I love analyzing poems, so thank you for giving me the chance to help you!
We can learn a lot about a poem by looking at the title: "Where the Rainbow Ends." What does this mean? The rainbow is a great (if overused) symbol representing "better times ahead" in that it is a promise that God will not destroy the world again by flood. The colors of a rainbow are glorious, and according to myth at the end of the rainbow we are supposed to find a pot of gold. The idea of the rainbow is usually a positive one.
On the other hand, a person can never actually FIND the end of a rainbow, so it can be a symbol of futility and frustration. Where does that leave us? We can guess that the poem is going to either have a theme of promise and hope, or one of futility.
"I saw the sky descending, black and white
Not blue, on Boston..."
When the sky descends it usually means a storm is coming. Boston appears to be the setting.
...where the winters wore
The skulls to jack-o-lanterns on the slates,
A rite of fall is Halloween (hence the pumpkin carving.) These jack-o-lanterns seem to have been left out on the "slates" (porches?) too long and are now husks embraced by the winter storm.
"And hunger's skin and bones retrievers tore
The chickadee and shrike."
Both are birds found in Boston, and both migrate South during extreme cold weather. This storm has caught these birds who would rather have migrated to warmer climate. Perhaps the storm came early in the year...that would also explain the pumpkins being left out.
"...The thorn tree waits
its victim, and tonight
The tree with thorns seems to imply it has no leaves, or no fruit to offer as food for the birds. Instead of a symbol of life, it is bad news for the birds.
The worms will eat the deadwood to the foot
Of Ararat: the scythers, Time and Death
Helmed locusts, move upon the tree of breath;"
Mount Ararat is purported to be the mountain on which Noah's ark came to rest. That ties in with the rainbow in the title. Time and Death are seen here as negative forces, locusts that consume all in their path.
"...the wild ingrated with the olive and the root
Are withered, and a winter drifts to where
The Pepperpot, ironic rainbow, spans
Charles river and its scale of scorched earth miles..."
The winter seems to be smothering the trees and plants. A "pepperpot" can be a small lighthouse, which seems to be casting a light across the barren Charles River.
"I saw my city in the scales, the pans
of judgment rising and descending."
The scales of justice, weighing good and bad, against Boston.
Piles of dead leaves char the air -- and I am a red arrow on this graph
I'm not so sure here...the dead leaves are a part of fall/winter. He seems to think of himself as the pointer on the scale of justice.
Every dove is sold.
The chapels sharp-shinned eagle shifts its hold
On serpent time, the rainbows epitaph.
The dove is a peace bird, and the bird that Noah let out from the ark. Compare that with the powerful eagle. Boston is being judged harshly.
I don't have the space left to quote the last stanza, so you'll have to look it up. It seems to be saying that while the city of Boston has been judged harshly in a biblical apocalypse, the narrator is able to transcend those things and remain a force that praises God and operates for "good." Jesus has helped the narrator to rise above the harshness of Boston.
I hope these ideas help you!