You can put a rain jacket in the backpack you're carrying and you're ready for a rainy day. But if you need one that you can put on without taking off your backpack, have a look at the Funnell Eject | Wear Backpack and Jacket system.
It comes in two versions: the city version and the sports version for use its respective settings as the name implies. Once deployed the waterproof jacket also covers the backpack that it came from, protecting it from the rain.
Inside the storage compartment for the jacket, which is separate from the main compartment, is also waterproof so you can store the jacket even when it's wet. In a main compartment is a laptop holder and plenty of internal pockets.
$188 during its failed Kickstarter campaign.