200 designated survivors
Suppose there is an apocalyptic scenario on the horizon: in six months, e.g. a red giant only six light years away will go supernova. Scientists have already known that for a few years, but everything is kept secret to prevent panic. Your scientists haven't found any solution that would allow Earth's population to survive. It is estimated Earth will become uninhabitable for at least a century. They consider to preserve the human race, by sending out a space ship with a mixed population of ca 200 humans, initially in hibernation, to find shelter behind Jupiter for several months, to wait out the effects of the supernova.
The pilot made a mistake
At some point underway to Jupiter, your pilot accidentally touches a switch, which is decompressing the stasis compartments, exposing them to space. The stasis containers break. Everyone in hybernation dies, only the pilot survives. Watching the Earth burn.. A lonely and desperate human.
It will depend on your story what happens next. Earth is dead now, there were a few other humans in space (ISS), but they don't have the means to survive. When the pilot is male, it would certainly be the end of mankind. Your evacuation ship could have a female pilot. She'll improvise with frozen sperm, to get herself pregnant.. Best case, she knows about the subject of genetics and pick some diversity (say, 6-7 children from different sperm samples).
Some notes about surviving a supernova on Earth
Putting the above answer, I had not realized your survivor protagonist resides in California. To work that out for my scenario.. First, to get an idea of the effects,
Supernova - 30 light years
.. on 30 light years distance, the oceans would be depleted of food and the atmosphere and O3 layer will be destroyed. Mass extinction follows. But that would not immediately result in a complete wipe out of civilization. Humans will manage below the surface, plans could be in place to restore the Ozone layer in some way (hand wave) and after some time, your planet would get repopulated with humans. After spending a century or two underground, part of Earth's surface and Earth's atmosphere would have to be re-terraformed to repair the damage ! In this case, there would be millions of survivors. 30 light years won't do, for a plausible single survivor.
Supernova - 6 light years
Now suppose the Supernova would occur at only 6 light years distance, like I've proposed, the Earth would be within the Supernova radius itself. That would not allow survival on Earth, I assumed zero human survivors on Earth.
Huge effects, like deadly X-Ray and gamma radiation spikes, electrocuting EMP's.. and after 3-5 years, you'll get core fragments and debris impacting on Earth. A Russian roulette, that could yield a ravaged Solar system.
All humans on Earth will have become infertile, due to the excessive radiation. Now, some families in California could have survived undergrounds, when lucky. Most will die, because they can't return to the surface. Maybe someone very smart invents a way, to keep his family alive longer.. The youngest son could survive his family for ca. 60 years and become the (very) lonely single human surviving on Earth.