Three short lines, and the whole mood turns on one surprise. We expect the end of winter to feel like relief: the ice is gone, spring is coming. But the poem skips past the melting and lands on the city as "a treasury / of ash." Instead of fresh green, we get gray leftovers.
The word "treasury" is the interesting choice. A treasury is where valuable things are stored, yet here it holds ash, which is what remains after something has burned. That clash makes the city feel both precious and spent. Maybe the speaker sees the bare, soot-colored streets once the snow and ice stop covering them. The poem doesn't say, so that's a guess from the imagery.
The speaker's feeling is hard to pin down. It isn't plain sadness or plain joy. It reads as quiet, observant, a little wry, and slightly hollow, like noticing that what the thaw uncovers isn't beautiful yet. The mood is stark and still, with a faint sense that something has to begin again.