What does it mean "torpid and simmering May"?

I am trying to understand this sentence, but I don't understand how "may torpid and simmering" in this context:
"Then the city enters another torpid and simmering May". Maybe weather in St. Louis special or somehting?
from this story of Pizzolatto
https://www.theatlantic.com/magazine/archive/2003/10/ghost-birds/302804/