Typhoon Hagupit (Nina) – September 22nd, 2008
On September 14 a tropical disturbance formed to the northeast of Guam. Over the next few days it slowly developed, with the JMA designating it as a minor tropical depression on September 17. Later that day the Joint Typhoon Warning Center (JTWC) then issued a Tropical Cyclone Formation Alert on the developing tropical depression. Late the next day the JTWC designated the depression as 18W as it began to issue advisories on the depression. Early on September 19 the JMA began to issue full advisories on the depression as it moved in to PAGASA’s Area of Responsibility and was named Nina by PAGASA. Later that day both the JMA & the JTWC upgraded the depression to a tropical storm. The JMA named the storm as Hagupit, and assigned the international number of 0814.
Typhoon 18W (Hagupit) has rapidly intensified over the past 12 hours while turning increasingly westward under the steering influence of a building subtropical ridge to the north. Poleward outflow has increased ahead of a mid-latitude trough to the northeast while vertical wind shear has decreased with the development of an enhanced anticyclonic outflow layer over
The current position is based on satellite fixes from PGTW and RJTD, several recent microwave satellite passes, and radar fixes from Taiwan. The current intensity of 85 knots is consistent with Dvorak t-numbers of 4.5 from PGTW and 5.0 from RJTD. Analysis of recent dotstar dropsonde data from around the storm circulation lend high confidence to the wind field depicted in the current warning. A ragged and fairly broad eye is now evident in multispectral satellite imagery. A rather symmetric convective structure surrounds this eye. The deepest convection is located on the southern periphery of the Typhoon, where the low level circulation center has begun to interact with the high terrain of northern Luzon.
The forecast track has been nudged slighlty equatorward based on analysis of the anticipated steering flow pattern.
Typhoon Hagupit will continue to track generally west-northwestward over the next 72 hours under the steering influence of a strong and continuous subtropical ridge to the north. Vertical wind shear should remain low and poleward outflow strong through tau 24, allowing Hagupit to intensify slowly despite some disruption of the circulation through interaction with the Island of Luzon to the south. After tau 24, Hagupit will move equatorward of a syntopic scale, upper-level anticyclone center. The peripheral flow associated with this anticyclone to the north will introduce increasing vertical wind shear and begin to disrupt poleward outflow. Thus, the current forecast anticipates a weakening trend beginning around or just after tau 24 and continuing through landfall just after tau 48.
After making landfall, the Typhoon will weaken considerably. The current forecast is in good agreement with the closely-packed numerical model guidance. However, the UKMET, JGSM, and ECMWF guidance suggest that the Typhoon may follow a track slightly farther south, keeping the circulation over water for a longer period. In this case, the intensities depicted in the current forecast may be too low. However, unfavorable synoptic influences and almost certain land interaction will still produce considerable weakening between tau 48 and tau 72 even if the system skirts the chinese coast and does not track fully inland.
Under the current track scenario, Hagupit will continue dissipate below the warning threshold intensity of 25 knots by tau 96 while tracking generally westward over land.