[1][2] The developing system drifted eastward and acquired a surface circulation, which prompted the Joint Typhoon Warning Center (JTWC) to issue a Tropical Cyclone Formation Alert at 03:00 UTC on September 17.

However, deep convection, originally confined to the southern semicircle of the system, began to wrap around the eastern quadrant as an anticyclone developed aloft.

This improving presentation on weather satellite soon yielded the development of an 19 km (12 mi)-diameter eye around 00:00 UTC on September 20, prompting the JTWC to upgrade Bart to a typhoon at this time.

While Bart attained increased in intensity, one of the subtropical ridges to its north weakened, causing the system to become nearly stationary while it was positioned about 370 km (230 mi) southwest of Okinawa.

[1] A climatological rate of strengthening ensued as the cyclone slowly began to move northward late on September 21, and Bart reached winds of 240 km/h (150 mph) at 18:00 UTC that day, making it a super typhoon based on JTWC classification.

By 06:00 UTC on September 22,[2] with an eye that had grown to 39 km (24 mi) and excellent outflow in all directions,[1] Bart attained peak one-minute sustained winds of 260 km/h (160 mph), equivalent to a Category 5 hurricane on the Saffir–Simpson scale.

An approaching trough in the mid-latitudes and an intensifying ridge to its northeast caused the cyclone to accelerate in that direction, while microwave imagery depicted a newly formed, symmetric 74 km (46 mi) eye.

This movement brought Bart into the Sea of Japan to the west of Hiroshima by 00:00 UTC on September 24, around which time it further decayed into a tropical storm.

[1] The JTWC declared the system an extratropical cyclone at 15:00 UTC on September 24,[2] and the JMA followed suit over subsequent hours as the non-tropical low moved into the Sea of Okhotsk east of northern Hokkaido.

[2] As Bart overspread much of Japan and combined with a cold front,[4] it yielded numerous reports of rainfall accumulations in excess of 510 mm (20 in).

[6] The cyclone also produced numerous tropical storm- or typhoon-force winds across the island, peaking at 151 km/h (94 mph) across Seto in the Ehime Prefecture.

[13] In Okinawa, downed trees, flooded roads, and overturned cars were prevalent, forcing the U.S. military to restrict personnel and their families to their homes.

In the Kumamoto Prefecture, more than 30 police officers were sent to Shiranui,[19] conducting search and rescue in homes that were flooded to their rooftops by a storm surge up to 3.5 m (11 ft) that funneled into Yatsushiro Sea.

The presence of high convective available potential energy and strong wind shear favored the development of supercell thunderstorms across Japan on September 23–24.