Tropical Storm Sepat - Tropical Cyclone Advisory #4, 0000 UTC 6/24/25

…Sepat beginning to unravel…

Current storm information:

• Position: 29.4°N 140.9°E

• Movement: NW at 13 knots (25 km/h)

• Intensity: 35 knots (65 km/h)

• Central pressure: 1004 hPa

Trend:

• Weakening is forecast during the next 24 hours, and Sepat could weaken to a tropical depression later today.

Watches and warnings:

• No land-based watches or warnings are in effect.

Hazards affecting land:

• Interests in Honshu should monitor the progress of this system. U.S. military interests can get additional information from the Stars & Stripes Pacific Storm Tracker: https://www.stripes.com/theaters/asia_pacific/storm_tracker/

Discussion:

Sepat has fallen apart fast, a consequence of it beginning to lose its upper-level support. The tropical storm has lost virtually all of its banding structure, with just one skeletal band of convection loosely wrapping into an increasingly exposed circulation center. The convection is also not flaring over the center but rather to the south, which is suggestive of a system beginning to decouple in the face of increasing shear and subsidence. HYSAT scatterometer data suggests the wind field has shrunk quite a bit as well, with only a tiny area of 35 kt winds just east and south of the center. Dvorak estimates also range from T2.0 to T2.5, and so the intensity is lowered to 35 kts.

Sepat is not going to be able to hang on as a tropical cyclone for much longer. Although it remains over SSTs of about 26-27°C, the shear will only get stronger as Sepat accelerates northwestward to northward during the next 24 hours and runs headlong into a deep-layer trough over western Japan. This shear should make quick work of the rest of Sepat’s convection, and it will likely weaken to a tropical depression later today and degenerate into a remnant low soon after. The remnant low will probably stick around for a little while and race northward toward Honshu, but it will likely dissipate near that island on Wednesday.

Forecast positions and maximum winds

• 00 hrs: 29.4°N 140.9°E – 35 kts

• 12 hrs: 31.0°N 140.1°E – 25 kts

• 24 hrs: 32.3°N 139.7°E – 25 kts Post-tropical/remnant low

• 48 hrs: 34.5°N 140.5°E – 20 kts Post-tropical/remnant low

• 72 hrs: Dissipated