The poem begins with a storm raging outside "[o]nce more," a phrase that suggests that there was a brief ebb in this storm before the poem began (and, perhaps, that the speaker has been sitting with his daughter for quite some time). The speaker's young daughter is "half hid" in her cradle and "sleeps on," apparently oblivious to the chaos of the wind "howling" outside.
Right away, note how the daughter is at once protected (she's indoors with the speaker, and she's halfway sheltered by her cradle) and vulnerable (she's only halfway sheltered by her cradle, which is to say that she's also halfway exposed, and unaware of the danger that swirls around her).
The speaker, for his part, is clearly aware of his daughter's vulnerability to this storm, worrying that "[t]here is no obstacle" apart from a nearby forest and a single "bare hill" to shelter their residence from the violent weather. ("Gregory's wood" refers to an actual forest on the estate of Lady Gregory, Yeats's close friend and literary associate.) The storm outside has gotten this speaker pretty worked up, and he paces and prays anxiously "[b]ecause of the great gloom that is in my mind."
By this point, readers might start to sense that this storm is not just a literal one, but also symbolic of the worldly dangers that await the speaker's daughter. Yeats's first child, a daughter named Anne, was born just two days before this poem was written, in 1919, and readers can assume that the poem evokes his personal anxieties as a new parent.
This eight-line stanza also establishes the poem's rhyme scheme, which runs AABBCDDC, and meter, which is a loose mixture of iambic pentameter and tetrameter. An iamb is a poetic foot with a da-dum rhythm; pentameter means there are five of these iambs per line, while tetrameter means there are just four. This meter adds some steadiness and rhythm to the poem, though Yeats stretches and plays with the meter a lot. For example, listen to lines 1 and 2:
Once more | the storm | is howl- | ing, and | half hid
Under | this cra- | dle-hood | and cov- | erlid
Notice that both lines have five feet (pentameter) and that, overall, the iambic da-dum rhythm predominates. Yet clearly not all the feet here are iambs (starting with the poem's very first foot: "Once more")! The point of this metrical flexibility is simple: it adds emphasis to certain words and ideas, like "Once more" and "half hid." The steady pulse of the iamb, meanwhile, also helps create an ominous, threatening atmosphere, which makes the "great gloom" in the speaker's mind seem more heartfelt, urgent, and intense.