unannounced, unanticipated, unexpected, unforeseen

awesome, awful, breathtaking, fabulous, heart-stopping, marvelous (or marvellous), miraculous, portentous, prodigious, staggering, stupendous, sublime, wonderful, wondrous

extraordinary, phenomenal, rare, sensational, spectacular

befuddling, bewildering, confounding, confusing, discomfiting, disconcerting, dismaying, flustering, muddling, nonplussing (also nonplusing), perplexing, upsetting

incomprehensible, inconceivable, incredible, unbelievable, unimaginable, unlikely, unthinkable

singular, uncommon, unique, unusual, unwonted

conspicuous, notable, noticeable, outstanding, remarkable

impressive, striking