That seems very possible. When budgies get stressed/scared, their poops can be very watery. It happens to our Penry on occasion. The fact that Bertie's droppings normalized makes me think there was a stressful period when the storm was happening that made his poop watery. If you think about it, this happens to humans too-- when humans are anxious/nervous/stressed it can be reflected in their poop!
Sometimes, I even think Penry will get a little over-excited and drop a poop that way.
If he sees a bath, or gets let out of his cage after a while inside, he will do a funny little dance and poop immediately. Crazy birds!