Some areas of the state are dry and one farmer was asking if he should leave off the Dual Magnum and Valor because rain was not projected over the next five days.  It is indeed a guessing game.  I think it is better to continue with a proven weed management approach (including a PRE treatment) and not try to out guess the rain.  Even if the herbicide sits on the surface for a while you will get some out of it,  Of course the longer it sits the less control you get. The alternative is not applying a PRE and then getting the surprising rain.  You will have a full crop of weeds at that point to deal with.