I've been unable to find a definitive answer anywhere, so here's my speculation!:
This is a type 2 auto ATIS that specifies precipitation type ie RA/GR/PL. This is where the MM is so could be the discriminator needs maintenance. I know this is usually indicated by a $ sign for the whole system, but could this just be the discriminator part?
Either that or storms originating in Mexico FIR.
