By PDC’s Senior Weather
Specialist Glenn James
The Pacific Disaster Center’s (PDC Global) Wednesday, August 23, 2023, Tropical Cyclone Activity Report…for the Pacific Ocean, the Indian Ocean, and adjacent Seas
Current Tropical Cyclones:
Tropical Cyclone 08W…is located approximately 340 NM east-northeast of Andersen AB, Guam
Tropical Cyclone 09W…is located approximately 416 NM south-southeast of Kadena AB
Northeast Pacific Ocean:
>>> South of Southern Mexico…
A broad area of low pressure located several hundred miles south of the coast of southern Mexico is producing some disorganized showers and thunderstorms. Environmental conditions appear favorable for gradual development of this system, and a tropical depression is likely to form within the next two to three days while it moves slowly to the northwest or north-northwest. Interests in southwestern Mexico should monitor the progress of this system.
* Formation chance through 48 hours…medium…50 percent
* Formation chance through 7 days…high…80 percent
>>> Central East Pacific…
Showers and thunderstorms are showing signs of organization in association with a low pressure system located well south of the southern tip of the Baja California peninsula. Environmental conditions are expected to be conducive for development of this system, and a tropical depression could form in the next few days while it moves west-northwestward over the central portion of the
tropical eastern Pacific.
* Formation chance through 48 hours…medium…40 percent
* Formation chance through 7 days…medium…60 percent
Western Pacific, Indian Ocean and adjacent Seas:
Tropical Cyclone 08W
According to the JTWC Warning number 5…
Sustained winds were 25 knots…with gusts to near 35 knots
Animated satellite imagery shows a disorganized but consolidating system with the main convection, although sheared eastward, encroaching into but still partially exposing the low level circulation center (llcc). The satellite loop also shows the steady influx of cool dry air from the north as evidenced by a stream of stratocumulus streaks wrapping into the llcc.
Analysis indicates a marginally favorable environment with strong poleward outflow and very warm sea surface temperatures offset by the dry air intrusion and moderate vertical wind shear aloft.
TD 08W will remain nearly stationary over the next 24-36 hours. Afterward, the ridge will build and become the dominant steering mechanism and slowly drive the cyclone northward. After 72 hours, a secondary deep-layered ridge to the northeast will assume steering and drive the cyclone northwestward toward Japan.
The marginally favorable environment will fuel a gradual intensification to 45 knots during the stationary phase. Afterward, increasing poleward outflow and diminishing dry air intrusion at the low levels will promote further intensification to 65 knots by 120 hours.
Worth mentioning at this early stage is that TD 08W will track along the eastern boundary of a gyre that includes multiple vortices over a wide area bounded by Japan, Taiwan, and the Philippine island of Luzon. The wind field of this system is forecast to become very expansive with subtropical characteristics.
Tropical Cyclone 09W
According to the JTWC Warning number 2…
Sustained winds were 35 knots…with gusts to near 45 knots
Animated enhanced infrared satellite imagery depicts a quickly consolidating low level circulation center (llcc) with flaring, disorganized convection, scattered across the southern portion of the rotation. A bullseye satellite image depicted a small, but well-defined llcc with 25-30 knot winds extending from southwest to east of the circulation center. A color-enhanced microwave image shows what could be a nascent microwave eye feature and well-defined banding features wrapping into the center.
The system lies in a favorable environment, with good outflow, low (10-15 knot) vertical wind shear, very warm sea surface temperatures. The system is currently moving towards the north-northwest along the western side of a subtropical ridge centered to the east near 135E.
By 24hours , multiple, fairly drastic changes in the upper-level pattern begin to emerge. First, the ridge center currently steering the system is forecast to quickly weaken and disappear around 36 hours. Second, the ridge will be replaced by a monsoon gyre that sets up to the east in the deep-layer mean pattern, with the outlines of the cyclonic flow reaching almost 1700 NM. Third, a new ridge center develops near Hong Kong. The combination of this trifecta will result in the system slowing as it reaches the poleward inflection point around 24 hours, hang out there for a few hours, then turn sharply equatorward as steering transitions to the gradient between the strong gyre to the east and the ridge center to the west.
The system tracks generally southward about 96 hours, where it encounters another seismic shift in the steering pattern as the gyre dissipates and is replaced by a monstrous ridge centered around 40N 160E. The far western extension of this ridge will assume the dominant steering role after 96 hours, and the system will begin to slowly turn back to the north by the end of the forecast period.
In terms of intensity, the favorable environment will fuel steady intensification through 72 hours, where an influx of northerly shear will lead to a 24 hour period of stagnation in the intensification trend. Intensification resumes after 96 hours through the remainder of the forecast. There’s a high probability of rapid intensification between 48 and 72 hours (80-100 percent). Therefore, there is a possibility of a higher peak in the forecast than is currently anticipated, and the potential for rapid intensification after 48 hours.