed the central Philippines in mid-October, strong vertical wind shear and a vigorous northeast monsoon kept the basin free of tropical storms for two months, apart from a few weak depressions, until Washi formed in mid-December. The archive's last tropical-stage observation is on 19 December.

Three Category 5 typhoons

Songda, Muifa and Nanmadol each reached one-minute winds of 140 kt, Category 5, with a minimum pressure of 918 hPa in the archive. Songda was the first of them and is the season's listed strongest storm, peaking east of the Philippines on 26 May after rapid intensification; it was also the season's first typhoon. Muifa reached the same peak on 30 July over the Philippine Sea before an upper-level trough weakened it, and Nanmadol followed on 26 August just before it reached northern Luzon.

Nanmadol's strengthening was exceptional even by these standards. The warning center found that its intensification exceeded four Dvorak T-numbers in two and a half days, nearly double the standard rate, helped by low wind shear, strong outflow enhanced by an upper-level low to its northeast and very warm ocean water. The center raised Nanmadol to super typhoon status in its post-season analysis.

Below the three leaders, Nalgae peaked at 130 kt and 926 hPa on 1 October, just short of Category 5, while Ma-On, Roke and Nesat each reached 115 kt, Category 4. Sonca peaked at 90 kt over open water, and Merbok and Nock-Ten were minimal typhoons.

Landfalls and impacts

Songda stayed offshore but still caused serious damage. Its outer rainbands caused flash floods and landslides that killed four people in the northern Philippines. On Okinawa, which the center passed about 40 nautical miles to the north-northwest on 28 May, Kadena Air Base recorded sustained winds of 74 kt with gusts to 95 kt, and losses on the island were estimated at 23.2 billion yen. As Songda turned extratropical it brought flooding rain to mainland Japan, where 13 deaths were reported and about 400,000 people were evacuated around Tokyo.

Nanmadol struck northeastern Luzon, crossed the southern tip of Taiwan and faded in the Taiwan Strait. The warning center cites at least 35 deaths and 34.5 million US dollars of damage in the Philippines, at least one death and 500 million dollars in Taiwan, and two deaths and 48.5 million dollars in China, all in 2011 dollars. Muifa enhanced monsoon rain over Luzon and then turned toward Okinawa before, much weakened, it came ashore at the Yalu River estuary on 8 August.

Japan took two damaging blows in September. Tropical Storm Talas moved slowly and came ashore on Shikoku on 3 September; Wikipedia's season table lists 82 deaths and about 600 million US dollars of damage. Typhoon Roke followed on 21 September, making landfall near Hamamatsu after explosive intensification, with 13 deaths and about 1.7 billion dollars of damage listed in the same table, in 2011 dollars.

Luzon was hit twice within a week. Typhoon Nesat came ashore early on 27 September, driving storm surge over seawalls in Manila and flooding neighborhoods and a Manila hospital before going on to strike Hainan and northern Vietnam; Wikipedia's table lists 98 deaths and about 2.1 billion US dollars of damage across the affected countries. The stronger Nalgae crossed northern Luzon on 1 October, destroying nearly 2,900 homes, killing at least 18 people and causing about 115 million pesos of losses in the Philippines.

Tropical Storm Washi was only a moderate storm, peaking at 50 kt, but it was by far the deadliest of the season. It crossed Mindanao on 16 December, and overnight flash floods after about ten hours of heavy rain swept through Iligan and Cagayan de Oro. Wikipedia's season table lists 2,546 deaths, and more than 9,400 houses were destroyed.

Forecasting lessons and retired names

The warning center singled out two storms for review. Songda proved highly predictable as it curved around a persistent subtropical ridge, and the center's track errors for it were 50 to 67 percent below the season average at three to five days. Even so, the storm destroyed the Doppler weather radar at Kadena Air Base. Nanmadol was the opposite: most models wrongly expected it to interact with Talas and turn northeast, so it moved farther west than forecast and produced the center's largest track errors for any storm that year. The center also changed its formation outlook from poor, fair and good to low, medium and high on 1 June.

Washi's death toll led the Typhoon Committee to retire the name, replacing it with Hato. The Philippine weather agency retired Bebeng, Juaning, Mina, Pedring and Sendong, its local names for Aere, Nock-Ten, Nanmadol, Nesat and Washi, and also dropped Nonoy because it sounded like the nickname of the sitting president.

Why the counts differ

This page counts storms with the US agency's one-minute winds, which gives 18 tropical storms and 10 typhoons, the same totals the Joint Typhoon Warning Center reported. The Japan Meteorological Agency, which uses ten-minute winds, named 21 tropical storms but ranked only 8 as typhoons. The three extra names, Sarika, Tokage and Banyan, reached tropical storm strength on the Japanese scale but peaked at 30 kt in the US record. Ten-minute averages run lower than one-minute peaks, so fewer systems reach typhoon strength on the Japanese scale.

The warning center also counted four super typhoons, a category that starts at 130 kt, so it includes Nalgae alongside the three Category 5 storms. On the Saffir-Simpson scale used here, Nalgae ranks as Category 4.