Sudden :  adj : happening without warning or in a short space of time; "a sudden storm"; "a sudden decision"; "a sudden cure" [ant: gradual]

aback, abrupt, abruptly, accelerated, breakneck, breathless, brisk, electrifying, expeditious, extraordinary, fast, fleet, forthwith, hasty, headlong, hurried, immediate, impatient, impetuous, improbable, impulsive, more than expected, nerve-shattering, out-of-the-way, overhasty, panting, past expectation, precipitant, precipitate, precipitous, quick, rapid, rash, reckless, rushing, shocking, short, speedy, startling, suddenly, surprising, swift, unannounced, unanticipated, unanticipatedly, unaware, undivined, unexpected, unexpectedly, unforeseeable, unforeseen, unguessed, unheralded, unhoped for, unlooked-for, unpredictable, unpredicted, unprepared for, unwonted, without warning,