If you guys are really wanting to hit PD Canyon, go for it. Just have access to AM radio at least, and tune 710am out of Amarillo for weather reports.
The typical way the storms come through is to build up with afternoon heating and blow through from the SSW to NNE. So your mornings ought to be dry, and the storms over by 9pm or so. So keep your head up, don't get into bad situations, you'll probably be fine.
If a flash flood is coming, the rangers will roust you out to higher ground.
Obviously, there is no way to predict, judge for yourself, etc. etc.
If it rains in the afternoon, drive into Canyon and visit the
Panhandle-Plains Historical Museum, a really fine museum. Or eat at Fat Boys BBQ, Feldman's Wrong Way Diner, Pepitos Mexican, or our personal favorite, Sayakormarns. It's on the square downtown, and the best Thai food you'll find anywhere. Really big city quality in a hole-in-the-wall.
Also can't recommend personally, as I have yet to go, but
Palace Coffee, also on the square, has a really good reputation. They recently won some sort of coffee shop competiton that makes them one of the 10 best in the nation.
To whet your appetite, check out the
Palo Duro Canyon Photo ContestThere are links on that page to the 2013 winners, as well as the 2012. I've got one in the 2012 contest.