By PDC’s Senior Weather
Specialist Glenn James
The Pacific Disaster Center’s (PDC Global) Wednesday, August 30, 2023, Tropical Cyclone Activity Report…for the Pacific Ocean, the Indian Ocean, and adjacent Seas
Current Tropical Cyclones:
Tropical Cyclone 09W (Saola)…is located approximately 231 NM east-southeast of Hong Kong
Tropical Cyclone 10W (Haikui)…is located approximately 420 NM southeast of Kadena AB
Tropical Cyclone 11W (Kirogi)…is located approximately 566 NM east of Andersen AFB, Guam
Northeast Pacific Ocean:
There are no tropical cyclones nor any areas of disturbed weather under investigation by the NHC
Central North Pacific…
There are no tropical cyclones nor any areas of disturbed weather under investigation by the CPHC
Western Pacific, Indian Ocean and adjacent Seas:
Tropical Cyclone 09W (Saola)
According to the JTWC Warning number 30…
Sustained winds were 135 knots…with gusts to near 165 knots
09W (Saola) remains a super typhoon (sty) but is staring to show signs of weakening across the spectrum of available data. Animated enhanced infrared satellite imagery depicts a 20 NM wide stadium eye with eye temperatures maintaining consistent over the past six hours. Animated radar data and imagery confirm three important facts; the eye has begun to shrink, the northern eyewall has eroded significantly over the past six hours, and the secondary eyewall is beginning to dominate the overall structure.
Environmental conditions remain favorable, however the onset of the eyewall replacement cycle (ewrc) is proving to the primary factor in the weakening of the system.
STY 09W will continue to track towards the west-northwest for the next 24 hours along the southwest side of the ridge far to the northeast. STY 09W will turn westward between 36 and 48 hours under the influence of this transient anticyclone.
From an intensity perspective, the system will slowly but steadily weaken through tau 48 first in response to the ongoing ewrc, from which the system will not have time to recover, and second in response to an expected uptick in southerly wind shear after tau 36. After 48 hours, the influence of a dry air entraining into the core of the system, increased vertical wind shear and disruption of the low-level flow due to terrain interaction, will lead to a faster pace of weakening.
As the system quickly weakens as it passes south of Hong Kong, the steering level will steadily progress to lower levels of the atmosphere, where the steering gradient will be much weaker as a weakness in the steering pattern develops. As the system weakens below tropical storm strength, it will come under the steering influence of the low to mid-level southwesterly monsoonal flow, and turn sharply back towards the east-northeast by the end of the forecast period.
Tropical Cyclone 10W (Haikui)
According to the JTWC Warning number 11…
Sustained winds were 50 knots…with gusts to near 65 knots
Animated enhanced infrared satellite imagery shows that tropical storm 10W (Haikui) continues to slowly but steadily improve structurally.
Conditions remain generally favorable now that the northerly shear has begun to relax a bit. Sea surface temperatures are very warm but outflow remains weak and relatively constrained to the equatorward direction.
TS 10W is forecast to continue tracking
towards the northwest along the southwest side of the strong ridge through 72 hours. The system is expected to pass south of Okinawa by 48 hours, then enter the East China Sea as the ridge builds towards the southwest and links up with a transient anticyclone moving east behind a mid-latitude trough. Landfall along the coast of eastern China south of Shanghai is expected near 96 hours, with the system continuing inland to the west of shanghai through the remainder of the forecast.
A sharp sea surface temperatures gradient along the coast of China, and increased shear will lead to a general weakening trend prior to landfall, with additional rapid weakening due to land interaction after the system goes feet dry.
Tropical Cyclone 11W
According to the JTWC Warning number 5…
Sustained winds were 55 knots…with gusts to near 70 knots
Tropical storm 11W continues to consolidate with improved spiral bands of persistent deep convection obscuring the low-level circulation center (llcc), evident in animated enhanced infrared. Animated water vapor imagery also indicates an increase in central convection with an increase in poleward outflow as the system taps into an upper-level trough to the northeast; thus environmental conditions continue to improve.
Environmental conditions are favorable with moderate equatorward outflow, improving poleward outflow into a upper-level low position to the northeast, and low vertical wind shear.
Tropical storm 11W is forecast to track generally northwestward along the western periphery of the subtropical ridge (str) positioned to the northeast through 48 hours. Environmental conditions are forecast to remain favorable through 72 hours with moderate equatorward outflow, improving poleward outflow into an upper-level low which is expected to shift westward, along with low vertical wind shear.
After 72 hours, the ridge is forecast to strengthen to the northeast and become oriented more east-west, which will ultimately turn and steer the system west-northwestward through the end of the forecast period. However, track uncertainty increases in the later part of the forecast as the system could interact with another developing low-pressure system in the vicinity of 11W. Additionally, in terms of intensity, after 72 hours, outflow becomes less favorable while vertical wind shear and dry air entrainment increase resulting in a gradual weakening trend through 120 hours.