msgid/genadmin/navmarfcstcen Pearl Harbor hi/jtwc// subj/prognostic reasoning for typhoon 15w warning nr 15// rmks/ 1. For meteorologists. 2. 12 hour summary and analysis. A. It appears that a period of weakening associated with an eyewall replacement cycle has ended with the appearance of a solid ring comprising the new, secondary eyewall. The original eye feature, which has wobbled within the larger outer eyewall throughout much of the past 12 hours, is still visible in animated multispectral satellite imagery. B. The current position is based on high confidence Sat- ellite fixes from pgtw and rjtd and radar fixes from taiwan and Japan. The current intensity estimate is consistent with Dvorak T-numbers of 6.0 from both pgtw and rjtd. TY 15w is continuing slowly poleward between subtropical ridge centers to the west and to the east of the cyclone. Outflow aloft remains strong. In fact, a developing upper level trough to the east of TY 15w appears to have spurred the development of a poleward outflow channel. Ocean heat con- tent is favorably high. 3. Forecast reasoning a. Changes in the storm structure, including the eyewall replacement cycle discussed above, necessitated a downward adjustment in the near-term intensity forecast for warning number 14. However, the overall forecast reasoning remains unchanged from that presented in the previous prognostic reasoning message. B. TY 15w will continue to track slowly poleward in the competing steering environment between ridge centers to the east and to the west over the next 48 hours. A weakness in the subtropical ridge induced by a mid-latitude trough to the north has persisted, and the subtropical ridge to the east of the storm has emerged as the slightly more dominant steering influence in the resulting flow pattern. As the trough to the north moves eastward, an extension of the subtropical ridge is expected to extend westward along the poleward side of the typhoon. This ridge extension should nudge the cyclone westward a bit as it continues poleward toward the Point of eventual recurvature. A second mid- latitude trough will further weaken the ridge as it pro- gresses eastward to the north of the storm around tau 48. This trough is expected to influence the steering environ- ment sufficiently to induce recurvature, a scenario supported by the majority of the available numerical model guidance. JGSM and NOGAPS remain as noteworthy exceptions to this scenario. Both take the system farther westward to eventual landfall along the northeast coast of Taiwan. However, the JGSM solution has not verified well over the past 24 hours and the NOGAPS tracker has shown a westward track for only the past two runs. Therefore, this forecast favors the majority model grouping and is hedged accordingly against the model consensus. With excellent upper level outflow and favorable ocean heat content, TY 15w should maintain intensity or slowly intensify over the next 48 hours as the current forecast reflects. Enhancement of outflow by an upper level trough to the east may drive intensification higher, though, particularly if the large outer eyewall contracts over time. An increase in vertical wind shear after recurvature around tau 48 will begin a steady weakening trend. C. Considerable uncertainty remains in the extended fore- cast period. However, the most recent numerical model guidance has shown an encouraging trend toward closer agreement. The EGRR, ECMWF, GFDN, and GFS trackers now depict recurvature between tau 48 and tau 72 followed by a track in the tau 96 to tau 120 period that passes across or along the southern coast of the major islands of Japan. With the zone of enhanced mid-latitude westerly flow now extending through Southeast Asia toward southern Japan, this general track scenario appears increasingly probable. However, there are significant differences in the extended period track speeds among these models. The current forecast speeds represent a reasonable hedge against the consensus. Increasing westerly vertical wind shear and the introduction of a mid-latitude airmass by an approaching mid-latitude trough will drive both weakening and the first phase of extratropical transition during the extended period. Forecast team: Bravo//