Their coat change appears to be triggered by the length of the day, moreso than temperature changes. They can be prevented from growing a coat by keeping them under bright lights until summertime sunset time, and they can be encouraged to shed early by putting them under bright lights in the evening in the spring. Some show barns leave the lights on late and turn them on early.
Course, if they do that, they also blanket them.
I wonder if it has something to do with the amount of light too? Okie had a much thicker winter coat in Oklahoma than here. Our hours are pretty much the same but they have more overcast days.