In chapter 15 Jem says he's "just got this feeling." What events does this quote foreshadow?

Jem has always been a very observant young man. He has the uncanny ability to read people's expressions, interpret their behavior, and sense when something's not quite right. We see this illustrated in chapter 15 of To Kill a Mockingbird. One evening Jem answers the door: it's Sheriff Heck Tate. He doesn't come inside the house but instead Atticus goes outside to meet him. A group of men have shown up outside along with Heck; this is usually an ominous sign.
Initially, though, Jem seems to have got the wrong end of the stick. He thinks that the men standing talking to Atticus are some sort of gang, or maybe the Ku Klux Klan. It turns out that they're just regular folk, friends of Atticus such as Link Deas and Dr. Reynolds. Far from being some kind of lynch mob, they're actually trying to prevent one. They've heard whispers that a group of men from the country are going to descend on the courthouse to lynch Tom Robinson. So naturally everyone is concerned.
Although Jem may have misinterpreted what was happening outside, he's still incredibly uneasy. Later that night, Scout finds him lying on his bed, deep in thought. Jem tells Scout that he's scared something bad might happen to Atticus. His concerns are raised further when Atticus departs from his normal routine. After supper one evening, Atticus says he's going out, but instead of walking like he normally does, he's going to drive. Jem knows there must be something wrong, so he leaps out of bed to get dressed. He's going to get to the bottom of things. Scout agrees to accompany Jem on his little adventure—Dill too. When they're all outside Jem openly voices his concerns:

"I’ve just got this feeling," Jem said, "just this feeling."

Jem has every right to be worried; his dark sense of foreboding has proved to be entirely accurate. After a long search, the children eventually track down Atticus; he's sitting outside the jailhouse, his chair propped up against the front door. The atmosphere is tense, thick with drunken menace. As Heck Tate had feared, a mob of boozed-up country men have descended upon the jailhouse to lynch Tom Robinson. Despite Atticus ordering him several times to go home, Jem stands firm; he's not going to see his father get hurt. Yet it's Scout who saves the day by reaching out to Walter Cunningham Sr., the head of the lynch mob, and engaging him in conversation about the legal work that Atticus has done for his family.