Early the following day, Cosme attained its peak intensity as a Category 1 hurricane on the Saffir–Simpson scale with maximum sustained winds of 85 mph (140 km/h) and a minimum barometric pressure of 980 mbar (hPa; 28.94 inHg).

However, Cosme then began to encounter stable air and lower sea surface temperatures, causing the system to weaken to a tropical storm late on June 26.

The system continued to weaken and degenerated into a remnant low pressure surface trough about 690 mi (1,110 km) west-southwest of Cabo San Lucas, Mexico, on June 27.

[2] During the morning hours of June 20, the National Hurricane Center (NHC) began monitoring a broad area of disturbed weather several hundred miles southeast of Acapulco, Mexico.

[3] Assessed with a low chance of tropical cyclone formation within a two-day interval, conditions at the time were unfavorable but expected to become more conducive for development over subsequent days.

Following organizational improvement on satellite and microwave images,[6] the system became Tropical Depression Three-E at 12:00 UTC on June 23 while located about 500 mi (800 km) south of Manzanillo, Colima.

"[7] The depression did not become better organized initially, with meager central convection and a lack of well-defined cloud features; by early on June 24, however, shower and thunderstorm activity began forming over the center.

A large area of very cold cloud-tops developed over and just southeast of the center of the storm, and convective banding wrapping into the mass began to form, mainly in the north and east semicircles.

[10] Though the cloud pattern became elongated north to south during the pre-dawn hours of the following day,[11] a rapid improvement of the inner core on microwave and the structure on satellite imagery became apparent.

A timely AMSU pass indicated an eyewall nearly closed with an accompanying intensity estimate of 77 mph (124 km/h), and a transient eye became visible on early morning satellite images.

[13] Around 00:00 UTC on June 26, Cosme attained its peak intensity with maximum sustained winds of 85 mph (140 km/h) and a minimum barometric pressure of 980 mbar (hPa; 28.94 inHg).

[16] At approximately 12:00 UTC on June 27, Cosme degenerated into a remnant low pressure area while situated about 690 mi (1,110 km) west-southwest of Cabo San Lucas, Baja California Sur.

[19] When Cosme became a tropical storm on June 24, the Mexican National Weather Service (Servicio Meteorologico Nacional NWS) reported that it could produce heavy rains in the western area of the country.

The Mexican NWS also that "because of its wide circulation, [Cosme] generates significant potential for landslides and intense rain...waves 4 to 5 meters high and strong gusts of wind along the coast.