JaccoW

I'll echo what others have said here; layers and a waterproof jacket with some mechanical ventilation in the form of pit zips.

I prefer the full lenght hiker type that sort of starts at your lower rib, all the way up to your armpits and down to you biceps. Jackets that only go to just below my armpits are still too hot whereas bigger zips can give you a lot more flexibility in what part you ventilate.

So less:

Decent but not great ventilation

And more:

Much better ventilation, or even the option to wear it as a bodywarmer when needed.

Ultralightweight jackets, especially wind breakers are a great addition to carry when you just need some protection from the wind but no insulation. Which is more often than you think.

As for legs, I have several waterproof trousers, some from Gore, some from other brands but unless I'm riding all day in the cold pouring rain I would rather wear a quick-drying pair of trousers or just add a pair of Rainlegs to just shield from most of the cold and wetness.

P.s. Don't forget that a longer back is glorious in the rain.