Afterward, encroaching dry air and shear caused the cyclone to begin weakening and turn extratropical, before it struck Japan on October 23 as a weaker typhoon.

[1] After the slow consolidation, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert to the elongated system early on October 14,[2] shortly after the Japan Meteorological Agency (JMA) started to monitor it as a low-pressure area.

[4][5] In the afternoon, the JTWC also upgraded it to a tropical depression assigning the designation 25W, which formative but shallow convective bands had become more organized, and symmetrically wrapped into a defined low-level circulation center.

[6] About three hours later, the JMA upgraded it to the twenty-first Northwest Pacific tropical storm in 2017 and assigned the international name Lan, when it was located approximately 310 km (190 mi) to the northeast of Palau.

[7] Early on October 16, the JTWC upgraded Lan to a tropical storm based on T-number 2.5 of the Dvorak technique,[8] shortly before it entered the Philippine Area of Responsibility and received the name Paolo from PAGASA.

[9] In an area of low to moderate low vertical wind shear, convection over Lan's center was occasionally displaced, but strong poleward outflow enhanced by a tropical upper tropospheric trough (TUTT) as well as sea surface temperature (SST) over 30°C with high ocean heat content (OHC) contributed to the intensification,[10] resulting in being upgraded to a severe tropical storm by the JMA at around 00:00 UTC on October 17.

[19] Early on October 21, when Lan accelerated north-northeastward along the western periphery of the deep layered subtropical ridge, the JTWC reported that its one-minute maximum sustained winds reached 250 km/h (155 mph), a high-end category 4 of the Saffir–Simpson scale, ranging from T6.5 to T7.0 of the Dvorak technique.

[21] For increasing vertical wind shear, Lan began to weaken and undergo the extratropical transition early on October 22 with significant erosion of its eyewall, after maintaining the super typhoon status as well as peak intensity for over one day.

Despite excellent poleward outflow tapping into the mid-latitude westerlies over Japan, the satellite imageries revealed cold-air stratocumulus streaming southward over the western semi-circle of the typhoon, which was associated with advection of cooler, drier air.

[27] Lan entered the Pacific Ocean shortly before 09:00 JST (00:00 UTC) and continued accelerating northeastward within the westerlies, displaying a well-defined frontal structure with an exposed, broad center and rapidly decaying deep convection sheared to the northeast.