The Ashfall Series by Mike Mullin
For Alex, being left alone for the weekend means having the freedom to play computer games and hang out with his friends without hassle from his mother. Then the Yellowstone supervolcano erupts, plunging his hometown into a nightmare of darkness, ash, and violence. Alex begins a harrowing trek to seach for his family and finds help in Darla, a travel partner he meets along the way. Together they must find the strength and skills to survive and outlast an epic disaster.
The Yellowstone supervolcano nearly wiped out the human race. Now, almost a year after the eruption, the survivors seem determined to finish the job. Communities wage war on each other, gangs of cannibals roam the countryside, and what little government survived the eruption has collapsed completely. The ham radio has gone silent. Sickness, cold, and starvation are the survivors' constant companions.