How to say goodbye to parents before moving? I want to meet up

My step dad is usually calm and level headed compared to my mom who hears one word and runs with it. A few days ago I mentioned moving to Washington and how I'll be doing it in a few weeks. My mom was not pleased and told me I'm not financially responsible enough and I'm not ready. She assumed she'd have to pay even though I told her it's already covered. Haven't mentioned it since, but it's happening. Boyfriends grandma is letting us live in one of her houses rent free which will save a lot of money! Uhaul and packing will all be covered! My only issue is I want to say bye and I don't know how. Do I wait a few days before moving to say bye? Do I tell them now and risk my mom interfering with the flow? How do I approach them? I have been living on my own for 2 years and she has discredited me getting my own apartment and paying for my bills and being able to do it on my own. I want my goodbye to be a good one but I know my mom will flip out