Tropical Storm Co-May (11W) – Tropical Cyclone Advisory #9: 1500 UTC 27 July 2025

Co-May regenerates into a tropical storm east of Okinawa.

Author

Will Weaver
July 27, 2025

…Co-May regenerates into a tropical storm east of Okinawa…

Himawari-9 infrared satellite image (CyclonicWx)

Current storm information:

  • Position: 26.1°N 129.6°E

  • Movement: ESE at 9 knots (15 km/h)

  • Intensity: 40 knots (75 km/h)

  • Central pressure: 992 hPa

Trend:

  • Some strengthening is possible during the next 24 hours.

Watches and warnings:

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

Hazards affecting land:

  • Co-May is likely to produce heavy rainfall and gusty winds over portions of the Ryukyu Islands, including Okinawa, beginning on Monday. U.S. military interests in Okinawa can get additional information from the Stars & Stripes Pacific Storm Tracker: https://www.stripes.com/theaters/asia_pacific/storm_tracker/

Discussion:

Himawari-9 infrared satellite image (CyclonicWx)

Co-May managed to retain its identity after degenerating into a remnant low the other day. Interaction with the convergent wind flow of the monsoon trough caused the system to redevelop convection early on Saturday, and this convection has persisted. Meanwhile, Co-May’s circulation merged with another monsoonal low east of the Ryukyu Islands (invest 91W), and satellite imagery indicates that Co-May was the dominant circulation. Given that the convective structure has continued to improve since then and scatterometer data indicates that the system is producing 35 to 40 kt winds, Co-May has regenerated into a tropical storm, and its intensity is increased to 40 kts.

Synthetic 89 GHz microwave image, showing convection wrapping into Co-May’s center (RAMMB/CIRA)

Co-May is currently moving eastward within the monsoonal flow. As the monsoon gyre flattens out, Co-May should do a sharp u-turn and turn back toward the west-northwest on Monday under the influence of strong subtropical ridging to the north. The environment will likely be favorable for strengthening with low shear and very warm waters, and some strengthening does appear likely. In fact, CIMSS AI-RI and RAMMB RIPA rapid intensification guidance show rather high likelihoods of rapid intensification (RIPA gives a 56.4% chance of a 45 kt increase in the next 36 hrs). While the forecast currently shows more modest strengthening, RI cannot be ruled out. By Wednesday, Co-May should encounter cooler waters and increasing shear as it approaches Eastern China, and this should cause some weakening before landfall on Thursday.

Forecast positions and maximum winds

  • 00 hrs: 26.1°N 129.6°E – 40 kts

  • 12 hrs: 26.9°N 129.3°E – 55 kts

  • 24 hrs: 27.3°N 127.3°E – 65 kts

  • 48 hrs: 28.9°N 123.7°E – 65 kts

  • 72 hrs: 30.8°N 121.9°E – 50 kts

  • 96 hrs: 31.5°N 121.2°E – 35 kts inland

  • 120 hrs: 32.5°N 120.6°E – 25 kts inland

JTWC forecast map