This is an interesting poem in that the revelation of what the "city planners" of the title are, in Atwood's view, trying to conceal or control in vain is delayed until the end of the poem. For much of the early part of the poem, the speaker's horror at the "dry" suburb, in which everything is "straight" and "discouraged" and where anything disturbing the clean lines—even a splash of paint or oil—is not defined as a reaction to global and environmental decay. As a reader, words like "hysteria" make me question whether Atwood is suggesting that the clean lines of the suburb are actually trying to conceal or contain the more disordered lives of the families who live within, as if straight lines and neat houses can make us all the same and therefore satisfy the desire for conformity expressed by the city planners.
Later in the poem, however, it becomes clear that the speaker's horror is based in greater things than these. Each of the city planners is pictured sitting within his own "blizzard," trying to draw lines and boundaries on a town as if this will somehow forestall the true tragedy which is coming—an environmental one, in which everything will eventually slip into the sea.
The themes of the poem, then, seem to be environmental disaster as well as the human desire to fiercely control small details rather than confront the real issues.