Original: When bands of precipitation near frontal zones approach steep topography, a low-level barrier jet stream forms parallel to and just prior to the mountain ridge, which slows down the frontal rainband just prior to the mountain barrier.
Simplified: When rainbands near frontal zones get close to steep mountains, a low-level barrier jet stream forms parallel to and just before the mountain ridge. This slows down the frontal rainband just before the mountain barrier.