I never know how to date these things. When in doubt I add a day, just because it seems to me that storms arrive later than predicted (a week out) and often head out to see faster than predicted 24 hours in advance. I have no idea, I'm leaving it.
EDIT to add: Right now it looks like your date is right. Storm to start wet on the 18th turn white after that.
"If it's my mountain, I'm installing a fixed grip and telling the lifties to stuff the beginners into chairs as aggressively as they have to, to keep it going full speed." —Brownski