An area of disturbed weather that would later become Tropical Storm Luke was first noted by the Joint Typhoon Warning Center (JTWC) early on September 13.
The disturbance was upgraded into a tropical depression by the Japan Meteorological Agency (JMA) the following morning.
As the disturbance tracked west-northwestward, passing near Saipan, improved upper-level anticyclonic outflow and falling surface pressures led to the issuance of a Tropical Cyclone Formation Alert at 11:30 UTC on September 14.
At 18:00 UTC, the JTWC designated the system Tropical Depression 20W, based on synoptic data that indicated the disturbance developed a closed surface circulation.
At the time of the upgrade, the depression's structure resembled that of a monsoon depression, with a ring of 55 km/h (35 mph) winds surrounding a large area of light and variable winds, indicating a broad and ill-defined circulation.
Early on September 15, the JTWC upgraded the depression into Tropical Storm Luke, with the JMA following suit six hours later.

The cyclone continued to slowly intensity over the next 48 hours as it tracked west-northwestward.
On September 17, satellite imagery indicated that the circulation had become less defined and the overall system had lost organization.
Despite this, the JTWC increased the intensity of the storm to 95 km/h (60 mph), its peak intensity.
Meanwhile, the JMA upgraded Luke into a severe tropical storm.
At 18:00 UTC, the JMA estimated that Luke reached its peak intensity of 105 km/h (65 mph).
Luke turned north-northwestward, only to make a turn to the east as a midtropospheric trough caused the subtropical ridge, which had been holding the system to a westward track, to recede eastward.
Meanwhile, the vertical wind shear caused by westerlies increased, causing Luke to transition into an extratropical cyclone.
The JTWC issued its final warning at noon on September 19, with the JMA declaring Luke extratropical that evening.
It was last noted late on September 21.
An area of disturbed weather that would later become Tropical Storm Luke was first noted by the Joint Typhoon Warning Center (JTWC) early on September 13. The disturbance was upgraded into a tropical depression by the Japan Meteorological Agency (JMA) the following morning. As the disturbance tracked west-northwestward, passing near Saipan, improved upper-level anticyclonic outflow and falling surface pressures led to the issuance of a Tropical Cyclone Formation Alert at 11:30 UTC on September 14. At 18:00 UTC, the JTWC designated the system Tropical Depression 20W, based on synoptic data that indicated the disturbance developed a closed surface circulation. At the time of the upgrade, the depression's structure resembled that of a monsoon depression, with a ring of 55 km/h (35 mph) winds surrounding a large area of light and variable winds, indicating a broad and ill-defined circulation. Early on September 15, the JTWC upgraded the depression into Tropical Storm Luke, with the JMA following suit six hours later.
The cyclone continued to slowly intensity over the next 48 hours as it tracked west-northwestward. On September 17, satellite imagery indicated that the circulation had become less defined and the overall system had lost organization. Despite this, the JTWC increased the intensity of the storm to 95 km/h (60 mph), its peak intensity. Meanwhile, the JMA upgraded Luke into a severe tropical storm. At 18:00 UTC, the JMA estimated that Luke reached its peak intensity of 105 km/h (65 mph). Luke turned north-northwestward, only to make a turn to the east as a midtropospheric trough caused the subtropical ridge, which had been holding the system to a westward track, to recede eastward. Meanwhile, the vertical wind shear caused by westerlies increased, causing Luke to transition into an extratropical cyclone. The JTWC issued its final warning at noon on September 19, with the JMA declaring Luke extratropical that evening. It was last noted late on September 21.
Tropical Storm Luke (1991)