The rain is a good daytime companion
When it rains, humans are usually much less interested in being outside, which means they'll be less interested in urban animals and our activities. Mild storms make for great cover.
This is the perfect opportunity to move around during the day. This can sometimes be a better time to find food that you don't normally come across, since some human places that have food are only open during the daytime.
A rainstorm comes with many dangers, whether during the day or night. Humans in cars can't see you as well, and may not even attempt to avoid you on the road. The sewers fill up with water, making a useful hiding place into a death trap.
The clever fox knows that rain is a clean source of water in the city, and will seek out places where it collects, such as on parked cars, in garbage cans, and within the holes in damaged roads.