Just a quick rain sprite I made during all this rain. I use it in CS but it's for any goldsource game. Set it's scale to somewhere around 1.5, stack them on top of each other and place them around your map. They are locked vertically. Use render mode additive with amount somewhere around 15.
This is great if you have sky lights or entities you don't want rain falling through. You can also have them placed under sky lights and have func_breakables trigger them on so if you shoot a sky light the rain begins poring inside.
They don't look super great but it looks ok I guess.