https://phys.org/news/2018-08-nasa-formation-eastern-pacific-tropical.html

Coldest cloud tops were near minus 70 degrees Fahrenheit (minus 56.6 degrees Celsius). Aqua observed fragmented bands of thunderstorms circling a low-level center of circulation and some stronger storms within them.

Storms with cloud top temperatures that cold have the capability to produce heavy rainfall. That heavy rain was falling over open ocean waters.