In the first stanza, we are introduced to a terrible storm, a storm that symbolizes the many potential problems that will be strewn across the path of the speaker's daughter when she grows up and which are set out in great detail in the remainder of the poem.
Yeats immediately emphasizes the fragility and vulnerability of his daughter as she lies sound asleep in her cradle. He does this by referring to the uncomfortable fact that only two features of the natural landscape—Gregory's wood and one bare hill—are capable of acting as obstacles to the storm raging outside.
And what a storm it is, too! This howling great tempest coming in off the Atlantic is so powerful that it levels haystacks and roofs. Not only that, but it screams upon the tower, under the arches of the bridge, and through the elms. In the storm, the speaker sees a grim presentiment of all the many obstacles that his daughter will have to overcome when she gets older. Given his gloomy sense of foreboding at what the future may bring, it is little wonder that the speaker spends so much praying for his daughter.