Q. Hello! In the examples in CMOS 8.48, can you clarify why Southern California, Northern California, West Tennessee, East Tennessee, and Middle Tennessee get capitalized, whereas western Arizona, eastern Massachusetts, southern Minnesota, northwest Illinois, and central Illinois do not?
A. Southern California and Northern California are capitalized because they have become recognized beyond their borders as the names of two geographic and cultural entities. East, West, and Middle Tennessee are capitalized as the names of the three “grand divisions” in that state, a usage that has become widely accepted; see, among other sources, the entry for “Tennessee” in Britannica. (Many sources style these as the Grand Divisions—with initial capitals—including this page from the Tennessee Historical Society. Tennessee’s state constitution, on the other hand, uses lowercase, and it doesn’t name the three divisions.)
As for the other examples in your question, each of those is more likely to be used generically (i.e., to refer to the western or other portion of a state) rather than as a proper noun. And though any of them might be capitalized in local usage (as in a travel guide extolling the virtues of a certain region), such usage shouldn’t necessarily determine your own (or that of your author). When in doubt, use lowercase for terms like western at the state level and caps for national or global regions.