Billieryder that's a bit extreme situation, which you normally never face. Thing is, snow straight out of snowgun is completely different material and can hardly be even called snow. You never ski on such snow, since this snow is always at least partially transformed when they "drive" over with pistenbullies, preparing track. Normally areas where high and low pressure snowguns are operating, are closed for skiing (except those where hmm... we call these things giraffe... it's few meters high tower where snow is falling from top down like from "shower"), so you normally don't ski on such snow.
Vsirin yes it's easy to pick about right wax, but only if you are waxing on morning when you go skiing, or maybe on evening previous day. Normally I don't do this. I wax skis after I come back from skiing, leave wax on, and I just scrap it off before I go skiing. Even with my experience it's pretty much impossible to know, what wax will do fine in 3 days, especially with weird weather we have last 2 winters.