"heavy downpour" is a perfectly valid phrase that is used in written English. It is used to refer to a large amount of rain that comes down in a short period of time, usually accompanied by thunder and lightning. For example, "The city was hit with a heavy downpour in the middle of the night, and the streets were flooded in no time.".
A heavy downpour?
An astonishingly heavy downpour flattened the seas.
A heavy downpour again fell on Dhaka.
Prepare for a heavy downpour of righteous rock'n'roll.
Williams called Costolo, who was driving home from the airport through a heavy downpour.
When a heavy downpour suddenly started, many listeners held their ground.
It may be "raining iron, first a light patter, then a heavy downpour".
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK