Impending, imminent are comparable when they mean very likely to occur soon or without further warning.
Both retain in this sense some feeling of now rare or disused senses in which they essentially denote being physically elevated and hanging over or projecting as if about to fall and, as a result, tend to convey an ominous or portentous note. Occasionally this feeling may be lacking and the words imply no more than the near futurity of the thing qualified.
Distinctively impending suggests that the thing likely to occur is foreshadowed far enough ahead to allow one time for worry and suspense or for aversive action.
Imminent usually implies greater immediacy and may suggest that the thing is on the point of happening, but imminent, unlike impending , may lose much or all of its suggestion of futurity and then attributes nearness in some other than temporal relation to the thing qualified.