。
I guess we should take an umbrella just in case.
Literal
By-chance [for-に備えて] umbrella [object-を] take-go way [subject-が] good [probably-だろうな].
持っていく ('to take [with you] and go') is the compound 持つ + 行く — 'take along.' ~た方がいい ('it's better to X,' 'should X') is the advice pattern, built from 方 ('side, way') + がいい. The final だろうな combines probability だろう + soliloquy な, giving a self-reflective 'I suppose.'