2. Typhoon SONCA (Bising/03w)                       
>> April 20-29, 2005

Sonca: contributed by Vietnam, is a singing bird known for its beautiful twittering sound and which lives in mountainous areas

Introduction

Typhoon Sonca became the surprise package of April, 2005, in the Western North Pacific, unexpectedly reaching a maximum intensity of 115 kts nearly 600 nm or so east of the Philippines. This storm well illustrates the unpredictability of tropical cyclones. Typhoon Sonca followed a track similar to that of Super Typhoon Sudal last year, but stayed well away from Yap Island. This is the third consecutive year in which April has produced a major typhoon with a MSW > 100 kts.

Storm Origins

Two suspect areas were being monitored in JTWC's STWO at 2000 UTC 16 April, one producing thunderstorm activity approximately 170 nm east- southeast of Palau while the second, pre-Sonca, disturbance was located 85 nm south-southwest of Chuuk. As the first was initially considered the more likely to develop into a tropical cyclone, a TCFA was issued at 18/0030 UTC. However, no further development occurred and the TCFA was cancelled at 19/0030 UTC. (This system was the one classified as a tropical depression by JMA on 18 April. See introductory paragraphs above.) The system had dissipated altogether by 19/0600 UTC. Meanwhile, the pre-Sonca LOW had been raised to a 'fair' development potential at 18/2200 UTC. An upper-level analysis depicted a low to moderate wind shear environment with favourable outflow aloft. When multi-spectral satellite imagery revealed convection organizing over a LLCC, a TCFA was released at 20/0530 UTC. This was replaced by the first warning at 20/1200 UTC, placing the centre of the newly formed tropical depression 270 nm east-southeast of Yap.

Synoptic History

At 1200 UTC 20 April Tropical Depression 03W was drifting westwards at 7 kts along the southwestern periphery of the mid-level steering ridge situated to the northeast. The storm showed little signs of developing into a tropical storm and remained a very slack, disorganized system during the 21st. Forecasts called for slow strengthening, but as this didn't seem forthcoming, JTWC intended the 22/0000 UTC warning to be the last. Just as this final statement was being released, TD-03W underwent a redevelopment phase and satellite images at this time showed that TD-03W had actually become much better organized. Realizing this, JTWC was forced to reinstate the storm only six hours after the last statement was issued so there was no break in transmission. There was a little intensification on the 22nd but the system remained at tropical depression status. After crossing into PAGASA's AOR at 22/1200 UTC, the name Bising was assigned by that agency for local use.

Tropical Depression 03W finally became a named tropical storm at 0000 UTC 23 April after both JTWC and JMA upgraded their respective MSW estimates to 35 kts. The newly-christened Sonca was then centered approximately 375 nm west-northwest of Yap. It was at this point that Tropical Storm Sonca really began to take off. An unexpected rapid strengthening phase ensued, and after a 23/1633 UTC AMSR-E microwave pass depicted a developing eye, Sonca was upgraded to a 75-kt typhoon at 23/1800 UTC while located about 450 nm west-northwest of Yap. After moving rather erratically on a west to northwest track for a couple of days, the storm turned north-northwestward and slowly moved in that direction awhile before veering northwards. Its intensity continued to climb alarmingly, reaching a peak of 115 kts at 24/1800 UTC. At this point, Sonca was located approximately 570 nm east of Manila and was beginning to recurve around the subtropical ridge.

(Editor's Note: The peak MSW assigned by JMA was 85 kts with an estimated minimum CP of 940 mb. PAGASA and the CWB of Taiwan also estimated the peak winds at 85 kts (10-min avg), whereas NMCC's was slightly higher at 90 kts.)

Even as Typhoon Sonca reached its maximum intensity at 1800 UTC 24 April, increasingly hostile environmental conditions were already distorting the appearance of the system. Its eye soon became cloud- filled at 25/0000 UTC and deep convection began to decrease in the western and southern quadrants. Sonca remained a major typhoon through the 25th until the MSW fell below 100 kts at 26/0600 UTC. The system began to accelerate northeastward into the mid-latitude westerlies and started to take on extratropical characteristics. At 26/1800 UTC Sonca was downgraded to a 55-kt tropical storm only 45 nm west-northwest of Iwo Jima, the island lying within the radius of 34-kt winds. JTWC issued their final warning at this time, but JMA maintained Sonca as a 65-kt typhoon. The latter agency finally classified the system as extra- tropical at 1200 UTC on 27 April with the 45-kt gale center located about 500 nm east-northeast of Iwo Jima and moving rapidly eastward. The final reference to former Typhoon Sonca in JMA's High Seas Bulletins was at 0000 UTC on 29 April and placed the weakening 25-kt LOW about 600 nm north of Wake Island.

A graphic depicting the track of Typhoon Sonca may be found by clicking this link.

Meteorological Observations

The only meteorological observations available were sent by Huang Chunliang--a couple of rainfall amounts from Chuuk, and these were from the formative stages of Sonca, several days before the depression stage began on 20 April.

     Only 24-hr amount(s) >= 100 mm listed:

  CHUUK, ECI (7.45N 151.83E)          116.1 mm [15/18-16/18Z]
  CHUUK, ECI (7.45N 151.83E)          102.6 mm [16/00-17/00Z]

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