Winter is loosening its grip in this short poem, and the speaker is driving through the thaw. The opening lines surprise us by calling the sound of slush "mellifluous," a word usually saved for music or honeyed voices. The pleasure is real, though it comes with a catch: the sound happens "just before" the tires start spinning, so the beauty sits right next to loss of control.
That tension runs through the middle stanza, where soft snow banks are compared to the Loreleis, the mythical singers who lured sailors to their doom. The snow looks gentle, but it can pull a careless driver in. By the end, the speaker is steering "gingerly" with the windows down, listening to the hiss of leaves and mud. Those sounds are welcome because they mean the road is clearing and spring is near. The mood is alert but quietly happy, a mix of caution and relief.