The United States Naval Research Laboratory (NRL) initially mentioned a tropical disturbance over Chuuk on October 11.
After the slow consolidation, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert to the elongated system early on October 14, shortly after the Japan Meteorological Agency (JMA) started to monitor it as a low-pressure area.
The agency upgraded it to a tropical depression almost one day later and began to issue tropical cyclone warnings since 06:00 UTC on October 15.
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.
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.
Early on October 16, the JTWC upgraded Lan to a tropical storm too, based on T-number 2.5 of the Dvorak technique, shortly before it entered the Philippine Area of Responsibility and received the name Paolo from PAGASA.

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 30ºC with high ocean heat content (OHC) contributed to the intensification, resulting in being upgraded to a severe tropical storm by the JMA at around 00:00 UTC on October 17.
Soon, the westward system became quasi-stationary due to a weakening subtropical ridge to the north.
When Lan developed into the ninth typhoon of 2017 at around 18:00 UTC, it had turned slowly northward under the steering influence of a building steering ridge to the southeast and east.
Early on the next day, the JTWC also upgraded it to a typhoon.
although it later became partially exposed for a half of day.
At 12:00 UTC on October 18, the JMA reported that Lan had become a large typhoon, with a diameter of about 1,310 km (815 mi).
Despite favorable conditions, Lan had struggled to intensify for two days, even while growing larger in diameter.

Lan started to rapidly intensify thanks to excellent outflow, particularly poleward, which gradually formed a large and symmetric eye with a diameter of 60 mi (97 km).
The JMA indicated that the typhoon had reached its peak intensity at around 18:00 UTC, with the central pressure at 915 hPa (27.02 inHg) and ten-minute maximum sustained winds at 185 km/h (115 mph); the JTWC also reported that Lan had intensified into a super typhoon at the same time.
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.
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.
As the result, the once large eye was quickly filled, and Lan was exhibiting frontal characteristics.

At 03:00 JST on October 23 (18:00 UTC on October 22), Lan made landfall over Omaezaki, Shizuoka Prefecture in Japan, with ten-minute maximum sustained winds at 150 km/h (90 mph) and one-minute maximum sustained winds at 165 km/h (105 mph), equivalent to a Category 2 hurricane.
At that time, its diameter of gale winds had expanded to approximately 1,700 km (1,055 mi).
Three hours later, Lan weakened to a severe tropical storm.
The United States Naval Research Laboratory (NRL) initially mentioned a tropical disturbance over Chuuk on October 11. After the slow consolidation, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert to the elongated system early on October 14, shortly after the Japan Meteorological Agency (JMA) started to monitor it as a low-pressure area. The agency upgraded it to a tropical depression almost one day later and began to issue tropical cyclone warnings since 06:00 UTC on October 15. 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. 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. Early on October 16, the JTWC upgraded Lan to a tropical storm too, based on T-number 2.5 of the Dvorak technique, shortly before it entered the Philippine Area of Responsibility and received the name Paolo from PAGASA.
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 30ºC with high ocean heat content (OHC) contributed to the intensification, resulting in being upgraded to a severe tropical storm by the JMA at around 00:00 UTC on October 17. Soon, the westward system became quasi-stationary due to a weakening subtropical ridge to the north. When Lan developed into the ninth typhoon of 2017 at around 18:00 UTC, it had turned slowly northward under the steering influence of a building steering ridge to the southeast and east. Early on the next day, the JTWC also upgraded it to a typhoon. although it later became partially exposed for a half of day. At 12:00 UTC on October 18, the JMA reported that Lan had become a large typhoon, with a diameter of about 1,310 km (815 mi). Despite favorable conditions, Lan had struggled to intensify for two days, even while growing larger in diameter.
Lan started to rapidly intensify thanks to excellent outflow, particularly poleward, which gradually formed a large and symmetric eye with a diameter of 60 mi (97 km). The JMA indicated that the typhoon had reached its peak intensity at around 18:00 UTC, with the central pressure at 915 hPa (27.02 inHg) and ten-minute maximum sustained winds at 185 km/h (115 mph); the JTWC also reported that Lan had intensified into a super typhoon at the same time. 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. 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. As the result, the once large eye was quickly filled, and Lan was exhibiting frontal characteristics.
At 03:00 JST on October 23 (18:00 UTC on October 22), Lan made landfall over Omaezaki, Shizuoka Prefecture in Japan, with ten-minute maximum sustained winds at 150 km/h (90 mph) and one-minute maximum sustained winds at 165 km/h (105 mph), equivalent to a Category 2 hurricane. At that time, its diameter of gale winds had expanded to approximately 1,700 km (1,055 mi). Three hours later, Lan weakened to a severe tropical storm.
Typhoon Lan