Were you able to predict emergence time by observing the colour of the imago through the pupal skin. I was able to do that some years ago with a Painted Lady here in the UK and then watched it for several hours to catch the actual emergence, which occurred just as I sat down to eat my lunch. I was amazed at how quickly it took place.
I take it you are on their migration route and that they breed locally? I have been fortunate to be able to see Monarchs here in the UK on a small number of occasions, the SW of England is well placed to receive any that make landfall after being blown across the Atlantic, an amazing occurrence and with their distinctive flight easy to spot as the flap past, once seen never forgotten, thanks for posting that.