It is NEMA 4X rated, so “water tight” and is fine for rain exposure (but not submersion!).
The instructions do say “don’t use in heavy rain”, and they also say “don’t use extension cords”... take that for what you will.
If you do opt for an extension cord, make sure it is if sufficient gauge for the current it will be carrying, and note that the connection between cords is not water tight, so plan/protect it accordingly.