Warning, don't visit these canyons if there's rain clouds to the east, anywhere in the sky.  As you can see in one of the pictures below the Navajo Nation accepts no responsibility for injury or death from flash flood.  They clearly post this warning at the entrance of the Lower Antelope parking lot so you can't miss it.  They are serious and you are on their land, under Navajo Nation law.  People have drowned in this canyon years ago by flash flood, so be aware of the danger and limit your stay to 30 min.  This is plenty of time to see the canyon and take loads of pictures while keeping things safe from incoming storms.  And don't be fooled, storms can occur any time of year, including summer.