I agree with the 'don't bother with a brolly' view, to be honest always carried one and it took up loads of space in my 2 rod quiver so instead I bought a fully water proof jacket with an optional fleece.
If I do know it's going to be raining heavily all day I'll chuck a golf type brolly in green of course
in the rod bag but usually I can shelter in heavy down pours or just ride it out.
I've often caught soon after a heavy down pour and think it sometimes livens a swim up especially if there is lots of trees around the rain must wash lots of goodies in for the fish to investigate