When she went missing, we had weather that wasn’t too bad. I’d have to go back and look, but I think the temp stayed around the 50s. The extremely cold weather came in the following weeks. If he buried her within days, the ground shouldn’t have been frozen yet.
I wonder if it was already dirt in the area he dug the grave in - meaning it would be mud in January. Easier to dig through and very easy to disguise any disturbance.