The subject of Amy Lowell's poem "St. Louis" is not only about the city of St. Louis, but it is also about New England. Her present situation (St. Louis) is contrasted to her past (New England). The first stanza vividly depicts the summers of St.Louis, while the second reflects upon New England. In the third stanza, the two different locations are shown using rapid images of each.
Readers know that she is located in St. Louis currently because she uses the words "now" and "here." Both of these words allow the reader to know that St. Louis is where she is currently at.
Basically, even though Lowell never specifically mentions St. Louis in the body of the poem, readers can assume that she is talking about the location from her descriptions of the landscape. In the same way, Lowell alludes to New England. She speaks of "lands of hills", "tired farm-houses", and "old meeting houses."
Lowell is reminiscing about her time in New England contrasting the differences between her current location and her past location (the one she dreams of and misses deeply).