The Collaborative International Dictionary of English v.0.48:

Alarm \A*larm"\, v. t. [imp. & p. p. Alarmed; p. pr. & vb. n.
   Alarming.] [Alarm, n. Cf. F. alarmer.]
   1. To call to arms for defense; to give notice to (any one)
      of approaching danger; to rouse to vigilance and action;
      to put on the alert.
      [1913 Webster]

   2. To keep in excitement; to disturb.
      [1913 Webster]

   3. To surprise with apprehension of danger; to fill with
      anxiety in regard to threatening evil; to excite with
      sudden fear.
      [1913 Webster]

            Alarmed by rumors of military preparation.
                                                  --Macaulay.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alarming \A*larm"ing\, a.
   Exciting, or calculated to excite, alarm; causing
   apprehension of danger; as, an alarming crisis or report. --
   A*larm"ing*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alarming
    adj 1: frightening because of an awareness of danger [ant:
           unalarming]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "alarming":
   awing, bad, chilling, critical, dangerous, dangersome, daunting,
   deterrent, deterring, disconcerting, discouraging, disheartening,
   dismaying, disquieting, explosive, fear-inspiring, fearful,
   fearsome, fraught with danger, frightening, frightful, jeopardous,
   menacing, overawing, parlous, periculous, perilous, scaring, scary,
   serious, startling, threatening, ugly