(1) When the skies are grey, and there's that annoying rain which feels like spittle on your face. You feel like a fool for having an umbrella up because it isn't helping.
(2) When a situation is really bumming you out.
Grim + dismal = grismal.
Guy 1: Put your umbrella away, dude, it won't help you. It's grismal out there.
Guy 2: Great. I'll need windshield wipers for my glasses.