The worst case, given the specified uncertainty, would be that the approaching object is already fractured and weak enough to explode at high altitude. Such an airburst can spread its energy out over a larger area and will be more damaging than a crater-forming impact. The worst-case scenario would be a high altitude airburst releasing about 10.6 Mt of energy. The best estimate would be that the fragment is a slightly less dense 50-meter object. Even a relatively strong object of this size is likely to explode at high altitude; although it is possible some fraction of it could reach the ground and form a crater. This best estimate is almost identical to current understanding of the Tunguska explosion. [...]
Since the uncertainty in impact location is extensive, we also provided a map showing the damage footprint at several locations within the ellipse, which we then we convolved with the footprint (Figure 3). We advised participants that preliminary evacuation plans for an airburst over land should be in a lateral direction into area known already to be at no risk, but that detailed plans should wait until radar data becomes available (about 6 days before impact) [...]
Much of the uncertainty ellipse spans parts of the Gulf of Mexico. An impact within that part of the ellipse would produce a tsunami, and would affect the whole of the coastline from Texas to Florida. According to calculations by Souheil Ezzedine of Lawrence Livermore National Laboratory (LLNL) a tsunami generated by an impact in the easternmost part of the ellipse would have wave heights of 3 to 10 m and would arrive at the coast over a time spanning from 1 to 4.25 hours after the impact. The tsunami would first reach the Louisiana coastline, causing near total destruction to the barrier islands, The wave run-up would extend inland as far as 16 km.
When planetary physicists start role-playing, you end up with the darkest tabletop strategy game ever.