1. "Pending his availability" means that you are unsure of whether the person will be available for the meeting/event/get-together, so you cannot make plans until you get a definite answer from him as to whether or not he will be available to attend.
2. "then, there is the weather..." This is an idiomatic expression, which means that the weather also must be taken into account. Perhaps bad weather has been forecast. This expression means that the weather could pose another potential obstacle, over and above the questions concerning the invitee's availability.