Hour after hour, Tyree Jackson would sit watching YouTube highlights of great quarterbacks at his home in Norton Shores, Michigan. Tom Brady was a favorite. Jackson would watch the way he moved. The angle of his arm. How he gripped the ball. He'd memorize every detail. Then he'd move to the backyard, where he had a tire hanging by a rope from a tree, and throw and throw and throw and turn himself into a quarterback.

That's how he built the arm that led to one of the greatest careers in Michigan high school history, starting all four years at Mona Shores High and finishing fourth in state history in yardage and third in touchdowns. The arm that got him recruited by the University of Buffalo, where he started as a redshirt freshman and earned MAC Offensive Player of the Year honors for his 2018 junior season. The arm makes him potentially the biggest surprise of the 2019 NFL draft class.        

But that also is the source of the biggest concern over Jackson as a prospect—the reason he'd be viewed as a surprise and not a lock despite his arm strength, drive, size (6'7", 249 lbs) and combine results that have been making NFL coaches and scouts drool.

Most elite quarterback prospects work with private coaches from an early age to hone their instincts and technique. Jackson? "The first time he was ever on the field with a private QB coach was last July at the age of 20," says former NFL QB Jordan Palmer, the private coach working with Jackson now.

Will all those years of figuring it out on his own lead to debilitating issues at the pro level?

Palmer thinks it could actually be the opposite. "Tyree had a very limited development history, so he was able to make big gains in a short period," he says. "He's incredibly thorough, hungry and athletic. That allows him to improve really quickly."

My latest mock draft has Jackson as a third-rounder. He could be the type of player a team overpays for and ends up getting a scouting staff fired. Or, if Palmer's right, he could be the steal of the draft that late.

"Incredible where he could go with this," Palmer says.

The 2019 Senior Bowl roster was set. Executive director Jim Nagy had already put together an impressive roster of eight quarterbacks—the max in previous seasons—who would come to Mobile, Alabama, to work out for NFL teams' scouts and coaches.

Then Jackson decided to enter the draft following his redshirt junior season.

"He was too talented not to bring to Mobile," Nagy says. "In terms of pure tools, he is one of the most intriguing players, regardless of position, in this year's draft."

So Nagy broke the eight-quarterback rule, and in late January, Jackson took Mobile by storm.

Jackson's play seemed to improve daily, even though he was throwing to unfamiliar wide receivers and playing in uncooperative weather. And he impressed on a personal level too. "After spending a week with him down in Mobile, I can tell you he's got great energy about him...the type of guy that can connect with different types of people, which is critical for the QB position," Nagy says.

His draft stock started to climb.

Then in late February, when the NFL took over Indianapolis for the scouting combine, folks really woke up to his awesome potential.

One quarterback coach approached me in a crowded bar the night before the QB workouts there, just to pull me close and say: "Get Tyree up your board. He's going to dominate here."

And he did.

Jackson ran a 4.59 second 40-yard dash—the same number Cam Newton posted at the 2011 combine. And Jackson is even bigger than Newton, who measured in at 6'5", 248 pounds in '11.

Per the NFL database at MockDraftable.com, Jackson's 40-yard dash performance put him in the 91st percentile for quarterbacks who have worked out at the combine. In fact, he was in the 90th percentile or higher for his height, weight, wingspan, arm length, hand size, 40 and broad jump:

"Everyone thinks Josh Allen was this great athlete last year," one coach told B/R after Jackson's jaw-dropping performance in Indianapolis, "but let me tell you: Jackson is better."

Allen is a common comparison for Jackson, given their size, athleticism, small-school backgrounds and arm strength that scouts describe as the strongest they've ever seen. Patrick Mahomes and Newton are the only other NFL quarterbacks they see as having similar arm strength. And it's not just Jackson's ability to launch a deep ball but also his velocity and accuracy in throwing underneath that distinguish him.

During combine workouts, NFL Network analyst and former All-Pro receiver Steve Smith Sr. even approached Jackson to tell him he was throwing too hard—something no one had ever seen during the on-field workouts in Indianapolis.

Par for the course for Jackson.

"I've worked with some of the brightest young QBs in the NFL, and Tyree's upside is unparalleled," Palmer says. "His size, arm talent and instincts put him on a trajectory that I haven't seen since Patrick Mahomes was a soph at Texas Tech."

Adds Nagy, "He's big, athletic, and he has a hose for an arm."

That simple? Of course not.

Traits matter, and Jackson's blend of size, speed, arm strength and personality are enticing enough to excite evaluators who see elite-level athleticism and arm talent. But overlooking the weaknesses in his game when your job is on the line is another story.

All of those traits will mean nothing if there isn't accuracy to go with them. And that's where NFL scouts get worried about Jackson.

"[Jackson] is scary because you saw him against these small-school dudes, and he still couldn't hit 60 [percent] of his passes," one scout says. "Yeah, he has a big arm, but he's not NFL-ready, and accuracy is one thing you can't coach up."

Making the throws isn't a problem with Jackson's arm strength, but getting the ball in the right area code has indeed been troublesome. His career completion percentage of 55.8 is well below the threshold the league likes (60 percent). And even in his breakout junior season, his 28 touchdowns and 3,131 yards came on 225 of his 407 passing attempts (55.3 percent).

That's why NFL teams need to be prepared for the prospect of his accuracy not improving, which is why an elite athlete with an arm that will immediately be top-five in the NFL is considered a Day 2 or Day 3 prospect.

It's also why phrases like "boom or bust" come up often when evaluators talk about Jackson.

"Traits" is a dirty word when talking to some coaches.

Says one AFC team's quarterbacks coach: "Traits get you fired, man. You can talk about traits in the media, but we want guys who've shown they can make the throws."

Still, betting on a player with some obvious traits but also areas of less certainty has worked before.

Most evaluators considered Mahomes a second-round pick before the Chiefs traded up to draft him No. 10 overall. Cam Newton was drafted first overall based on his athletic marvels and the belief in his upside as an athlete and a passer after starting just one year at Auburn. Russell Wilson fell to the third round because he lacked ideal height at 5'11", but the Seahawks saw his leadership and football IQ traits, plus a world-class arm to go with elite speed, and let him start as a rookie.

Is Jackson the next in that line?

"There is some rawness that will put him in the developmental category for most teams," Nagy says. "But he has the personality and the smarts, so some team is going to want to invest time into him."

The difference between one of these bets paying off and not can come down to the particular player's work ethic—his desire to improve in the areas of need.

The young man who trained himself to be a quarterback with YouTube, a rope and a tire ended up at Buffalo—bypassed by all the in-state colleges and Power Five schools in his recruitment process.

And if his lack of traditional QB grooming makes him more of a question mark than an answer for teams, it's also the reason not to doubt his ability to improve. He will always have the drive.

As Jackson told NFL.com's Brooke Cersosimo at the NFL combine, "I'll always have a chip on my shoulder, being from a small school."

Palmer sees it too. "The chip on Tyree's shoulder is massive," he says. "No one recruited him, everyone doubted him, and he continues to take what's given to him and create opportunity in what's not given."

Heading into the draft, it's more of the same. Jackson's arm is better than that of Missouri's Drew Lock, who has similar accuracy concerns. He's bigger and faster than Duke's Daniel Jones. And yet those QBs, coming out of the SEC and ACC, respectively, are first-round locks. Jackson's not.

He doesn't need to figure it out on his own anymore. He has Palmer, and soon he'll have an entire organization around him committed to helping him succeed.

Incredible where he could go with this.

Matt Miller covers the NFL for Bleacher Report. Follow him on Twitter: @nfldraftscout.