A good question. Living in the great State of Lousy-anna (we've got the best politicians and Police that $$$$ can buy), my preparations are geared towards:

Highly Possible:
Hurricanes; Hazardous Materials Incident (from Railroad or Interstates); National Financial Meltdown*1; Martial Law*2; High Criminal Activity*3; Supply Shortages**1; Food Shortages**1;

Possible:
Tornadoes; Nuclear Incident of any kind; Civil Unrest*3; Flooding; Local Financial Meltdown; Foreign Invasion; Home Fire; Major Medical Emergency***1; Other Severe Wind/Electrical Weather; Being recalled to Active Duty****1; Ice Storm; Yellowstone Cauldera Eruption; West Coast Volcanic eruption causing a disruption;

Unlikely Possibility:
Tsunami; Terrorist Incident;

Not on the RADAR Scope at all:
Earthquakes; ExtraTerrestrial Invasion; Zombie Uprising; Local Volcano Eruption;

* 1) In the process of occurring, 2) Currently being used as a threat, 3) Logical result of *1.

** 1) Beginning stage currently being observed in stores;

*** 1) Currently have 1 elderly and 1 severely handicapped, adults living in my home;

**** 1) Would result in Family Financial Crisis;

This question has proven much more worthy of the time I've spent answering it than I first thought!! It has helped get the STOBAR in a different light. This list could/does change day by day as current events evolve and dissolve.