L’Eau,
God is omniscient because God knows everything that can be known. God does not know the future for certain, but can make very accurate predictions about it, because of God's complete knowledge of the present.
Stop digging! If this omniscient “God” of yours “has complete knowledge of the present” as you put it, then he knows every possible sub-sub-sub (down to bedrock) particle of matter and force there is, in which case he can also calculate exactly how they’ll interact and thus what they’ll produce in the future!
If you do want to get of this hook of your own making, you’ll have to drop either (or both) contradictory position: posit a god who knows quite a bit but isn’t omniscient at all, or posit a god who is omniscient and thus knows everything that has been, that is,
and that will be (in which case he has an awful lot to answer for).
Just now what you have is a bugger’s muddle – an “omniscient” god who’s actually just a sort of smart bookie with a stubby pencil and a pork pie hat: pretty good at working out the odds, but every now and then a rank outsider will come up on the rails and take him unawares.
Is “I believe in an omniscient god who can be surprised” really where you want to be?
Really though?