Looking on the internet deeply has found these results:

it was raining heavily is the most popular phrase on the web.

it was heavily rainy. 11 results on the web	it was raining heavily 118,400 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
9 Eyl 2011 ... these notes into a novel� One night, it was heavily rainy and my notebook got soaked and it torn down. It is a pity that I lost my notes like that.	It was raining heavily as I was walking up the hill towards the station at six o'clock on a Saturday morning. At this early hour there wasn't much traffic and there�... It was raining heavily when I got up this morning. (CK); [S] � [T] We were late for school because it rained heavily. (CK); [S] � [T] It began to rain heavily more than �... It was raining heavily that day when my brother arrived. ... The first sentence mean, that it was raining heavily when the brother arrived. At 9pm it was raining heavily and we all huddled together like a rugby scrum, heads to the center to try and sleep. Equally, the device of raining down trash�...

it was heavily rainy.

11 results on the web

it was raining heavily

118,400 results on the web

More popular!

9 Eyl 2011 ... these notes into a novel� One night, it was heavily rainy and my notebook got soaked and it torn down. It is a pity that I lost my notes like that.

It was raining heavily as I was walking up the hill towards the station at six o'clock on a Saturday morning. At this early hour there wasn't much traffic and there�...
It was raining heavily when I got up this morning. (CK); [S] � [T] We were late for school because it rained heavily. (CK); [S] � [T] It began to rain heavily more than �...
It was raining heavily that day when my brother arrived. ... The first sentence mean, that it was raining heavily when the brother arrived.
At 9pm it was raining heavily and we all huddled together like a rugby scrum, heads to the center to try and sleep. Equally, the device of raining down trash�...