Now is a good time to sit down, figure out what you need to move out, and then figure out what you need to do to make that happen. Be sure that removing your name from the lease once you move (If it's on the lease now) is on that list. Otherwise if she falls behind on the rent her landlord could go after you for the money!
You don't say anything about how you think she'll take it. You don't have to let her know until she comes home one day and you're not there, if you feel she may turn dangerous. Likewise, if she isn't the sort who would care at all, you *could* tell her tonight - but personally, I wouldn't say anything about it until you have at least an emergency backup plan in case she acts up worse than you estimated.
AlAnon is a great resource for those affected by someone's alcoholism. I know you have only recently discovered she's an alcoholic, and I know you are already looking into leaving - I still suggest attending AlAnon, especially while you're stuck living with her, as it will help you remain centered and focused on what you need to do. The more educated you are about the disease, the less likely you'll be to dig your hole deeper while you're mapping your way out!