[susan53]

Again, it's an adjective and not part of the noun. The "southern US" is just a description of a rough area, not an official name, and therefore not part of a proper noun. Compare this headline from the Guardian newspaper....
Flash flood warnings for southern England
with this one from a police website ...
The Western Isles, also known as the Outer Hebrides, are a chain of islands some 30 miles off the north-west coast of Scotland.
Notice the lack of capitalisation for southern - where again it's just a geographical description, and the capital in Western - where the word is part of the official name.
If you see northern/southern/ eastern/western capitalised when the word is not part of an official name, then it's a mistake.