the resident seattlite can chime in to affirm Psimet's opinions... (except that we are so used to racing with snorkels.and.masks that we don't worry about needing clean air.)

Right as stuff gets wet it will be the slickest as all the dust/oil/grime will still be on the road.
Once it has been raining for a while, all those bits will get washed away and the road will be fine. There's not that much traction difference as long as you have good tyres and you avoid paint/manholecovers/etc