If you play as Woodie and transform into a form while in a sandstorm, he will move at incredibly slow speeds, much slower than the 60% reduction there should be.
However, if you transform outside of the desert and enter it during the sandstorm, there is much less effect.
From my testing, this includes transforming in the oasis while not effected by the sandstorm: you just have to be in the desert somewhere and transform.
Transform as Woodie in a desert during a sandstorm.
Transform as Woodie outside of a desert, then enter it during a sandstorm.