What you want is a lightweight gore-tex (or equivalent) shell, sized to fit over your max clothing.

In warm weather, armpit zippers are a VERY big plus in helping make things cooler.  After having used them, I wouldn't buy any rain jacket, unless it was for the most short-term, casual use, without pit-zips.

Probably the easiest thing to do is buy a 1st gen USGI gore-tex pants and jacket, and clean them with the right stuff that will not only clean them, but remove any UV brighteners that some idiot added when washing with regular laundry detergent.  Besides that, the good stuff will clean the pores of the gore-tex, and allow them to work.  Google "Sport Wash", and have at it.

Trust me, if it's raining that bad, you will want the pants also.

Pay very close attention as to how the vendor describes the sizes offered, and when in doubt, go  at least  one size larger, since this will also have to fit over winter clothing, in a pinch. Most GI outerwear automatically comes sized larger than normal for this reason, bur just in case, make sure of return priveleges.

A good, re-habbed GI gore-tex outfit will cost WAY less than a new, civvy outfit, and probably outlast the civvy unit by years.

If on a budget, or going ultra-light, a contractor's trash bag, cut with arm-and head-holes will shed a lot of rain, just make sure to have something for your head, preferably very wide-brimmed, and with a chin-strap.  Your hat's no good to you if it blows away in the dark, and goes over a cliff.