Synonyms for Unforeseen:

adj.

surprising (adjective)
surprising, sudden, out of the blue, unanticipated, shocking, unexpected, unlooked-for, abrupt, astounding, startling, astonishing.

adv

unpredicted (adverb)
out of the blue.

n.

trouble is brewing (for), Houston, we have a problem, uneasy, acrimonious, stormy, one thing after another, in the wrong place at the wrong time, the matter, it's not someone's day. unforeseen (noun)
sudden, unlooked-for, out of the blue, unexpected, unanticipated.

Other synonyms:

uneasy, acrimonious, stormy. Other relevant words:
stormy, acrimonious, unlooked-for, out of the blue, abrupt, uneasy.

Usage examples for unforeseen

  1. Like the low notes in a treble voice, there is that that seems to vibrate in our hearts at a most susceptible moment, and with the force of an unforeseen contrast; and it was thus that, in her graver times, she won over me an ascendancy, and inspired an interest which, had I been other than a mere boy, had certainly been love. – That Boy Of Norcott's by Charles James Lever
  2. The driver was in possession of the papers necessary for the journey, and, unless something unforeseen should happen, nothing could prevent the fugitives reaching the frontier in safety. – Princess Maritza by Percy Brebner
  3. He reached Greenock on the 20th of March, but because of unforeseen delay it was not until April 11th that he embarked for Canada. – McGill and its Story, 1821-1921 by Cyrus Macmillan