TOKYO - Typhoon No. 30 was rapidly weakening over the East China Sea at 4 p.m. on October 25 and is expected to become an extratropical cyclone by 3 a.m. on October 26, according to the latest weather forecast, although its merger with an autumn rain front could trigger prolonged heavy rainfall across western and eastern Japan, particularly along the Pacific coast and around the Izu Islands.
The Japan Meteorological Agency's latest analysis, issued at 3 p.m. on October 25, placed the storm's central atmospheric pressure at 996 hectopascals, with maximum sustained winds near its center of 20 meters per second. The typhoon has weakened considerably as lower sea surface temperatures and an influx of dry air from the west have disrupted its circulation.
Satellite imagery captured at 4:07 p.m. showed that the storm had lost much of its organized cloud structure, with only a relatively shallow circulation of low- and middle-level clouds remaining near its center. Infrared observations indicated that the high-altitude clouds normally associated with a developed tropical cyclone had largely disappeared, making the storm's center difficult to distinguish.
Additional satellite observations tracking atmospheric moisture revealed that dry air had penetrated deep into the storm's circulation from the west. This process, combined with cooler ocean temperatures, is accelerating the breakdown of the typhoon's tropical structure as it approaches the autumn rain front.
The storm is forecast to complete its transition into an extratropical cyclone by 3 a.m. on Monday, October 26. By 3 p.m. that day, the resulting low-pressure system is expected to reach an area north-northwest of Makurazaki, a city in Kagoshima Prefecture in southern Kyushu.
Although the system is not expected to produce storm-force winds by the time it reaches that area, rough seas will remain a concern. Waves exceeding four meters are forecast in some surrounding waters through the first half of October 26, with hazardous conditions also possible along coastal areas of Kyushu, other parts of western Japan, and the Pacific coast of eastern Japan.
Weather conditions are expected to become increasingly complex as the weakening storm merges with the seasonal rain front extending across southern Japan.
Weather charts analyzed at noon on October 25 showed the typhoon approaching the autumn rain front, with forecasts indicating that the merger would be largely complete by the morning of October 26. However, atmospheric conditions could also support the development of a separate, smaller low-pressure system along the front.
The potential secondary system is expected to form somewhere between the Pacific waters south of Shikoku, the seas south of the Kii Peninsula, and the Izu Islands. Forecasters warned that this developing system could become an important driver of heavy rainfall independently of the remnants of Typhoon No. 30.
By 9 a.m. on October 26, dry air is expected to extend around the southern side of the former typhoon's circulation. Along the leading edge of this dry air, atmospheric conditions could favor the formation of additional rain clouds and a new low-pressure disturbance farther east.
Rainfall forecasts based on the Japan Meteorological Agency's five-kilometer-resolution weather model indicate that the autumn rain front will gradually shift southward around eastern Japan and the waters off the Kii Peninsula during the evening of October 25.
During the daytime on October 26, the front is expected to become nearly stationary across an area extending from Kyushu toward Shikoku, allowing rain clouds to strengthen and potentially produce sustained downpours.
Two distinct areas of developing rainfall are anticipated. One will form northeast of the low-pressure system originating from Typhoon No. 30, while another will develop farther east, stretching from waters off Shikoku through the seas south of the Kii Peninsula toward the northern Izu Islands.
Although the two areas of precipitation may eventually become connected, forecasts suggest that rain cloud development near the northern Izu Islands could become particularly pronounced from the afternoon of October 26, potentially exceeding the intensity of rainfall associated directly with the former typhoon.
This means the heaviest rainfall may not necessarily occur close to the center of the weakening storm. Instead, the interaction between the autumn rain front, incoming dry air, and newly developing low-pressure systems could produce concentrated areas of heavy precipitation at some distance from the original typhoon.
Western Japan, the Pacific-facing areas of eastern Japan, and the northern Izu Islands are among the regions facing an elevated risk of intense rainfall. Some locations could experience heavy or extremely heavy rain over an extended period as rain clouds repeatedly develop along the front.
The changing atmospheric structure also creates uncertainty over exactly where the strongest rain clouds will form and how long they will remain over particular locations.
Residents in potentially affected areas are advised to monitor updated weather forecasts, rainfall warnings, and coastal wave conditions throughout October 25 and October 26, even as Typhoon No. 30 loses its tropical characteristics.
Source: ウェザーニュース
