Flowers definitely looks like mustard, but the leaves seem a bit off. Have you tried tasting one of the leaf ? You might need to do it several time if you don't taste anything, but if it taste a bit spicy it should be mustard.
If you wait a while, it should go to seed and look even easier to recognize.
I remember because of the snow. Do you remember tiny ad?
Your suggestions have been mashed into the PIE page - wuddyathink?