Tropical Depression (#97W) – Special Tropical Cyclone Advisory #1: 1800 UTC 5 September 2025

Tropical depression forms west of Luzon. Likely to bring heavy rainfall to Hong Kong and Macau early next week.

Author

Will Weaver
September 05, 2025

Current storm information:

  • Position: 17.6°N 117.6°E

  • Movement: W at 8 knots (15 km/h)

  • Intensity: 30 knots (55 km/h)

  • Central pressure: 1004 hPa

Trend:

  • Some gradual strengthening is forecast during the next 24 hours and the depression could become a tropical storm on Sunday.

Watches and warnings:

  • There are no land-based watches or warnings in effect.

Hazards affecting land:

  • Interests in southern China, including Hong Kong and Macau, should monitor the progress of the depression. It could produce periods of heavy rainfall in the region beginning early on Monday and continuing through midweek.

Discussion:

GPM GMI 89GHz microwave satellite image

The complex monsoonal area of low pressure that initially formed east of Luzon (invest 97W) has crossed Luzon and merged with another low off Luzon’s west coast (invest 96W). Since this merger, the low’s circulation has become better defined, and bands of convection have begun wrapping into it, although the convection is generally lopsided to the west of the center. Most of the convection is being sustained by the system’s interaction with a monsoon trough to the south. Scatterometer data from Oceansat-3 indicates that there is an area of 25 to 30 kt winds to the south of the center, as well. These data, as well as Dvorak estimates of T2.0 from KNES and RJTD, indicate the system has become a tropical depression, and its initial intensity is set to 30 kts.

The depression is currently located in a rather favorable environment of low shear and very warm waters, and so gradual strengthening appears likely. The depression should move generally west-northwest to northwestward under the influence of a relatively weak mid-level ridge positioned northeast of Taiwan which is also retrograding westward. TD 97W will likely become a tropical storm by Sunday morning. As 97W consolidates, it could begin intensifying at a faster rate beginning later on Sunday, and most of the model guidance brings 97W to severe tropical storm criteria before it makes landfall on Monday. As a result, the wind values given in the forecast below are higher than in the JTWC forecast. It should be noted that the forecast track may need to be adjusted somewhat to the west in future advisories as much of the model guidance - GFS, ECMWF, and Google DeepMind ensembles - suggest a track further to the west as the ridge builds westward.

Forecast positions and maximum winds

  • 00 hrs: 17.6°N 117.6°E – 30 kts

  • 12 hrs: 18.5°N 117.0°E – 30 kts

  • 24 hrs: 18.9°N 115.7°E – 40 kts

  • 48 hrs: 20.8°N 114.1°E – 50 kts

  • 72 hrs: 23.1°N 112.9°E – 25 kts inland

  • 96 hrs: Dissipated

JMA forecast map