Believe it or not, the decision to cancel a trip due to rain in the forecast can sometimes be a hard one. There is a lot to consider - the size of the drainage, the ability to get out of the way if rain/flooding threatens, the likelyhood that it will rain at all, the ability of the group, etc. We are always conservative with this decision, for obvious reasons.
Today, we're psyched that we cancelled and just drove around checking out waterfalls instead! Rain in the desert is such a special thing.