Tropical Storm Hagupit
Tropical Storm 18w (Hagupit) has intensified slowly over the past 12 hours while continuing generally northwestward toward a weakness in the subtropical steering ridge to the north.
The low level circulation center (LLCC) has further consolidated, as evidenced by a more symmetric convective tructure. Vertical wind shear has decreased slightly, and recent animated water vapor imagery indicates a marked improvement in poleward outflow.
The intensity estimate of 60 knots is consistent. Convection has increased near the circulation center, although the deepest convection is still located on the western and southern peripheries of the LLCC.
Tropical Storm Hagupit has moved into an area of decreasing vertical wind shear closer to an upper level ridge axis. In addition, the flow associated with a tropical upper tropospheric trough (TUTT) and associated TUTT cell to the northeast is enhancing poleward outflow.
Forecast track speeds have been increased slightly for the current warning. Tropical Storm Hagupit is expected to turn toward an increasingly westward track through tau 72 as the mid-latitude trough inducing the weakness in the steering ridge to the north lifts northeastward and continuous, zonally-oriented subtropical ridging builds along the poleward periphery of the tropical storm. The numerical model guidance envelope continues to shrink, lending increased certainty to the track forecast.
The current forecast track remains slightly south of the model consensus, favoring the well-performing ecmwf and ukmet solutions. Tropical Storm Hagupit will steadily intensify through tau 48 as low vertical wind shear, excellent upper level outflow, and passage over high ocean heat content take effect. Between tau 48 and tau 72, ts 18w will begin to weaken in response to decreasing ocean heat content, suppressed poleward outflow, and land interaction.
In the extended period, Tropical Storm Hagupit will progress westward into southern China under the continued steering influence of sub-tropical riding to the north. the track is expected to remain over land, which should induce dissipation below the warning threshold intensity of 25 knots by tau 120. However, a small equatorward jog would allow the system to remain over water and consequently progress closer to Hainan Island and northern Vietnam as a stronger system than currently forecast. Some adjustments to the extended forecast are likely as the anticipated orientation of the steering flow pattern during that period becomes clearer.