By PDC’s Senior Weather
Specialist Glenn James
The Pacific Disaster Center’s (PDC Global) Monday, November 13, 2023, Tropical Cyclone Activity Report…for the Pacific Ocean, the Indian Ocean, and adjacent Seas
Current Tropical Cyclones:
Tropical Cyclone 17W…is located approximately 398 NM east-southeast of Koror – Final Warning
Tropical Cyclone 02P (Mal)…is located approximately 295 NM northwest of Suva, Fiji
Northeast Pacific Ocean:
There are no tropical cyclones, nor any areas of disturbed weather under investigation by the National Hurricane Center (NHC) at the time of this writing.
Central North Pacific:
There are no tropical cyclones, nor any areas of disturbed weather under investigation by the Central Pacific Hurricane Center (CPHC) at the time of this writing.
Western Pacific, Indian Ocean and adjacent Seas:
Tropical Cyclone 17W – Final Warning
According to the JTWC warning number 6
Sustained winds were 20 knots…with gusts to 30 knots
Tropical depression 17W has pulled a fast one, and emerged in animated shortwave infrared and visible as now being a full exposed low level circulation center (llcc) moving south-southeastward. This motion is confirmed by a bullseye pass, resulting in a second relocation in the last 12 hours.
The system clearly has lost its battle with the persistent easterly shear, which proved just too much to overcome. Now being an exposed llcc confined to the low levels, the system has turned southward, likely influenced by the enhanced pocket of winds to the northwest, as the overall environmental steering flow should be to the southwest.
In the near-term, TD 17W is likely to continue to weaken below warning criteria as it remains quasi-stationary and drifting in a looping manner. No significant change is expected in the strength of the easterly shear, and while sporadic convection is likely to flare along localized convergent flow lines, it is unlikely to be able to consolidate into a coherent core.
But while the system is forecast to weaken below warning criteria, it is also expected to regenerate after 36 hours. By 36 hours the steering gradient is expected to strengthen, and TD 17W will kick out to the west, accelerating past Palau by 60 hours before slowing and turning more poleward by the end of the forecast period.
Model fields suggest a weakening of the deep-layer shear after 36 hours, which should allow the system to make another run at intensification, albeit slowly and weakly. By 36 hours the system is forecast to regain 25 knots, then slowly increase to 35 knots by the end of the forecast period, as it approaches the eastern portion of the Philippines. However, if convection fails to develop or the system remains stalled near its current position, then the system will fail to re-intensify and dissipate within the next 36 hours.
Tropical Cyclone 02P (Mal)
According to the JTWC warning number 5
Sustained winds were 65 knots…with gusts to 80 knots
Animated enhanced infrared (eir) satellite imagery depicts a rapidly developing system, with well-formed bands of deep convection wrapping into an obscured low level circulation center (llcc). The most recent eir imagery shows intense convective towers firing off just southwest of the assessed llcc, forming a small inner central dense overcast (cdo) feature. The center of this cdo is offset west-southwest due to moderate amounts of shear in the mid- to upper-levels, while overall deep-layer shear remains low. A microwave image showed a formative eye, with deep convective banding features primarily evident on the north and eastern sides, having yet to fully close off an eye.
The system continues to track quickly to the southeast along the west side of a deep-layer subtropical ridge (str) to the east. Environmental conditions continue to improve, with low deep-layer vertical wind shear, warm sea surface temperatures and strong poleward outflow. The only hindrance is a moderate amount of vortex tilt brought on by the moderate mid-level shear from the northeast.
Tropical cyclone 02P (Mal) will track southeastward along the western side of the steering ridge through the entirety of the forecast period. Some slight deviations east or west of the forecast track are possible as the ridge expands or retreats, but overall the track forecast is straight forward. The system is expected to pass about 60 NM west of Nadi, Fiji by 24 hours, then move into the open waters of the South Pacific through the remainder of the forecast.
Due to a slow relaxation of the steering gradient, the system is forecast to slow down over the next 48 hours before picking up a bit more speed by the end of the forecast. While the vortex is still tilted a bit due to some persistent mid-level northeasterly shear, global model fields suggest the shear will weaken over the next few hours, which should allow the convection seen firing off now, to help align the vortex.
Once this occurs, the system will rapidly intensify through 24 hours, reaching at least 75 knots as it passes west of Nadi. The window of opportunity for intensification will be fleeting however, with shear expected to sharply increase by 36 hours. By 48 hours, the system will be well into cooler waters, will be decapitated by the very high westerly shear and will be in the process of being smothered by very dry mid-level air. This will mark the beginning of a rapid subtropical transition, which will be completed no later than 72 hours.