If it is that warm I would stick with nickers and a nice breathable shell.

Add a fender and it will keep your butt dry and happy. REI sells a few tights that have water resistant front panels down to the knees.

Add a good pair of rain booties and your GTG.

In PDX the rain comes down and it is 40 deg or colder. So I wear a Mountain Hardware Dragon Jacket with an old pair of snow pants and a good set of booties. I throw some underarmor underneath and some running tights and ride captain ride. You will warm up. Does this keep my desert dry? No, but I am not soaked either. Yesterday we had a soaker. Rain drops the size of half dollars with pea sized hail. My bike shorts and underarmor shirt were dry, sweaty, but dry. My tights and socks were wet. I have a hole in my booties.