Impending, a.
Hanging over; overhanging; suspended so as to menace; imminet; threatening. An impending brow. Hawthorne. And nodding Ilion waits th' impending fall. Pope.
Syn. -- Imminent; threatening. See Imminent.
Shades of meaning
Fernald has no entry of its own for impending; he lists it under imminent.
Where he writes about it:
Impending, also from the Latin, with the sense of hanging over, is closely akin to imminent, but somewhat less emphatic. — from imminent
Imminent is more immediate, impending more remote, threatening more contingent. — from imminent
An impending evil is almost sure to happen at some uncertain time, perhaps very near; an imminent peril is one liable to befall very speedily; a threatening peril may be near or remote, but always with hope that it may be averted. — from imminent