If you'd've told me a couple of years ago that I'd be able to write a synopsis before writing an entire novel, I would have laughed in your face and scoffed, 'Inconceivable!'
If you'd've told me a couple of years ago that I'd actually be following that synopsis, pulling it out every few scenes to check it and make sure I was still on track, I'd have laughed again. 'Inconceivable!'
And if you'd've told me I'd be walking down the street thinking to myself, in these exact words, 'Well, I've finished Act 1 at the end of chapter three where the Inciting Event happens, which is a bit late but never mind, and now I've written two of the three Turning Points so all I have left is one more Turning Point and then I can write the Climax!' I would have done more than laughed; I would have guffawed in a very loud and raucous manner and shouted, 'INCONCEIVABLE!'
But we all know what happens when you shout something like that. Inigo Montoya (or indeed even @IamInigoMontoya) pops up and says:
'You keep using that word. I do not think it means what you think it means.'
Turns out, it doesn't.
I've discovered that writing a synopsis before writing the book isn't impossible for me. I have to do some legwork and thinking beforehand, and I might have to come up with several unworkable drafts first. But I can do it.
And because of my knowledge of story structure—where the acts are in my writing, where the turning points are, how I follow a natural progression—I can feel more confident that I know where this book I'm writing right now is going. I don't know exactly how the story fits together, scene by scene, but I do know the type of scene I need at any given point. And that makes brainstorming easier, it makes my characters clearer, it makes dealing with subplots much simpler.
Usually at the midpoint of a book, I reach my 'suckage point'. I don't know where I'm going, I don't know whether what I've written is any good at all, I am pretty sure I should jack it all in and become a plumber or something.
I haven't hit a suckage point with this book. I've hit some places where I need to think, but no suckage point. Not yet. And because the book's structure is sound, because I tested this synopsis over and over again, I suspect—I don't know, but I suspect—that I might not have a proper suckage point this time round.
But even if I do, I've discovered that my process isn't as written in stone as I thought it might be. I've discovered that maybe, if I need to, I can try different ways of working, even to the extent of becoming a bit of a planner instead of flying by the seat of my pants.
Maybe, it's not quite inconceivable.