I'm no expert, but that's my understanding of it too. When you see snow falling on puddles or wet grass and it kinda does nothing for ages but then all of a sudden everything goes white, that's the point at which wet bulb temperatures are approached freezing. Locally, the ground level air mass can be cooled enough by heavy precipitation for wet bulb temperatures to change, but in showery, windy, mobile conditions, wet bulbs are mostly determined by the overall air mass and therefore those wet bulb temperatures are harder to shift downwards.