Alarming, a.
Exciting, or calculated to excite, alarm; causing apprehension of danger; as, an alarming crisis or report. -- A*larm"ing*ly, adv.
Shades of meaning
Fernald has no entry of its own for alarming; he lists it under awful.
Where he writes about it:
Awful should not be used of things which are merely disagreeable or annoying, nor of all that are alarming and terrible, but only of such as bring a solemn awe upon the soul, as in the presence of a superior power; as, the awful hush before the battle. — from awful