Final answer:
The appropriate synonym for the given sentence is 'abate'.
Step-by-step explanation:
The appropriate synonym to complete the sentence, 'If the storm doesn't _________, we'll have to spend the night in the cabin,' is abate. Abate means to become less intense or widespread, which would indicate that the storm is diminishing or decreasing. In this context, it implies that the storm is weakening and will not pose a threat to their plans.