The storm prudently strengthened on September 22, and the following morning, attained peak intensity following a pressure drop of 100 mbar (3.0 inHg) in slightly less than 24 hours.

Forrest then moved north, impaling the Japanese archipelago before transitioning into an extratropical cyclone on September 28, before eventually dissipating on October 4.

Hurricane hunters investigated the system four times from September 17–20, though none of them were able to identify a closed atmospheric circulation.

[2][nb 1] During the evening hours of September 20, the JTWC started issuing warnings on the system after the low developed a central dense overcast.

Initially, only gradually strengthening was expected by the JTWC, but this did not occur and by the morning hours of September 21, Hurricane Hunters measured winds of 95 to 115 km/h (60 to 70 mph).

[2] By 1800 UTC that day, both the JTWC and the JMA upgraded Forrest to typhoon status as the storm developed an eye.

[1][2] After moving away from Guam, Forrest continued deepening;[2] by the evening hours of September 21, Hurricane Hunter data indicated a minimum barometric pressure of 976 mbar (28.8 inHg).

A mere 11 hours later, however, the aircraft reported a pressure of 926 mbar (27.3 inHg), which prompted the JTWC to increase the intensity of the cyclone to 225 km/h (140 mph).

[4] This marked the end of the fastest pressure drop ever recorded by a tropical cyclone—100 mb (3.0 inHg) in just under 24 hours.

[4] After a brief turn towards the west-northwest, the JMA reported that Forrest attained its peak intensity at 0000 UTC on September 23, with winds of 205 km/h (125 mph) and a minimum central pressure of 885 mbar (26.1 inHg).

[2] By this time, Forrest was moving northwest, and the JTWC expected the storm to re-curve due to a weak spot in the subtropical ridge.

[1][2] Thereafter, several ships reported storm and gale force winds while the system moved towards the east-northeast as it approached southwest Alaska.

[1] While weakening and passing 200 km (125 mi) southwest of Okinawa, gusty winds and heavy rains were recorded.

[9] When the storm posed a threat to Kyushu, five ships were evacuated to an air force base that was considered "safe typhoon haven" by the JTWC.