【Surprise】 ,  【astonish】 ,  【astound】 ,  【amaze】 ,  【flabbergast】  can mean to impress one forcibly because unexpected or startling or unusual.

【Surprise】 can mean to come upon another suddenly and with startling effect, or, more broadly, it can apply to an unexpected or unanticipated development that tends to arouse some degree of 【surprise】 , amazement, or wonder; both senses imply a lack of preparation or a reversal of what is anticipated.

【Astonish】 can imply a dazing or silencing or it may mean to 【surprise】 so greatly as to seem incredible or sometimes merely unusual.

【Astound】  stresses a stunning or overwhelming emotional effect and usually implies so great a difference between what one believes possible and what one discovers to be true that one can find no precedent for it; thus, a piece of news surprises one when it is unexpected; it astonishes when one finds it hard to believe; it astounds when one cannot account for it by any previous knowledge or experience; the successful laying of the Atlantic cable astounded everybody, while its later breaks astonished no one, but after it was finally in operation, many said that no future human invention could  【surprise】 them.

【Amaze】 , though it carries an implication of astonishment, stresses rather bewilderment, perplexity, or wonder.

【Flabbergast】  is a picturesque and often hyperbolical synonym of  【astonish】  or  【amaze】 ; it suggests vividly the physical signs of a sudden dumbfounding.