"Rain and snow are falling", which means that on the way back from the wind and snow, it is raining and snowing.
The full sentence is "In the past, I left, and the willows were Yiyi. Now I come to think about it, and the rain and snow are falling." This is one of the famous sentences in the Book of Songs. When the soldiers went to war, it was the spring of Willows Yiyi. When they came back, It's winter time. It highlights the long time apart and also incorporates the emotions during the expedition and return home.
I think of the time when I went out for the expedition, where the willows were surrounded by lush branches and leaves. But now I was returning home in the snow, the road was long, the weather was cold, I was hungry and thirsty, it was very embarrassing and miserable. It is like a painting that vividly expresses the mood of a soldier (or traveler) who has been away fighting for many years.