Typhoon Ampil (2024)

Ampil affected Japan during the Obon holiday week, shortly after Tropical Storm Maria caused unprecedented rainfall to northern Japan. As the storm neared the coast, hundreds of thousands in Tokyo and nearby regions were urged to evacuate. Numerous modes of transportation, such as flights and trains, were canceled due to the approaching typhoon. The NHK reported that Tokyo suffered minimal damage, while four people were injured across the Kantō region. However, Japan was struck by a more powerful cyclone named Typhoon Shanshan one week later. Later, the remnants of Ampil caused a high surf advisory, a high wind watch, and a coastal flood advisory in Alaska, bringing strong winds and coastal waves to western Alaska. Ampil contributed to an atmospheric river when its moist core entered a low-pressure system and merged with the Pacific jet stream, which was anticipated to reach California.

Meteorological history

Typhoon Ampil originated from an area of convection 976 km (606 mi) east of Kadena Air Base on August 3. At 18:00 UTC on August 11, the Japan Meteorological Agency (JMA) designated it as a tropical depression. Soon after, the United States Joint Typhoon Warning Center (JTWC) began to re-monitor it, noting that it was in a marginal favorable environment for development. At 07:00 UTC on August 12, the agency issued a tropical cyclone formation alert on the disturbance. A few hours later, they recognized the system as a tropical depression, designating it as 08W. Soon after, the JMA noted that it had intensified into a tropical storm, with convective bands wrapping around a well-defined low-level circulation center, and named it Ampil.

Afterward, it moved north-northeastward, primarily influenced by the strengthening low-level flow. A central dense overcast was obscuring the low-level circulation center; however, Ampil became distinctly better defined, featuring a well-defined center and spiral banding. The JMA upgraded the system to a severe tropical storm on August 13. Around 15:00 UTC on August 14, the JTWC reported that Ampil had intensified into a minimal typhoon, as animated infrared imagery indicated the development of a pinhole eye feature, which exhibited excellent equatorward outflow and good poleward outflow. The JMA then reported that Ampil had intensified into a typhoon due to warm sea surface temperatures and low vertical wind shear on August 15.

The JMA reported that Ampil reached its peak intensity at 12:00 UTC that day with 10-minute sustained winds of 155 km/h (100 mph) and a central pressure of 950 hPa (28.05 inHg). Ampil then turned northward, along the western periphery of a mid-level subtropical ridge. Ampil eventually peaked at Category 3-equivalent intensity on the Saffir-Simpson scale at 03:00 UTC on August 16, with 1-minute sustained winds of 205 km/h (125 mph) and a large eye measuring 74.1 km (46.0 mi) in diameter before making its closest approach to Japan. Operationally, the JTWC classified Ampil as a Category 4-equivalent cyclone, with winds of 215 km/h (130 mph). This made Ampil the first storm in the 21st century in the Western Pacific basin to reach the 34th parallel north while at or above Category 3-equivalent strength, with only two others having been recorded—Typhoon Carmen in 1965 and Typhoon Oscar in 1995. Additionally, the JTWC assessed the lowest barometric pressure as 939 hPa (27.73 inHg).

Ampil degraded due to internal fluctuations and a symmetric central dense overcast, while convection was confined to the southern semicircle. Satellite imagery depicted a well-defined 17 km (10 mi) wide eye surrounded by deep convection, though the banding diminished after encountering cold, dry air. Ampil was beginning to undergo an extratropical transition, as its convective structures had dissipated by August 17. The JTWC then ceased issuing advisories on the system the next day as it turned east-northeastward and merged with the mid-latitude westerlies. The JMA reported that Ampil had transitioned into an extratropical low on August 19. The extratropical storm entered the Bering Sea on August 20, crossed the Aleutian Islands, moved inland over the Russian Far East and Arctic Alaska, emerged into the Arctic Ocean between August 21–22, and dissipated on August 23.