Explain the line "The trumpet of prophecy" from "Ode to the West Wind".

The "trumpet of prophecy" is a critical component of Shelley's closing in "Ode to the West Wind."  He seeks to link the natural phenomena of storms and the natural changing of the seasons to the his own hopes of achieving poetic immortality from obscurity.  Essentially, through a natural experience, Shelley seeks to link it to his own evolution as both thinker and artist.  Shelley sees the power of the winds, as it scatters leaves and hopes that it will "drive my dead thoughts over the universe," an allusion to the fact that while he might not be appreciated now, Shelley hopes to achieve poetic immortality over time.  As "ashes and sparks" are scattered, his hopes are that his worlds will follow accordingly.  He closes the poem with the idea that the "trumpet of a prophecy" is the wind, and its signal for change, for growth, for evolution.  As the seasons change, from winter to spring, his hopes are that his measure as a poet and thinker will also evolve, and the trumpet is the natural evolution from destruction to creation.  The prophecy aspect makes this as something known and understood, what he seeks for both his words and stature as a poet.