Well, to keep you dry in a really heavy downpour that lasts all day long, you’d probably need a jacket with a hydrostatic head rating somewhere over 30,000mm
…it’s also need to have a breathability rating similar to that (if not higher) to cope with the amount of sweat you’ll produce while riding, otherwise you’ll get wet from the inside out
…of course, it’ll also need to have a good/strong enough DWR treatment on the outside of it to keep the rain beading, rather than soaking into the face fabric for the whole time you’re out – otherwise the outer fabric will get saturated, creating another layer on the jacket, stopping your sweat from escaping, thus leaving you soaked from the inside out again
Hmm
good luck finding that! 😆