Q. Does CMOS have guidelines about using “where” to refer to something other than a place? For example, “This is a situation where extra care is needed.”
A. We don’t, but Bryan Garner, the author of chapter 5 in CMOS (on grammar and usage), does. See Garner’s Modern English Usage (5th ed., Oxford, 2022), under “where.” That entry has two relevant subentries: (1) on where for in which and (2) on where for when.
The first subentry explains that in formal prose, the locative where shouldn’t be used as a relative pronoun, as when a phrase like “a case where” is used instead of “a case in which.” But in prose with a “relaxed tone,” including text with contractions (like many of the answers in this Q&A), where is not only acceptable but may even be preferable.
Your example (“a situation where”) seems perfectly OK to us, but if you are going for a formal, elevated style, change “where” to “in which.”
As for where in place of when (which is temporal rather than locative—i.e., referring to time instead of place), Garner says that writers who do this have “misused” the word. So, for example, you should write “a year when,” not “a year where.”