2009 Pacific typhoon season

A below-normal count with a violent finish

By the numbers, 2009 sat a little under the long-term norm. Its 23 tropical storms, 15 typhoons and accumulated cyclone energy (ACE) of 274.5 compare with 1965 to 2008 averages of about 26.6 tropical storms, 16.7 typhoons and an ACE near 300 quoted by the Tropical Storm Risk consortium. The Joint Typhoon Warning Center (JTWC) likewise described the year as a continuation of below-normal activity, with 28 cyclones against a long-term average of 31.

What the season lacked in numbers it made up for in intensity and impact. Seven storms reached major typhoon strength, and five of those came between mid-September and late November. Seasonal forecasters had expected more: Tropical Storm Risk raised its outlook in May after La Niña conditions faded and it grew confident they would not return, and by July and August it was calling for activity about 20 percent above normal. The count never caught up with those forecasts, but the late-season storms were both intense and deadly.

How the season unfolded

Only weak depressions formed over the first four months, and the archive's first tropical storm observation falls on May 2. Typhoon Kujira developed near the Bicol region of the Philippines and became the first typhoon on May 4, reaching 115 knots later that day before turning into an extratropical low. Typhoon Chan-Hom followed from the South China Sea, tracking east to cross Luzon through Pangasinan on May 7. June brought Typhoon Linfa toward Taiwan and southern China and Tropical Storm Nangka across the central Philippines.

July stayed modest, with the weak Tropical Storm Soudelor and Typhoon Molave both heading for southern China. The pace picked up in August as Tropical Storm Goni, Typhoon Morakot and Tropical Storm Etau formed within about a week of each other. Typhoon Vamco then grew to 120 knots over the open ocean on August 21 and later crossed the International Date Line as an extratropical low, while Typhoon Krovanh closed out the month.

September was the busiest month, with seven systems in JTWC's monthly tally. Typhoon Choi-Wan and Typhoon Koppu formed in mid-September, and between September 25 and 29 Typhoons Ketsana, Parma and Melor all developed. October added Tropical Storm Nepartak, Typhoon Lupit and Typhoon Mirinae, and November brought an unnamed tropical storm and Typhoon Nida. A final unnamed tropical storm in early December ended the season, with the last tropical observation on December 5. Typhoon Parma was the longest-lived storm, its archived track covering about 426 hours.

Four Category 5 typhoons in ten weeks

All four Category 5 storms formed in the second half of the season. Typhoon Choi-Wan peaked at 140 knots and 918 hPa on September 15, the same day it crossed the Northern Mariana Islands. Typhoon Melor reached 150 knots and 911 hPa on October 4 southeast of Okinawa. Typhoon Lupit reached 140 knots and 918 hPa on October 19 in the Philippine Sea, and Typhoon Nida topped them all on November 25 with 155 knots and 907 hPa, the season's highest winds and lowest pressure.

Typhoon Parma came close, peaking at 135 knots and 922 hPa on October 1, just below the Category 5 threshold. Earlier in the year, Typhoon Kujira in May and Typhoon Vamco in August had already reached Category 4. Nida formed southeast of Guam and intensified rapidly, then sat nearly stationary for more than two days before northeast monsoon wind shear broke it down in early December.

Floods in Taiwan and Luzon

Typhoon Morakot was only a Category 1 storm at its 80-knot peak, but its passage in early August brought extreme rain to southern Taiwan. Rainfall in Pingtung County was estimated at up to 2,500 mm, breaking Taiwan's single-storm records, and nearly all of southern Taiwan flooded. Taiwan counted 461 deaths and 192 missing. JTWC called Morakot the most destructive cyclone of the year, with more than 650 deaths reported, mostly from landslides, and heavy damage in Taiwan and the Chinese provinces of Fujian, Zhejiang and Jiangsu. Total damage is listed at US$6.2 billion (2009 dollars).

Typhoon Ketsana brought Metro Manila its heaviest 24-hour rainfall on record on September 26. The PAGASA Science Garden in Quezon City measured 455 mm in 24 hours, including 341 mm in six hours, and 25 provinces plus Metro Manila were placed under a state of calamity. The storm then went on to Indochina. Ketsana is blamed for 921 total deaths across the Philippines and Indochina, including precursor and extratropical stages, and US$1.15 billion in damage (2009 dollars).

Only days later, Typhoon Parma struck northern Cagayan on October 3. Caught in weak steering between a mid-latitude low, the subtropical ridge and Typhoon Melor, it criss-crossed northern Luzon three times over the next week. It then crossed Hainan on October 12 and came ashore in northern Vietnam on October 14. At least 500 people were killed, and damage totaled US$617 million in damage (2009 dollars).

Several other storms caused deadly damage. Typhoon Melor came ashore in Japan on October 7 as a minimal typhoon and was the only 2009 system to make landfall near US military installations. Melor caused three deaths and about US$1.5 billion in damage in Japan (2009 dollars). Tropical Storm Etau never made landfall, but its outer bands dropped up to 326.5 mm of rain on Japan and caused floods and mudslides, especially in Hyōgo Prefecture, killing 28 people. Typhoon Mirinae crossed Luzon at the end of October and later hit Vietnam, with 162 deaths across the two countries. In May, Typhoon Kujira left 28 dead in the Bicol region by the Philippine disaster council's count of May 12. Typhoon Choi-Wan destroyed almost every structure on Alamagan in the Northern Marianas, although no casualties were reported.

Retired names, counting rules and forecasting lessons

The ESCAP/WMO Typhoon Committee retired Morakot, Ketsana and Parma, replacing them with Atsani, Champi and In-fa. PAGASA retired its local names Ondoy and Pepeng, used for Ketsana and Parma, after each storm caused more than 300 deaths and over 1 billion pesos of damage in the Philippines. The names Molave, Mujigae and Mirinae were used for the first time in 2009.

Other agencies count the season differently. The Japan Meteorological Agency, which uses ten-minute winds and names only the storms it tracks at tropical storm strength, recorded 22 tropical storms and 13 typhoons. It named Mujigae, which never exceeds 30 knots in US-agency one-minute data, but did not name the November tropical storm or the December one, both of which reach at least 34 knots on the one-minute scale. JTWC's annual report lists 28 numbered cyclones, with 15 typhoons and 22 tropical storms or stronger based on its warnings. It kept the December system at 30 knots after a single warning, while the archived best track gives it 35 knots. JTWC also counts five super typhoons because its threshold of about 130 knots includes Parma, which falls below Category 5.

Two storms stood out to JTWC forecasters. Typhoon Lupit made an S-turn in the Philippine Sea and then turned sharply northeast near northern Luzon, producing large four- and five-day forecast errors and forcing six US Navy ships to divert. Parma's weak steering near Luzon, and its uncertain interaction with Melor, made it the other major challenge. The season also marked the end of an era for satellite wind data: the antenna motor on NASA's QuikSCAT failed on November 23, 2009, cutting off a long record of ocean surface wind measurements.