PORTRUSH, Northern Ireland -- Shane Lowry is way out in front after three impressive days at Royal Portrush. But there are some big names -- Brooks Koepka the biggest among them -- lurking on the leaderboard. Oh, and there's bad weather expected to roll in -- rain and wind and who knows what else -- on Sunday.

So what's going to happen? We answer the biggest questions heading into the final round of The Open.

1. Shane Lowry has a 4-shot lead. What stops him from holding the Claret Jug on Sunday?

Bob Harig: Nerves. This is a huge moment for a guy who, candidly, has not won as frequently as expected. Before winning in Abu Dhabi earlier this year, Lowry's last win was in 2015 at the WGC-Bridgestone. And there's that four-shot lead he had through 54 holes at the 2016 U.S. Open. Plenty to process.

Michael Collins: Mother Nature and nerves. Since he's never won a major before, Saturday night will be a tough night for Shane. If the full force of what's expected shows up in Sunday's final round, it becomes a battle for Lowry to slow down and not watch the scoreboard to know who's coming at you. This will be a huge ask with two experienced major winners (Brooks Koepka and Justin Rose) just on the fringe.

Ian O'Connor: Emotion. Too much emotion. Irish fans behind the 18th green were going nuts Saturday night when Lowry appeared after his round. It felt like a premature coronation. He's already blown a 4-shot lead in a major, and he's never won a big one. So Lowry's going to be dealing with a whole lot of things that have nothing to do with the weather. Lowry said he's learned from his failure at Oakmont and will show up Sunday a different man. We'll see.

Mark Schlabach: I think Lowry is the only one who can stop him from winning. He has a 4-shot lead. Can he hold it? If Lowry handles his nerves and stays composed, the Irishman is going to win the first Open Championship played on Irish soil in 68 years. He's going to have nearly everyone at Royal Portrush cheering for him. He's been here before, though, and it didn't end well.

Nick Pietruszkiewicz: Letting those bad memories from 2016 come back to haunt him. He's been down this road before. He led the U.S. Open by 4 back then at Oakmont after a third-round 65. He shot 76 on Sunday and lost by three. And this one will be even tougher to close. He's played three great rounds. He's coming off a scintillating 63 (with a back-nine 30 on Saturday at Royal Portrush). Sometimes there is a letdown after super-low rounds. And there's perhaps the biggest piece of the equation: He's pretty much got everyone on the grounds in Northern Ireland rooting for him. That's a lot to handle.

2. How far is too far back? What player could make the huge move?

Harig: You have to look at the score in second place, and that is Tommy Fleetwood's 12 under. If Lowry falters, then that becomes the number, and anyone who is within six of that conceivably has a chance, especially if they shoot a number early and the weather turns frightful, as has been predicted. Jon Rahm is at 7 under and will tee off four groups prior to Lowry. He'll need a low one, but it's certainly possible.

Collins: Don't think for a second that Justin Rose or Brooks Koepka are too far back. While 7 shots seems like an insurmountable number, in really bad conditions those shots can be gone in three, four or five holes. I think Rickie Fowler and Lee Westwood (both 8 back) are too far behind, but that's also because of the mental baggage they carry from not having won a major. But if it gets crazy coming down the stretch, be glad you're on the sofa or in a chair and not inside the ropes wanting to scream for help.

O'Connor: I think 7 under, 9 shots back, is where the line should be drawn, giving the likes of Spieth, Rahm and Tony Finau a chance. Of course, Lowry has to falter, and one of those players has to catch fire and shoot 63. I think Fowler, at 8 under, makes a big move. Nearly everyone thinks he's pretty much out of it, and that might allow him to play relatively pressure-free golf in pursuit of that first major.

Schlabach: I think anybody within 8 shots might have a chance -- if Lowry gives some back. That leaves seven players in the mix. Maybe Jordan Spieth (9 back) or Henrik Stenson (10 back) can get back into contention if Lowry completely falls apart and the weather is really bad. Still, that seems like quite a stretch. If the weather makes Lowry lose his mind, it's not going to be easy for anyone else to play well and make up ground, either.

Pietruszkiewicz: This is going to sound nuts, but I'll say anyone at 6 under or better. Yes, I'm aware 6 under is 10 shots behind Lowry. The group at 6 under only has 11 players with better scores in front of them. While that is a lot of shots, that is not a lot of people. Strange things can happen in the final round of a major. Strange things can happen at The Open. Strange things can happen when the weather gets bad. And the weather is supposed to get bad. If 10 shots is indeed in play, that puts Rahm and Jordan Spieth (9 back) and Stenson and Matt Kuchar (10 back) in that you-never-know kind of position. But the guy who is going to come from the pack is Rickie Fowler, who is 8 strokes behind. But we'll get to him in a second

3. The forecast is calling for rain and possible wind gusts up to 40 mph. Is it mean to root for bad weather?

Harig: Heck, no. That's what The Open is so often about. We're allowed to enjoy the havoc nasty weather brings to the best in the world. It creates its own kind of drama and has a way of shining a light on how good these guys really are, as nobody typically can imagine being able to produce good shots under such conditions.

Collins: That's like asking if it's mean to root for someone getting knocked out in a fight! It's the Open Championship -- bad weather is supposed to be part of the deal here. I do love the fact that when the weather is perfect (for this part of the world), the best players can show just how great they are. But I'd be lying if I said I didn't want Mother Nature to punch the field in the face on Sunday to see who's left standing.

O'Connor: Never! To the average fan, golf is the most maddening game in the world. Fans want to see these highly skilled craftsmen, these conquerors of an unconquerable game, suffer like they do. When you're sitting on your couch at room temperature, what better TV entertainment could you find than a bunch of coddled multimillionaires getting drenched by rain and battered by wind?

Schlabach: I think the players know what's coming after Saturday's conditions. They got theirs on a dry day with practically no wind. Now, Royal Portrush is going to get its 10 pounds of flesh on Sunday. The Open Championship isn't supposed to be as easy as it was on Saturday. Half of the field -- 30 out of 73 players! -- shot under par. Harry Colt, the original architect of the famed links course, was probably rolling in his grave. The rain is going to come down in buckets, and the wind is going to howl. It's going to be magnificent.

Pietruszkiewicz: It's been mostly benign conditions for a week here in Northern Ireland. Everyone has had the picturesque views of the coast. The postcard looks at nearby Dunluce Castle and Giant's Causeway. We've all gotten to see gorgeous Royal Portrush in all its beauty. It's time for some madness. Let it rain. Let the wind blow. Let 80 be considered a decent score.

4. Tiger Woods won the Masters. Brooks Koepka won the PGA. Gary Woodland won the U.S. Open. Pick the American on the leaderboard who could give the U.S. its first major sweep since 1976.

Harig: Koepka. He's spent three days watching putts not go in, and yet he's still 9 under and tied for fourth. If Lowry had not gone on that late run, Koepka would be right there. As it is, he's a Lowry implosion from being right there.

Collins: Koepka. Remember, before dominating on the PGA Tour, he cut his teeth on the European Tour. That means he's played in conditions PGA Tour pros would normally never play in. Could J.B. Holmes catch Lowry? Only if Mother Nature makes a left and doesn't show up. Even so, Brooks' putter has yet to get hot. When/if that happens ... watch out.

O'Connor: You still have to go with Koepka over Fowler for the obvious reasons. BK finally made a putt on the 18th hole Saturday. If that wakes up his putter Sunday, watch out. Koepka has won four majors, or four more than the combined total of the three men ahead of him on the board. A second straight two-major year would be a remarkable feat for a guy who treats regular tour events like silly season exhibitions.

Schlabach: I think Koepka -- who else? -- is the American with the best chance to catch Lowry. If Koepka's putter was hot, he'd probably be in the lead, or at least closer than seven behind. He hasn't made much of anything all week, and yet he's still tied for fourth at 9 under. He didn't putt well at the U.S. Open at Pebble Beach and couldn't hunt down Gary Woodland in the end. Bottom line: He has to make putts to have a chance to win.

Pietruszkiewicz: I picked Rickie Fowler to win before this thing started. I'm not jumping ship now. Sure, he's 8 shots out, but he is tied for sixth. That's not a lot of people between him and Lowry. Basically, he's going to need help from Lowry. He's going to need help from Fleetwood, who is 12 under, 4 shots out of the lead, 4 shots ahead of Fowler and alone in second place. He's going to need help from Mother Nature.

5. OK ... who wins?

Harig: Lowry. Not a very daring choice, but with a 4-shot advantage, he has "created so many winning options for himself,'' as Padraig Harrington said. While poor weather could cause him some problems, it will for others, too. And perhaps grinding out a bunch of pars will be the key to an Irish victory at Royal Portrush.

Collins: Shane Lowry. The key for Lowry will be the crowd chanting for him. When he finished his round on Saturday, there were hundreds (maybe a thousand) fans standing near the scoring area and interview area literally singing to him. The smile he had on his face throughout the day should be plastered there Sunday even through horrid conditions, because of the fans. An Irishman winning the Open Championship in Northern Ireland will be some kind of party.

O'Connor: Tommy Fleetwood. One birdie (for him) and one bogey (for Lowry) early in the round, and the lead is right back down to two. Fleetwood has a ton of talent, and he has a lot less pressure on him than Lowry does. Plus, his rounds keep getting better (68-67-66). I picked Fleetwood after my original pick, Rory McIlroy, went home for the weekend. I'll stick with him.

Schlabach: Lowry. The good news for him? There are only three players within 10 shots -- Koepka, Justin Rose and Jordan Spieth -- who have won a major championship. The rest are a bunch of players also trying to win their first major.

Pietruszkiewicz: Fowler. Getting that first major has been a struggle, so why not get it in epic fashion with a Sunday charge unlike anything ever seen at The Open? Something wild is coming Sunday. Maybe it's this.