TM, plan now, for rain. Put detailed plans in place. That uses Murphy's Law to actually reduce the likelihood of rain.
Now, what would be good rain plans?
Well, these are teen girls, right? They are not so predictable, they often like to do stuff that adults wouldn't do. And from my own experience of Aussie summer storms - dancing in the rain is a biggie. So prepare your play list. "It's raining Men" is a good start.
Put plans in place, girls like to dance in the rain down here, in summer. Bubbles are fun, a bucket of bubble mix with a few bubble wands, the kinds you wave around, goes down well. Just be careful of slips in the wet.
Water bombs are good. Set up teams and forts.
My favourite - sit a girl in a chair under a water-filled balloon. Blindfold another girl and the girl in the chair has to yell directions to her. Have multiple set-ups, maybe enough for all to play. The aim is to be the last seated girl getting wet (so you direct your blindfolded partner to other people's balloons!) You can use glitter instead of water if you prefer.
To keep girls dry if they really are wimps, use garbage bags as improvised raincoats. If you buy two different-coloured packets, you can have two teams for raining tug-o-war (across the pool, of course!). Cut a small hole in each corner of the base of the garbage bag (so you can stretch the bag into the arms) and a larger hole in the base for the head.
The more fun stuff in the rain that you plan, the less chance it will actually rain (Murphy's law).
And if the girls look dubious, tell them form me, in Australia our kids, especially our girls, dance in the rain in a summer storm. Just not if it's hailing. We wait under cover until the hail stops, because it stings.
Marg