Tropical Storm Co-May (#11W/Emong) – Tropical Cyclone Advisory #2: 1200 UTC 23 July 2025

Tropical Storm Co-May forms west of Luzon and is undergoing rapid intensification. Likely to bring strong tropical storm or typhoon conditions to Luzon on Friday. This is a particularly dangerous situation!

Author

Will Weaver
July 23, 2025

…Tropical Storm Co-May forms west of Luzon and is undergoing rapid intensification… …Likely to bring strong tropical storm or typhoon conditions to the northwestern coastline of Luzon on Friday…

…This is a particularly dangerous situation…

Himawari-9 infrared satellite image (Weathernerds)

Current storm information:

  • Position: 17.7°N 118.8°E

  • Movement: W at 8 knots (15 km/h)

  • Intensity: 40 knots (75 km/h)

  • Central pressure: 992 hPa

Trend:

  • Steady to rapid strengthening is likely during the next 24 hours, and Co-May could become a typhoon on Thursday.

Watches and warnings:

  • TCWS #1 (Philippines): Ilocos Norte, Apayao, Abra, Ilocos Sur, La Union, Benguet, and northern and western Pangasinan. Wind signals could be required for additional portions of northern and western Luzon on Thursday.

PAGASA wind signal map (click to enlarge)

Hazards affecting land:

  • Co-May, along with the large monsoonal system it is embedded in, is expected to produce extremely heavy rainfall over much of Luzon and Mindoro, particularly along the western coasts. Many of these areas have already seen rainfall of up to 300 mm. Additional rainfall of up to 500 mm is possible through Saturday before the monsoon gyre finally breaks down, and this rainfall is expected to cause widespread severe to catastrophic flooding. In addition, Co-May itself is expected to strengthen rapidly and bring tropical storm or typhoon conditions to portions of northwestern Luzon. This is a particularly dangerous and life-threatening situation! For additional information, including rainfall and flood warnings, refer to Weather Advisories and Severe Weather Bulletins from PAGASA or products from your local weather office.

Discussion:

Himawari-9 infrared satellite image (Weathernerds). Note the eye feature in the center of the main convective mass.

Tropical Depression 11W, now Tropical Storm Co-May, appears to be undergoing rapid intensification off the western coast of Luzon. Conventional and microwave satellite imagery indicates that the circulation of Co-May is spinning up quite fast, and a pinhole eye feature is already evident on 89GHz microwave imagery and clearing out on infrared satellite imagery. The system remains very compact, which likely explains how it is intensifying so quickly. The central dense overcast remains somewhat fragmented, but bursts of intense convection are continuing to flare in all quadrants, with a particularly strong burst wrapping its way around the center. The intensity is increased to 40 kts, but this is likely very conservative due to the fact that the Dvorak technique struggles with smaller scale systems, although the latest Dvorak estimates range from T2.5 to T3.0 from PGTW, KNES, and RJTD, and CIMSS ADT estimated 47 kts.

F-16 SSMI/S 91GHz microwave satellite image. Note the tight spiral of convection wrapping into the circulation, with a faint pinhole eye feature evident at the center

Co-May’s development trends are concerning. Before landfall, the models do not show Co-May getting very strong, with the GFS only bringing it up to about 50 kts or so. However, CIMSS AI-RI and RAMMB RIPA rapid intensification guidance indicates high probabilities of rapid intensification - RIPA shows a 44.8% chance of an increase of 55 kts during the next 36 hours while AI-RI shows a 59.8% chance of an increase of 45 kts during the same time period. Given that Co-May is already clearing out an eye and the environment is forecast to be quite favorable for rapid intensification up until landfall on Friday, it is very likely that Co-May will be a typhoon by the time it makes landfall, and the winds given below are higher than what are listed in the JTWC and JMA forecasts. After landfall, Co-May should weaken rapidly due to the rough terrain and increasing shear, and most of the model guidance decays Co-May into a trough east of the Batanes Islands by Saturday as it becomes reabsorbed into the monsoon gyre.

The track forecast has been nudged slightly to the west of the previous one, mainly due to what will likely be stronger southwesterly wind flow courtesy of Tropical Storm Francisco to the northeast, and this is in line with the model guidance. It should be noted that confidence is fairly low as the global models (GFS and ECMWF) are having trouble resolving Co-May’s small circulation, and show much less strengthening than indicated below as a result. However, the track still brings a typhoon over northwestern Luzon near Vigan overnight Thursday into Friday and over areas that have already seen extremely heavy rainfall this week. This is a dangerous and life-threatening situation and people in those areas need to take every necessary precaution.

Forecast positions and maximum winds

  • 00 hrs: 17.7°N 118.8°E – 40 kts

  • 12 hrs: 17.0°N 118.6°E – 60 kts

  • 24 hrs: 17.0°N 119.4°E – 80 kts

  • 36 hrs: 18.2°N 120.7°E – 50 kts inland

  • 48 hrs: 20.8°N 122.7°E – 35 kts 

  • 72 hrs: Absorbed by large monsoonal system

JTWC forecast map