There is really nothing that unnatural with your own words (which is often the case here; people tend to overthink things):
Most of the roads on my route have accumulated rain water and it is raining heavily.
I might reverse the terms, because and can suggest a cause-and-effect relationship or a Step-1, Step-2 sequence:
It is raining heavily and (to my knowledge) most of the roads on my route have accumulated rain water.
If in the US, you could add:
According to the National Weather Service there are Flash Flood Warnings in the area.
And remind your client of the possible peril:
I really don't think it is safe for either you or me to risk driving over a low-lying area where there is water over the road
All the above are natural phrases for a telephone conversation.
Or just keep it simple:
It's raining hard and there's a lot of water on the roads (in my area). [It's not safe to drive.]