Explanation:

- Option 1: Continuous heavy

- Grammatically correct. Indicates a prolonged period with a lot of rainfall.

- Option 2: further worse

- Grammatically incorrect. It should be "further worsen" instead of "further worse." "Worse" is an adjective or adverb, while "worsen" is a verb, which fits the context here.

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- Option 3: the flood situation

- Grammatically correct. It refers to the condition of the ongoing flood.

- Option 4: rainfall may

- Grammatically correct. Indicates potential outcomes related to rainfall.

Overall, the error is in Option 2, as "further worse" should be corrected to "further worsen" to maintain proper verb usage in the sentence.