Of all the major team sports, baseball’s draft is the most unpredictable and inexact science. It’s not uncommon for players to emerge out of obscurity in their draft year to become first-round picks; it’s also not uncommon for first-round picks to fail in the professional ranks at a relatively high rate.

As part of our extensive preview coverage of the 2009 draft, we’ve rolled out a revised list of the Top 250 Prospects. We initially compiled a list of the Top 500 Prospects in February to roughly coincide with the start of the 2009 college, junior-college and high-school seasons, and it’s interesting to compare the two lists to establish which players may have improved their draft worth over the course of the spring, which have more or less maintained a status quo, and which players may have even regressed.

Obviously, our lists of the top prospects represent our opinion, and we are quick to admit that they may vary to a degree from many major league clubs—just as the lists will deviate from one club to another—especially as we approach player No. 250.

But it’s still interesting to note that of the 10 players we ranked atop our top 500 list three months ago, nine are still in the top 10. The lone exception involves former Fresno State righthander Tanner Scheppers, who has improved from No. 12 to No. 2, and Oklahoma State lefthander Andrew Oliver, who has slipped from No. 10 to No. 22. But there’s no mistaking San Diego State righthander Stephen Strasburg, who began the season at No. 1 and has only strengthened his position atop the draft.

There have clearly been a number of players, however, whose draft stock has changed over the course of the spring, and we’ll focus on the ones who have helped themselves the most. We’ll look at players who were in the top 250 of our original Top 500 list, and players who were not in the top 250—or even ranked at all.

Three of the four players who have made the greatest strides since the start of the season are college lefthanders—Lipscomb’s Rex Brothers, who has improved from PG Crosschecker’s No. 162 prospect in February to the No. 11 prospect entering May; Kentucky’s James Paxton, who has gone from No. 472 to No. 16; and Texas A&M’s Brooks Raley, who has improved from No. 218 to No. 28.

All are now projected first-rounders, and if there is an obvious strength to this year’s draft, it may now be lefthanded pitching. There are now no fewer than eight lefthanders projected to go in the first round.

Brothers had a losing record in 2008, both as a sophomore at Lipscomb (4-5, 5.57) and in the Cape Cod League (1-4, 4.25), and walked a combined 81 in 133 innings. But he also punched out hitters at a high rate, mainly on the strength of two dominating pitches, a 92-94 mph fastball that topped out at 96 late last summer, and a hard, biting slider that overpowered hitters when he threw it consistently for strikes.

The key for Brothers has always been his control, and it got much sharper in the second half of the Cape League season as he gained confidence in his stuff, and it has continued to improve this spring. His slider, which has peaked at 88 mph, has also been a more dominant pitch than in the past.

Though the 6-foot-1, 205-pound Brothers, who went undrafted and was lighty recruited out of a Tennessee high school in 2006, still does not have a winning record (4-5, 3.01), he has pitched much more effectively this season. In 69 innings for a 17-27 team, he had allowed just 51 hits and struck out 101.

Paxton and Raley have shown similar improvement—Paxton in the velocity on his fastball, which has been up to 97 mph, and Raley in his overall development as a pitcher. He has an average fastball and an above-average slider, and depends mostly on pitchability for his success.

But for all the improvement shown this spring by the three college lefthanders, no one may have improved his stock for the 2009 draft more than Indiana righthander Eric Arnett.

The Hoosiers were expected to have two potential first-round picks in catcher Josh Phegley and lefthander Matt Bashore, and a couple of other possible early-round selections. But the 6-foot-5, 225-pound Arnett has blown past all his teammates and looms as a potential first-rounder himself.

After going undrafted out of an Ohio high school in 2006, Arnett went 2-2, 6.11 as an Indiana freshman and 4-5, 5.45 as a sophomore. Though his size and superior athletic ability—he won 10 letters in high school and holds school records for home runs, as well as for touchdown receptions and receiving yardage—intrigued scouts, Arnett had never made his mark on the mound until this season. In 76 innings as the Hoosiers ace, he is 9-1, 2.49 with 24 walks and 76 strikeouts.

Bigger and stronger this season, Arnett’s fastball has improved from 92 to 96 and he’s held his velocity deep into games. His slider has also become a dominant second pitch.

By contrast, players like Tennessee outfielder Kentrail Davis (No. 11 on Feb. 1, to No. 47 now), Florida shortstop/righthander Mychal Givens (No. 13 to No. 57), Baylor righthander Kendal Volz (No. 17 to No. 55), California righthander/outfielder Blake Smith (No. 18 to No. 64), Arizona righthander Jason Stoffel (No. 19 to No. 54), Phegley (No. 20 to No. 91) and Florida Southern shortstop Robbie Shields (No. 23 to No. 77) have essentially fallen out of the first round because of subpar or indifferent performances.

But no players have fallen quite as far as Northwest Florida State JC righthander Daniel Webb (No. 40 on Feb. 1) or Baylor righthander Craig Fritsch (No. 42), who have tumbled right out of the revised Top 250—and almost certainly out of early-round consideration by not coming close to living up to expectations. Cal Poly righthander Steven Fischback (No. 53) and Santa Clara catcher Tommy Medica (No. 88) have fallen out of the mix, as well, but their situation has been injury-related.

With Arnett and Brothers as the poster boys, here’s our double-take on the players who have done the most to improve their stock for the 2009 draft—from Feb. 1 to May 1.

First, we identify the players who were ranked among the top 250 in PG Crosschecker’s Top 500 at the outset of the 2009 season:

Second, we identify those players who were ranked in the bottom half of PG Crosschecker’s pre-season Top 500, or went unranked altogether at the outset of the 2009 season: