Complex sentence: 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.
Split into simpler sentences: 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.