Until you see the rain coming down, there's always a chance it will dissipate before it gets there. I've seen it happen time after time - don't forget that the green blobs are computer-generated composites and don't always show reality. I been at races that were run under a green blob all weekend long and no moisture was hitting the ground - I've also been at races where the radar was clear and it was raining hard enough to shut the race down.
So the rule is, "Look out the window. If it's not wet, it isn't raining."