Final answer:
The appropriate synonym to complete the sentence about the storm is 'abate', which accurately describes the reduction in storm intensity allowing for safe departure from the cabin.
Step-by-step explanation:
For the sentence, 'If the storm doesn't _______, we'll have to spend the night in the cabin.', the appropriate synonym to complete the sentence with the right flavor is abate. This word specifically means to lessen or diminish in intensity, which fits well in the context of a storm that needs to reduce enough for it to be safe to leave the cabin. While 'diminish', 'decrease', and 'dwindle' are also synonyms, 'abate' is the best choice because it is commonly used with reference to something unpleasant, such as bad weather, diminishing in force or intensity.