What Scout compares it to is the day that they had discovered the old, rabid dog Tim Johnson heading down their street, and were all just tense, watching and waiting, seeing what would happen. They knew something bad would happen, they just didn't know what or in what form. Of that day, Scout says, "Nothing is more deadly than a deserted, waiting street." Scout feels the same way waiting for the jury to come in. She says, "The feeling in the courtroom grew until the atmosphere in the courtroom was exactly the same as a cold February morning...a deserted, waiting, empty street." She compares everyone just waiting to see what the dangerous dog would to, to everyone just waiting for what the jury would do. It is a subtle foreshadowing of the negative verdict that was about to be announced; the dog situation didn't end well, and neither did the case, and Scout's emotional radar picked up on that very insightfully.
Scout is waiting for the jury to come out with the verdict in chapter 21. Moments before the jury comes in, Scout tells her readers that the atmosphere in the courtroom reminds her of a cold February morning from a previous year.
The feeling grew until the atmosphere in the courtroom was exactly the same as a cold February morning, when the mockingbirds were still, and the carpenters had stopped hammering on Miss Maudie’s new house, and every wood door in the neighborhood was shut as tight as the doors of the Radley Place.
Scout does not go into any further detail than that, but it is enough detail to remind readers of events that take place in chapter 10. Jem and Scout are out playing with their air rifles, and Jem sees a dog acting weird in the distance. The dog is Tim Johnson, and just the sight of his odd behavior is enough to spook Jem into going home to tell Calpurnia.
“Cal,” said Jem, “can you come down the sidewalk a minute?”
“What for, Jem? I can’t come down the sidewalk every time you want me.”
“There's somethin' wrong with an old dog down yonder.”
Calpurnia immediately recognizes that Tim Johnson is a mad dog and is a major threat to everybody in the neighborhood. She immediately begins making phone calls to ensure that everybody stays inside. She also makes a call to Atticus. Atticus and Heck Tate show up, confirm that the dog is mad, and Atticus shoots the dog dead with a single shot.
Before Atticus shoots the dog, Scout takes a moment to tell her readers what the entire atmosphere of the neighborhood felt like.
Nothing is more deadly than a deserted, waiting street. The trees were still, the mockingbirds were silent, the carpenters at Miss Maudie’s house had vanished.
The dog’s presence brought an ominous and foreboding feeling. Scout knows that things aren’t right. It feels as if something deadlier is going to happen, which is what happens. She gets that same feeling at the courthouse. She knows that something isn’t quite right, and she is reminded of the awful conclusion to the Tim Johnson event. The reminder serves as a way to foreshadow the jury’s negative verdict. Scout feels as if something bad is going to happen, and those feelings are soon confirmed.