You can usually say "is forthcoming", since "forthcoming" implies a future event already. Use "will be forthcoming" when the event may be further into the future, or you are really not sure when it will happen. Note: 'forthcoming' can mean "honest and open, straightforward", in this sense what is coming forth is one's self. Examples: "More news on the event is forthcoming." "At some point in Earth's future, another meteor disaster will be forthcoming." "They were forthcoming with their services." "She was a very forthcoming person."