Rachel's poem runs two stories side by side: a summer day that turns to storm, and a love that goes the same way. Each pair of lines is built to match. The morning promises warmth, and so does the new relationship. Both start out better than she expected, and she says she "couldn't get enough."
Then both begin to fade. The sun sinks, the clouds roll in, the breeze gets colder. Meanwhile the nights feel longer, smiling hurts, and his eyes stop "smoulder"ing. When the rain finally pours, her heart gives in with it.
The speaker moves from hopeful and happy to heartbroken and worn out. The mood shifts the same way, from bright and easy to gloomy and resigned. The repeated openings ("Soon enough," "Finally") make the ending feel slow and unavoidable, like weather you can only watch arrive. The title's "cool change" is the relationship cooling, too.