I have heard from various sources that the number of roof tops and blacktop roads creates a significant updraft of hot air pushing storms away from the area. If you have watched radar on the internet, storms frequently seem to split and go around TVs and reform on the backside. It takes a fairly storm rainstorm to disrupt the updraft and allow rain to hit the ground. It could also be raining but because of the hot air updraft, the rain evaporates before hitting the ground.
This is certainly an unscientific explanation, but it makes sense to me.
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