To investigate and evaluate the mechanics of how failures spread in power grids, this paper introduces a methodology for a credible list of critical N-k contingencies that would threaten the grids. The methodology is based on the concept of delta centrality. The list of critical contingencies will provide the look-ahead situation awareness for operators on any impending large power grid blackouts. We also propose novel indices which are simple to interpret and can be used to determine and identify locations of critical components caused by cascading failures. Build-ups of potentially high risk cascading failures can be monitored by these indices thus, providing wide area visibility of power system dynamics against threatening events. Results from 118 test bus system show a promising application for enhancing power system against rare and catastrophic outages.