The Collaborative International Dictionary of English v.0.48:

Thunderous \Thun"der*ous\, a. [Written also thundrous.]
   1. Producing thunder. [R.]
      [1913 Webster]

            How he before the thunderous throne doth lie.
                                                  --Milton.
      [1913 Webster]

   2. Making a noise like thunder; sounding loud and deep;
      sonorous.
      [1913 Webster] -- Thun"der*ous*ly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thunderous
    adj 1: loud enough to cause (temporary) hearing loss [syn:
           deafening, earsplitting, thunderous, thundery]
    2: extremely ominous; "world events of thunderous import"

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "thunderous":
   booming, cannonading, deafening, ear-piercing, ear-rending,
   ear-splitting, earthshaking, forte, fortissimo, full, fulminating,
   loud, loud-sounding, loudish, noisy, pealing, piercing, plangent,
   resounding, ringing, roaring, rolling, rumbling, sonorous,
   stentoraphonic, stentorian, stentorious, thundering, thunderlike,
   thundery, tonitruant, tonitruous, tumultuous, volleying,
   window-rattling