This is a great question. Horace is a wonderful poet, who write some of the best poetry in the Latin language.
This ode (1.2) begins on a somber note. One prodigy is mentioned after another. First, there is snow. Second, there is fierce hail. Third, there is lightning that strikes the sacred citadels. Finally, there is a flood. This concatenation of prodigies is so great that the city is in fear. In fact, according to Horace, the people may even be speculating about the inauguration of a new age. However, there is nothing desirable about this new age, since it is the age of Pyrrha, which is a cruel one.
According to mythology Pyrrha and her husband Deucalion were the only surviving humans after a global flood that destroyed all of humanity on account of their impiety. Destruction by water is underlined. Horace continues this motif for two stanzas. He writes of strange sights (nova monstra). Proteus, the guardian of Neptune’s seals in Virgil’s Georgics, is seen driving his herd of (presumably) seals to the mountain tops. Fishes are there too; they are caught on top of elms, while deer swim in terror.
Through these vignettes, Horace is able to set the stage of his ode. Times are dire. The mere mention of prodigies suggests this fact, since all prodigies presuppose a rupture in the pax deorum. In a word, the Romans are in trouble.