Get a pelican case for the laptop. Watertight panniers or not, that laptop will be dry. I do not carry a laptop but the pelican cases are great for most high tech gear (DSLRs, etc). I usually wimp out in pouring rain but am debating fenders on the new bike (Salsa Vaya) to make things less messy. Plus, my commute is nowhere near as long as yours...~15-20min. When I ride in the rain though, I do as others above have stated--wear something water resistant to have some breathability.