Post With Large Knot Cracking
This post has a large crack and will need to be
replaced. The crack was caused by a knot that over time loosened up and
started to separate, creating a large split in the post.
I would imagine that during construction, this split was not there or smaller than it is now. There are other ways to fix this post, but replacing it would be the best way to go.
Extreme temperatures can cause lumber to expand and contract. This is one possibility that could have created the post damage.