I do agree with @OLd Brixtonian. I just wanted to add that your sentence may mean different things depending on whether you use the article a or not.

The water level in the lake is much higher after a heavy rain [particular instance or after every heavy rain].

The water level in the lake is much higher after heavy rain. [every time it rains heavily, does not refer to a particular instance]

Compare:

Swimmers are being warned to stay out of the sea for at least 48 hours after heavy rain, due to pollution fears. (Newstalk)

and

The Met Office has warned there could be some damage to property and travel disruption in the event of a heavy rain. (express.co.uk)

Usually, heavy rain (without a) is used more commonly in weather forecasts.

I do agree with @OLd Brixtonian. I just wanted to add that your sentence may mean different things depending on whether you use the article a or not. And there are many contexts where uses with or without the article are interchangeable.

The water level in the lake is much higher after a heavy rain [particular instance or after every heavy rain].

The water level in the lake is much higher after heavy rain. [every time it rains heavily, does not refer to a particular instance]

Compare:

Swimmers are being warned to stay out of the sea for at least 48 hours after heavy rain, due to pollution fears. (Newstalk)

and

The Met Office has warned there could be some damage to property and travel disruption in the event of a heavy rain. (express.co.uk)

Usually, heavy rain (without a) is used more commonly in weather forecasts.