EMP: Nuclear Summer

The EMP electromagnetic pulse attack and ensuing nuclear bomb exchanged happened four years ago, taking out the electric power grids and electronics of much of the world. Ninety percent of the world's population has perished, those surviving being from the underdeveloped countries not targeted by a nuclear exchange that followed.
Two years ago, at the unused Yucca Mountain nuclear waste repository in Nevada, a group of survivors had briefly ventured out of their underground refuge only to face a battle with hostile foreign survivors seeking to take their refuge. The jet stream returned the fallout of the nuclear winter back to the region, but not before both replenish the food and supplies needed to carry them through another siege by the nuclear fallout.
The survivors in the mountain have the past two years prepared to repel attacks on the mountain and to battle the Islamic jihadists arriving from Central America seeking to control Hoover Dam for an Islamic State in Arizona.
Now, the nuclear winter that blanketed the globe has reduced to storms influenced by the jet stream. The survivors split, most leaving the mountain to establish a refuge in the Veteran's Hospital in North Las Vegas from future storm events. A small group remains at the mountain to defend it.
A battle ensues at the Hoover Dam just ahead of global weather changes that deliver radioactive dust from an El Nino storm that forces the survivors to seek cover. Tragedy strikes with unimaginable consequences.