Life as We Knew It by Susan Beth Pfeffer

Miranda's disbelief turns to fear in a split second when a meteor knocks the moon closer to the earth. How should her family prepare for the future when worldwide tsunamis wipe out the coasts, earthquakes rock the continents, and volcanic ash blocks out the sun? As summer turns to Arctic winter, Miranda, her two brothers, and their mother retreat to the unexpected safe haven of their sunroom, where they subsist on stockpiled food and limited water in the warmth of a wood-burning stove.
In the year that has passed since a meteor collided with the moon, Miranda's friends and neighbors have died, the landscape has frozen, and food has become increasingly scarce. The struggle to survive intensifies when Miranda's father and stepmother arrive with a baby and three strangers in tow. One of the newcomers is Alex Morales, and as Miranda's complicated feelings for him turn to love, his plans for his future thwart their relationship. Then a devastating tornado hits, and Miranda makes a decision that will change their lives forever.
Told in journal entries, this is the heart-pounding story of Miranda's struggle to hold on to the most important resource of all — hope — in an increasingly desperate and unfamiliar world.