What is the English term for gran evento pluvial?

The English term is “pluvial event” ; in geology, “pluvial episode” is also used, and a specific well-known example is the Carnian pluvial event.

Meaning

• Pluvial means related to unusually heavy or prolonged rainfall.
• So gran evento pluvial translates most naturally as major pluvial event or major rainfall event.
• If you mean a scientific/climate context, pluvial event is the most precise term.

Natural options

• Major pluvial event.
• Major rainfall event.
• Heavy rain event.
• Pluvial episode in a historical or geological context.

Best choice

For a general translation, use major rainfall event.
For a technical or scientific context, use pluvial event.

TL;DR: “gran evento pluvial” = major rainfall event ; in technical English, pluvial event.