Tropical Storm 02W formed from an extensive area of cloudiness associated with a near-equatorial trough. It was to become the third unnamed system on the trot, the previous being Tropical Storm 27W (December, 2003) and Tropical Storm 01W (February, 2004). JTWC began issuing STWOs on a persistent area of convection within the trough at 1130 UTC on 14 March located near 3.0N/142.6E, or approximately 460 nm southeast of Yap. Earlier, at 14/0812 UTC, a QuikScat pass showed a very large and elongated LLCC consolidating under the large area of convection which was situated south of the circulation near the strongest winds. The environment was assessed as moderately favourable with good diffluence and weak to moderate wind shear.
The potential for development at this point was judged to be poor, but this was upgraded to fair at 14/1400 UTC. The area of convection at this time was relocated to a position closer to Yap Island, approximately 350 nm to the southeast. The development potential remained fair through the 15th, and then was upgraded to good at 16/0900 UTC. A TCFA was issued as animated multi-spectral imagery revealed a better-defined LLCC associated with the system. The first warning on Tropical Depression 02W was issued three hours later.
At 1200 UTC on 16 March Tropical Depression 02W was moving westward at 6 kts, located approximately 150 nm east-southeast of Palau. The system continued to consolidate and was upgraded to tropical storm intensity at 16/1800 UTC. The third warning (issued at 17/0000 UTC) placed the centre 18 nm east of Palau, but the combination of visible and microwave satellite images and synoptic data from Palau indicated that the LLCC was further to the southeast than previously advertised. Tropical Storm 02W was thus relocated to a position 150 nm southeast of Palau. It was now moving somewhat faster (at 10 kts) towards the west-northwest, embedded in the steering flow of a mid-level ridge to the north. However, the LLCC soon separated from its associated area of deep convection, and TS-02W was downgraded to a depression at 17/0600 UTC. The earlier relocation meant that Palau had to endure a second approach, but this time not so close. By 17/1200 UTC the centre of the cyclone had passed the island to its south and was located 70 nm to the south-southwest. (PAGASA had initiated bulletins by this time, naming the system Butchoy.)
At 0000 UTC 18 March Tropical Depression 02W was moving west- northwestward at 17 kts from a position roughly 180 nm west of Palau. Animated multi-spectral imagery at this time depicted a complex system with up to three fully-exposed, weak LLCCs. An 18/0413 UTC AMSU pass gave the best indication of where the main LLCC was located, near 9.0N/130.0E, or approximately one degree southeast of the displaced deep convection. The synoptic situation with TD-02W began to improve as the system turned northwestward, and after the convection began to redevelop over the LLCC, the MSW was upped to 30 kts at 1200 UTC, and further to 35 kts (tropical storm status) at 18/1800 UTC.
At 19/0000 UTC Tropical Storm 02W/Butchoy was moving northwestward some 425 nm east-southeast of Manila with 35-kt sustained winds. At this time, animated multi-spectral imagery once again suggested the presence of multiple centres, and the 0000 UTC position was based on the LLCC nearest the deep convection. Forward motion fluctuated from 4 to 9 kts over the next couple of days as the difficult-to-pinpoint centre of TS-02W continued trekking to the northwest. A 19/2202 UTC AMSU pass showed the LLCC to be exposed on the south side of the deep convection.
By 0000 UTC on 20 March Tropical Storm 02W/Butchoy had closed the distance to Manila down to 295 nm (to the east-southeast). Initially, forecast models were predicting the system to cross the Philippines, but the 20/0000 UTC Prognostic Reasoning message called for a change to a northward track east of the Philippines, through a weakness, and finally recurving northeastward into the westerlies. But before that was to happen, TS-02W had one last, final fling. A TRMM pass at 20/1641 UTC showed improved organisation of the deep convection and the formation of banding features. The MSW was raised to 45 kts, and this was to be the peak intensity.
Microwave imagery at 21/0000 UTC showed that the LLCC had become partially-exposed again, indicating that shear was increasing from the west. Weakening had begun, and TC-02W was barely holding on to tropical storm status by 1200 UTC as it veered northeastwards, passing approximately 260 nm east-northeast of Manila, Philippines. The MSW fell below 35 kts and TS-02W was downgraded to a depression at 21/1800 UTC. At 0000 UTC 22 March the centre of the ailing tropical depression was relocated to a position approximately 240 nm northeast of Manila with the nearest deep convection sheared over 125 nm away to the northeast. Movement had become sluggish towards the north-northwest at 2 kts. As cool, dry air entrainment and high vertical wind shear were expected to continue, the final warning was issued by JTWC at 22/0600 UTC, locating the slow-moving and completely exposed LLCC of the dissipating storm approximately 220 nm northeast of Manila.
(Editor's Note: PAGASA was the only one of the Asian TCWCs to upgrade TC-02W to tropical status, assigning a peak MSW (10-min avg) of 45 kts.)
No damage or casualties are known to have resulted from Tropical Storm 02W/Butchoy.
(Report written by Kevin Boyle)
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