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During the rainy season it rains so much that it's hard to plan outings.
Literal
Rainy-season [topic-は] rain-falling [subject-が] much [connective-くて] outings [genitive-の] plans [subject-が] hard-to-make.
梅雨時 (つゆどき) appears again. 雨降り (あめふり, 'rainfall / rainy weather') is a simple noun for rain as a condition. ~くて links 多い ('many/much') to the consequence: 計画が立てにくい ('plans are hard to make'). ~にくい ('hard to do X') is the difficulty suffix attached to the verb stem 立て (from 立てる, 'to make/set up'). A practical, everyday observation about seasonal life.