Start creating a Sprite or a Tiled Object with the same width as your SCREEN SIZE and in a separated layer
i'v name this object Tempo(time)
Put the layer parallax with 0, 0
Create a Sprite and name it Rain, change the origin of this sprite to be at the left
Give the Sprite Rain the respective behaviors and turn it to the ground
in the event sheet create a event: System_every X seconds and a create object
PS: remember to put a condition to prevent a lot of rain drops outside of the screen
Use the same logic with the snow, just add the Sine behavior