2011 Pacific Typhoon Season

The 2011 Pacific typhoon season was a below average season that produced a total of 21 named storms, 8 typhoons, and four super typhoons. This season was much more active than the previous season, although both seasons were below the Pacific typhoon average of 26. The season ran throughout 2011, though most tropical cyclone tend to develop between May and October. The season's first named storm, Aere, developed on May 7 while the season's last named storm, Washi dissipated on December 19.

The season was also much deadlier and destructive than the previous season. Typhoon Muifa affected many countries during August. Tropical Storm Talas and Typhoon Roke made landfall over in Japan and were the most destructive since 2009. Typhoon Nesat was the most powerful to strike China since 2005. Tropical Storm Washi, a late but weak cyclone, affected southern Philippines and killed 2546 people.

The scope of this article is limited to the Pacific Ocean to the north of the equator between 100th meridian east and the 180th meridian. Within the Northwestern Pacific Ocean, there are two separate agencies who assign names to tropical cyclones which can often result in a cyclone having two names. The Japan Meteorological Agency will name a tropical cyclone should it be judged to have 10-minute sustained wind speeds of at least anywhere in the basin. Whilst the Philippine Atmospheric, Geophysical and Astronomical Services Administration assigns names to tropical cyclones which move into or form as a tropical depression in their area of responsibility located between 135°E and 115°E and between 5°N-25°N even if the cyclone has had a name assigned to it by the Japan Meteorological Agency. Tropical depressions that are monitored by the United States' Joint Typhoon Warning Center are given a number with a "W" suffix.

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Seasonal forecasts

During each season, several national meteorological services and scientific agencies forecast how many tropical cyclones, tropical storms, and typhoons will form during a season and/or how many tropical cyclones will affect a particular country. These agencies include the Guy Carpenter Asia-Pacific Climate Impact Centre (GCACIC), of the City University of Hong Kong, the Tropical Storm Risk (TSR) Consortium of the University College London, and the Taiwan's Central Weather Bureau.

During January 2011, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) predicted that between twenty and twenty-three tropical cyclones were likely to develop and/or enter the Philippine area of responsibility during 2011. On March 20 the Hong Kong Observatory, predicted that the typhoon season in Hong Kong would be near to above normal with six to nine tropical cyclones passing within of the territory against an average of around 6. On March 30, the TSR Consortium released their first forecast of the season and predicted that the basin would see a near average season with 27.8 tropical storms, 17.5 typhoons, 7.8 "intense" typhoons and an ACE index of about 275. In early April, the China Meteorological Administration (CMA) predicted that between 24 and 26 tropical storms would develop or move into the basin during the year, which it noted was higher than the previous total of 14. They also predicted that between seven and nine tropical storms would make landfall on China, with the first landing taking place before June 29 and the last landing taking place after October 7. On April 26, the Thai Meteorological Department predicted that two tropical storms would affect Thailand during 2011, with one affecting Upper Thailand during August or September, while one was expected to move through Southern Thailand during October or November.

During May within its first outlook for the year, the GCACIC predicted that the season would be near average with 31 tropical cyclones, 27 tropical storms and 17 typhoons developing during the season. They also predicted that seven tropical cyclones would make landfall on Southern China, between May and December, compared with an average of five while predicting that six tropical cyclones during the whole year compared to an average of four tropical cyclones. TSR revised its initial prediction during May and subsequently predicted that 28.0 tropical storms, 17.7 typhoons, 7.6 "intense" typhoons and an ACE index of about 266. In late June after a near-normal start to the season Taiwan's Central Weather Bureau predicted that the season, would be near average of 25.7 with 22–26 tropical storms occurring over the basin during 2011. Between three and five of the systems were predicted to affect Taiwan, compared to an average of around 3.6. Within its July forecast update, the GCACIC predicted that seven tropical cyclones would make landfall on Southern China, between July and December compared to an average of four and that there would now be 16 typhoons due to the strength of the India-Burma trough. They also predicted that seven tropical cyclones would pass within of the Korean Peninsula or Japan, during July and December compared to an average of around three. Within its July update, TSR predicted that the ACE index would be about 194, after raising its prediction for the number of tropical storms to 28.0, typhoons to 18.1 and intense typhoons to 8.4. On August 4, TSR subsequently slightly revised these predictions within its final update for 2011 to 28.2 tropical storms, 17.9 typhoons, 8.0 "intense" typhoons and an ACE index of about 281.

Seasonal summary

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During April two tropical depressions developed but they failed to intensify into tropical storms. Tropical Storm Aere (Bebeng) then developed on May 5, and after causing PHP 2.25 billion in damage to Northeastern Luzon and Eastern Visayas, the name Bebeng was retired by PAGASA. The second tropical storm of the season then developed on May 19, and affected the Philippines, Taiwan and Japan before becoming extratropical to the east of Japan.

After Tropical Storm Banyan dissipated during October 14, no tropical storms or typhoons were observed within the basin until Tropical Depression 27W developed into Tropical Storm Washi during December 15, due to high vertical windshear and a strong northeast monsoon.

Systems

Tropical Depression 01W

On April 1, the JMA reported that a tropical depression had developed within an area of moderate vertical wind shear about to the southeast of Ho Chi Minh City in Southern Vietnam. Over the next day, the system gradually developed further, before the JTWC initiated advisories on the system and designated it as Tropical Depression 01W. However, within hours of this, the depression became devoid of convection as wind shear buffeted the system. This prevented the cyclone from intensifying beyond depression status as it remained nearly stationary. The JMA continued to monitor the system as a tropical depression for another day before issuing their last warning on the system.

Tropical Depression 02W (Amang)

On March 30, the JMA began monitoring an area of low pressure located southwest of Yap. By April 2, the system developed a low-level circulation, though convection appeared disorganized. Exhibiting good outflow within a region of weak wind shear, the low was anticipated to develop further over the following several days as it drifted west-northwestward. After briefly stalling early on April 3, the storm turned towards the east. Additionally, the JMA considered the system sufficiently organized to be declared a tropical depression. As the system was located to the west of 135°E, PAGASA began issuing advisories on the depression as well, assigning it the name "Amang". Tracking northeastward, the depression eventually developed enough convection to be declared Tropical Depression 02W by the JTWC on April 4. However, this was expected to be brief as a decaying frontal boundary approached from the west and prompted the system to undergo an extratropical transition. This intensification prompted the National Weather Service (NWS) in Tiyan, Guam to issue a tropical storm warning for the islands of Agrihan, Pagan and Alamagan. Interacting with the front and high wind shear, the system became partially exposed and elongated as it moved over cooler waters. Early on April 6, the JTWC issued their final advisory on the depression as it began to dissipate over open waters.

Tropical Storm Aere (Bebeng)

On May 3, the JTWC started to monitor a tropical disturbance that had developed within a monsoon trough about to the west of Palau. At this time the disturbances low level circulation center was weak and unorganized, while a minimal amount of deep convection was observed around the system. Over the next couple of days the depression gradually developed further in an area of low vertical wind shear before it was declared a tropical depression by the JMA and the JTWC during May 6. In the same evening, PAGASA upgraded the low pressure into a tropical depression and assigned its local name 'Bebeng'. On the afternoon of May 7, JMA upgraded the depression to a tropical storm and assigned the name 'Aere'. During the early morning of May 12, the JMA downgraded Aere to a tropical depression while south of Kyushu Island as it became a weak extratropical cyclone. It's extratropical remnants finally dissipated on May 15.

Throughout the Philippines, multiple agencies activated their emergency plans as the storm approached. The Armed Forces of the Philippines, the Philippine National Police, and the Philippine Coast Guard were all placed on standby to deploy to areas struck by Aere once the storm passed. Several ports were affected by the storm, stranding 1,379 passengers by the afternoon of May 7. According to the National Disaster Risk Reduction and Management Council, at least 35 people have been killed and two more are missing as a result of Aere. Agricultural losses are estimated at PHP1.37 billion (US$31.7 million). Widespread flooding and landslides damaged homes, blocked off roads and severed communications. In Catarman, Northern Samar, of rain fell in just 24 hours, resulted in significant flash flooding.

Typhoon Songda (Chedeng)

A weak non-tropical system formed within the Intertropical Convergence Zone on May 17, as it moved in a westward direction. On May 19, the JTWC reported that an area of low pressure had persisted about to the southeast of Yap. As the system moved towards the northwest under the influence of a subtropical ridge of high pressure, it rapidly consolidated in an area of light to moderate vertical wind shear. The JMA then started to monitor the system as a tropical depression later that day, before the JTWC designated it as Tropical Depression 04W early on May 20. The JTWC then reported later that day that the depression had intensified into a tropical storm with wind speeds of , however, it later reported that it had overestimated the wind speeds and consequently lowered the storm's status to a tropical depression, based on observations from Yap island. Late on May 21, both the JMA and the JTWC reported that the depression had now become a tropical storm with the JMA naming it as Songda. Over the next couple of days, the system gradually intensified further while moving northwest into PAGASA's area of responsibility. PAGASA named it as Chedeng. At 1200 UTC on May 24, the JTWC reported that Songda had intensified into a typhoon. 12 hours later, the JMA followed suit while the system was located about to the southeast of Manila in the Philippines. It rapidly intensified into a Category 5 typhoon. On the afternoon of May 29, Songda became extratropical south of Shikoku Island. The extratropical remnants of Sonda later crossed the International Date Line, which was later absorbed by another extratropical cyclone on June 4, and later dissipated completely over Alaska.

Although Songda remained offshore, heavy rains within the typhoon's outer bands

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