It's misty this morning, the kind of mid-May "rain" that darkens the ground and makes the air smell flowery and forest-y. And does nothing to moisten the ground. Even last evening's solid rain did nothing more than wet the surface.
But this is an ideal time to water. The drops sink in more readily, the humidity is maxed, and you take advantage of whatever moisture is already there. Just before, during or just after a rain are all excellent.
Never assume a late spring or summer rain does anything for the roots but piggyback your own time with a hose or sprinkler for maximum effect.