Been begging the sun to come out and what do I get home to today, this ..What the heck is going on???

Most likely sunburn. I understand you have been having very hot temperatures. Also - and I can't repeat this enough - when you don't fill your pots all the way the reflected heat from the inside of the pot can cause sunburn.
If this is occurring all the way around your plant the cambium has been burned and your plant will die. You can save the top section above the burn.
Sorry to not have better news....

Mark, is this plant leafed-out?

Oh poor plant Mark, I would inspect the root zone and cut off the rotting part.

Are you more coastal side SD, or inland side SD?

Yes, that is sun burn.
I have had the same thing happen to me except it happened last year and mostly in the Fall