I want to take advantage of winter clearance right now and buy M some stuff for next winter including a coat. But I'm just not sure how fast they grow after they turn 1?

Currently she's 10 months and in 12 or 18 month clothing. So I lean towards buying her next winter's coat in a 3T but that sounds crazy because she'll be 18 months when the cold starts? But I do want the coat at least to last through the winter.

What size clothes and coat would you buy?