While the poem uses other devices like simile ("like the smoke from the chimney"), personification ("the snow came down, / Steadily all the day"), and imagery, the central metaphor is the snowfall itself, representing the grief and loss the speaker is experiencing over the death of his child.
This metaphorical connection is explicitly stated:
> "The snow came down, / Steadily all the day, /
> Even as our own grief had settled down /
> Upon our hearts that day."
The snowfall becomes a symbolic representation of the speaker's sorrow, the way it covers the landscape mirroring the way grief has settled upon the family. The poem continues to develop this metaphor, comparing the snow's blanket to the way the memory of the child will "cover" their lives.
While other elements are present, the extended metaphor of the snowfall as grief is the most prominent and drives the emotional core of the poem.