On this long storm the rainbow rose
Listless elephants
Clouds moving slowly across the horizon like tired elephants—the metaphor captures both their massive gray shapes and their exhausted pace after the storm.
Birds rose smiling
Pathetic fallacy—attributing human emotion to nature. The birds aren't actually smiling; this is how the living perceive the world after a storm.
Slow archangel's syllables
Resurrection theology—only Gabriel's trumpet at the Last Judgment can wake the dead. 'Slow' because it hasn't come yet, may never come.