A long range weather forecast should be obtained before leaving, as weather conditions are extremely unpredictable.
I pulled that quote from a Ron Jeffries sig file. It’s a great Zen-like nugget that sums up the typical approach to software development, i.e., it’s an unpredictable business, so we’ll do lots of upfront planning . . . ignoring the fact that the inherent unpredictability makes dependable upfront planning impossible.