You may have noticed that there's lots of imagery involving the weather in this book. (Not surprising, since "thunder" is actually in the title.) Taylor sprinkles loads of meteorological events throughout the novel: rain, thunder, lightning—pretty much everything but a tornado. What gives? Let's look at a couple of examples.
The Logan children (and the other poor black children) have to walk to school—rain or shine. And a lot of the time, it's in the rain:
That night when I was snug in the deep feathery bed beside Big Ma, the tat-tat of the rain against the tin roof changed to a deafening roar that sounded as if thousands of giant rocks were being hurled against the earth. By morning the heavy rain had become a drizzle, but the earth was badly sodden from the night's downpour. High rivers of muddy water flowed in the deep gullies, and wide lakes shimmered. (3.14)
Having to walk through that is bad enough, but they have the added indignity being splashed by the white-children-only school bus "zooming from behind and splashing [the children] with the murky waters of the road" (3.4).
The rain, then, is a physical force that reinforces the separation that exists between the black and white children because of their unequal educational facilities.
Think about the sound of thunder for a moment. It's loud. It's powerful. It can roll with gentle warning from off in the distance, or it can erupt suddenly, with an ear-splitting crack. Either way, it tells you that something big is on its way.
When we first meet Mr. Morrison, his voice is described as "deep [and] quiet [...] like the roll of low thunder" (2.34). This is linked to the song he later sings, "Roll of Thunder, Hear My Cry," which is a song of defiance against oppression. And when we first meet him, we already know that Mr. Morrison has been known to fight back against injustice.
The "low thunder" of his voice foreshadows the violence he will have to do later in the book. Remember that when the Wallace brothers attack Mr. Morrison and Papa, it's storming: '[D]idn't none of us hear [the truck] coming 'cause of the rain and thunder and all' (9.175).
It's not surprising, either, that thunder makes a conspicuous appearance right before the final chapter, when the book's tension is at its highest: "Thunder crashed against the corners of the world and lightning split the sky as we reached the road, but we did not stop. We dared not. We had to reach Papa" (11.106).
Check out what's happened: instead of a "roll of low thunder," we've moved on a "crash" against the "corners of the world." All the tension that's been building up throughout the novel culminates in an apocalyptic weather event—one that has changed Cassie's world forever.
Now that's what you call a symbol.