Not every top prospect hits the ground running the way recent Rookie of the Year winners like Evan Longoria and Ryan Braun did, stepping into a major league lineup and putting up All-Star caliber numbers. Sometimes it takes a couple of years’ worth of experience and adjustments for a high-upside player to reach his potential, but when he does, look out.

Our PECOTA projection system can help to identify such players via a trio of categories called Breakout, Improve, and Decline, which estimate the likelihood of a player’s production significantly rising or falling relative to his established baseline level. Breakout Rate is the percent chance that a hitter’s equivalent runs produced per plate appearance will improve by at least 20 percent relative to the weighted average of his performance over the last three years. A high rate generally indicates a high upside, though it’s worth noting the Ugueto Effect, in which the system will estimate a high rate for a horrible player simply because there’s nowhere else for him to go.

What follows are a handful of players-curiously concentrated among a small number of teams-whom PECOTA sees as excellent breakout candidates at the major league level this year, with Breakout Rates of at least 33 percent. Each is forecast for at least 400 plate appearances, a .275 Equivalent Average, and 2.5 WARP. Most are familiar names from recent Top 100 Prospects lists whom you’ll likely hear even more about as they approach their considerable potentials. Each is listed with his weighted mean triple-slash rates, his WARP forecast, and his Breakout Rate.

Justin Upton, Diamondbacks, RF (.278/.361/.506, 2.5 WARP, 59% Breakout Rate)

The first overall pick of the 2005 draft put up decent numbers for a 20-year-old last year (.250/.353/.463 with 15 homers in 417 PA), a sure sign that he’s a special talent. Nonetheless, his season was uneven; he clubbed five home runs in the first 11 games, but hit just .220/.345/.427 after April, spending most of July and August in Triple-A as he rehabbed an oblique injury and regained his confidence. PECOTA rates his breakout potential higher than any other qualifying player, and while he won’t turn into Ken Griffey Jr. overnight, he’s on his way.

Elijah Dukes, Nationals, OF (.278/.386/.486, 4.0 WARP, 54% Breakout Rate)

Dukes was dumped by the Rays due to some scary off-field problems, but the Nationals willingly took a chance by buying low, and their reward was the team’s most productive hitter last season, at least when he was available. Unfortunately, Dukes made three separate trips to the disabled list and underwent arthroscopic knee surgery, but he showed excellent power (13 homers), plate discipline (one walk for every 6.7 PA), and speed (13 steals in 17 attempts) in 81 games, while establishing some order in his life beyond the diamond. Forecast to match last year’s .302 EqA, the highest among qualifiers, he should provide a much-needed jolt to the Nationals’ offense, though as you’ll see, he’s not alone here.

Chris Young, Diamondbacks, CF (.268/.341/.487, 3.8 WARP, 43% Breakout Rate)

Let’s face it, PECOTA has a serious crush on the Diamondbacks, forecasting them for 87 wins last year and 92 this year thanks in part to the presence of multiple high-upside youngsters. Nonetheless, the Snakes fell short of their potential in 2008 because they ranked 13th in the league in Equivalent Average, with Young’s .315 OBP, mostly while batting in the lineup’s top two spots, one of their most glaring problems. His isn’t a simple case of plate discipline-Young actually improved his walk rate 27 percent from 2007, but contact problems and a low batting average on balls in play depressed his batting average well below what a player whose power and speed both profile above the 80th percentile should have. PECOTA retains plenty of optimism, hence his appearance here.

Lastings Milledge, Nationals, CF (.281/.352/.442, 3.3 WARP, 43% Breakout Rate)
Ryan Zimmerman, Nationals, 3B (.289/.358/.471, 4.2 WARP, 40% Breakout Rate)

Two more Nats crack this list, and aside from both being 24 years old, they’re as different as night and day. Milledge was a top prospect who had fallen out of favor with the Mets due to maturity issues; as with Dukes, general manager Jim Bowden took a worthwhile gamble by buying low. While his overall 2008 numbers weren’t inspiring, Milledge hit .318/.378/.485 over the final two months after returning from a groin injury, compared to .237/.300/.351 prior. As for Zimmerman, despite face-of-the-franchise status, he’s been receding since finishing second in the 2006 NL Rookie of the Year voting. A slow start and a shoulder injury took a huge bite out of his production last year, but a .306/.370/.455 second half heralded a return to form, and while he may not be David Wright, it’s far too soon to give up on him.

Edwin Encarnacion, Reds, 3B (.283/.365/.493, 3.3 WARP, 38% Breakout Rate)
Jay Bruce, Reds, RF (.283/.342/.509, 3.7 WARP, 36% Breakout Rate)

Despite last year’s spiking strikeout rate, a result of becoming too pull-happy, the 26-year-old Encarnacion has put up consistent .275-.280 EqAs over the past three years. Nonetheless, his managers have beaten him like a red-headed stepchild due to lousy play at the hot corner: 17 runs below average last year, 37 below for his career. The Reds’ crowded outfield and the presence of Joey Votto at first base preclude a much-needed position shift, so Encarnacion would benefit from a change of scenery, but even if that doesn’t happen, PECOTA remains confident that he can rake, foreseeing career highs in both on-base and slugging percentages. Meanwhile, Bruce topped our prospect list last year and hit .400/.500/.636 in his first two weeks after being recalled around Memorial Day. He struggled mightily thereafter (.232/.282/.425), and was eaten alive by lefties (.190/.263/.299), but PECOTA strongly believes he can overcome that drastic platoon split, forecasting a .257/.313/.468 line against southpaws.

Adam Jones, Orioles, CF (.278/.331/.470, 3.3 WARP, 36% Breakout Rate)

Acquired from Seattle last winter in the five-for-one Erik Bedard swap, the 23-year-old Jones is expected to be a cornerstone of the Orioles’ resurgence. His first year didn’t go quite as planned thanks to an ugly 4-to-1 strikeout-to-walk rate and a dreadful September (.228/.262/.380) following a return from a broken foot. Despite his problems controlling the strike zone, PECOTA sees enough in Jones’ combination of speed, power, and defense that his top comparables are Andre Dawson, Ellis Burks, Dwight Evans, and Garry Maddox, a quartet that combined for 13 All-Star Game appearances, 10 Top 10 MVP finishes, and 25 Gold Gloves. Speaking of the Oriole outfield, keep an eye on recent acquisition Felix Pie, who fell just short of this list and who will flank Jones in left field.