cataclysm, disaster, calamity, tragedy, supercatastrophe, megadisaster, megastorm, big one, superstorm, superexplosion, more...

blessing, boon

flood, earthquake, hurricane, tornado, tsunami, volcano, landslide, avalanche, more...

great, final, such, terrible, impending, major, natural, economic, national, nuclear, dreadful

the perfect storm,   complete debacle,   terrible event,   impending doom,   major event,   terrible situation,   cause of calamity,   bad end,   tragic end,   severe setback,   complete disaster