I had something similar with a flood this past weekend, @Pippa13. My cancellation policy is flexible, just because weather does play a part here. Two of my bookings rescheduled, one cancelled citing extenuating circumstances - which I proved with photos - but the fourth decided to come and assess, like yours. The water had fallen by Sunday, but the grounds were very sloppy - it had been river bottom the day before. I promised a credit for a future stay if they decided to go home, since we were well past the point of no return regarding a refund even with a flexible policy. (They did stay, and had fun combing the riverbank for flood flotsam.)
I err on the side of generosity when weather is involved and the guest communicates. I think you have to. If yours don't arrive, they will just use the extenuating circumstances clause, and will be annoyed with you because they had to do that. If you provide a refund, they will be happy. Either way you don't get paid, but the latter makes for less stress.
That said, if they offer some sort of split, take it!
I hope they come and all is well.