At 2000 UTC on 9 February JTWC issued a STWO on an area of convection located near 5.0N/153.0E, or approximately 250 nm west-southwest of Pohnpei. This area eventually developed into Tropical Storm 01W and was a twin to a disturbance in the Southern Hemisphere which became Tropical Cyclone Fritz. Animated infrared satellite imagery indicated cycling deep convection over a weak LLCC situated within the monsoon trough. Upper-air analyses indicated that the suspect area was located in the southwestern quadrant of the subtropical ridge with good divergence and moderate wind shear conditions. The potential for development for the next 24 hours was assessed as poor, and then upgraded to fair at 11/1300 UTC. At this time animated infrared satellite imagery indicated that the deep convection had persisted and consolidated over the weak LLCC. A TCFA was issued at 11/1500 UTC and soon followed by the first warning on Tropical Depression 01W at 1800 UTC.
At the time of the first warning, Tropical Depression 01W was located approximately 150 nm south of Agana, Guam, and moving toward the north- west at 12 kts . Deep convection continued to develop over the system and animated satellite imagery showed low-level cloud lines wrapping in from the southeast. After further intensification (and a turn to the west-northwest) TC-01W was upgraded to a 35-kt tropical storm, but the system was still being harassed by wind shear. Multi-spectral satellite imagery revealed an exposed LLCC south of the deep convection which had weakened by 12/1200 UTC, and as a result TS-01W was downgraded back to tropical depression status. By 13/0000 UTC the appearance of TD-01W looked worse for wear with only weak convection situated over a broad LLCC.
At 0000 UTC on 13 February the partially-exposed centre of TD-01W was moving west at 15 kts approximately 180 nm north-northwest of Yap. Most of the deepest convection was in the northern quadrants at this time. Things hadn't improved six hours later, but at 13/1200 UTC TD-01W was rewarded for its efforts and persistence. Deep convection began to increase once again and formed an impressive albeit sheared CDO as seen in 13/1800 UTC satellite images. TD-01W was re-upgraded to a tropical storm with the MSW increased to 40 kts. The system began to slow and meander toward the north-northwest under the steering influence of the low to mid-level ridge located to the northeast. (Note: Tropical Storm 01W was named Ambo by PAGASA, which issued only four warnings on this system from the 13th to the 14th.)
At 0000 UTC on 14 February Tropical Storm 01W (Ambo) had slowed to 5 kts, still trekking toward the north-northwest approximately 380 nm north-northwest of Yap. The system managed to strengthen a little more and the MSW reached a peak intensity of 45 kts at 14/0600 UTC. This was maintained for another six hours before TS-01W was abruptly down- graded to a tropical depression at 14/1800 UTC. At this time upper- level shearing had exposed the LLCC again with the nearest deep convection located over 65 nm away. Pressures had been building across the northern Philippines and this synoptic feature prevented the weak tropical cyclone from making any further progress toward the north. Instead, a very slow east-northeastward crawl had begun and this had turned east-southeastward by 15/0000 UTC.
Satellite images at 0000 UTC 15 February continued to show deep convection well-removed from the LLCC which, at this time, was located some 325 nm north-northwest of Yap. The intensity of TD-01W was hovering at 30 kts. Water vapour imagery at 0600 UTC showed a ball of deep convection developing over the LLCC, but this soon waned. Movement was slow throughout the day and toward the east or east-southeast, then south-southwestward at 15/1800 UTC as the ridge began to pull the system in toward the central Philippines.
By 16/0000 UTC TD-01W had finished its clockwise loop cycle and was accelerating south-southwestward at around 13 kts with the limited areas of deep convection mainly to the northwest of the exposed centre. JTWC issued the final advisory at 06/0600 UTC on the understanding that the system would continue to track through a poor sustenance environment, and by 1800 UTC satellite images showed hardly any trace of the tropical cyclone.
(Editor's Note: JTWC was the only warning agency to upgrade TD-01W/Ambo to tropical storm status.)
JTWC continued to monitor the remnants of TS-01W through STWOs, and at 2330 UTC on 17 February considered the development potential to be fair based on the increase of deep convection near the remnant LLCC, which had drifted southwestward to a position approximately 395 nm east of Mindanao, Philippines. This was downgraded to poor after the centre once again became fully-exposed to the east of the cycling deep convection. According to JMA's bulletins issued on the 17th the weak LOW was at a virtual standstill and remained stationary until late on the 18th when it began to drift slowly west. Further bursts of deep convection occurred as the system continued its way west at a quicker pace through the 19th, but at 20/0600 UTC JTWC ceased mentioning the remnants of TC-01W in their STWOs as the appearance and organization of the system deteriorated again.
What little convection remained in association with the wreckage of TD-01W continued to drift west towards the Philippines, but generally it came and went for several days. In fact, satellite images showed the disturbance re-organizing on the 23rd (although it was still being sheared) by which time it was moving northwestward and paralleling the east coast of the Philippines. On the 24th the system became exposed again. However, following another redevelopment phase early on the 25th, several bursts of deep convection appeared forming a cold CDO. Embedded within the overcast was a "pinhole" eye feature which appeared in satellite images for two hours. Shortly afterwards, cloud top temperatures warmed and the disturbance was completely stripped of convection by upper-level shearing. Accelerating towards the northeast the disturbance completely dissipated and had been absorbed into a frontal system by 0000 UTC on 27 February. (In an e-mail, Roger Edson noted that he'd carefully looked at satellite imagery of the pinhole eye feature referenced above, but could not discern any rotation. Without rotation of the cloud system, of course, the feature would not have been a true eye.)
This disturbance may be at least partially related to the remnants of TC-01W/Ambo, and satellite animations covering the period after Ambo's dissipation certainly suggest that it was the rejuvenation of Ambo. (Huang Chunliang pointed out that only CWB was following the system as a LOW during the time that the "eye" was seen and that CPHC was the only agency to classify the disturbance as a weak depression and to mention the fact that it had intensified during the previous 12 hours. (JMA at the time was following another LOW centre further to the south.)
There were no reports of damages or loss of life in association with Tropical Storm 01W/Ambo.
(Report written by Kevin Boyle)
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