Future 50: Gray challenging Appel

Jonathan Gray's ascent gives the Astros a strong alternative to Mark Appel at No. 1 overall. Andrew Woolley/Four Seam Images/AP Images

Welcome to the second installment of the Future 50 for 2013, my ranking of the top 50 prospects for this June's Rule 4 draft. (For version 1.0 of the Future 50, click here.)

This is not a projection or "mock" draft -- I'll start doing those in May, once team preferences start to narrow around specific sets of players. This list is a ranking by talent and upside based on my own scouting of players (live and off video) and conversations with scouts from all over the country.

As with my ranking of the top 100 pro prospects, I use the 20-80 grading scale in these comments to avoid overuse of the terms "average" and "above average" across the 100 player comments. On that scale, a grade of 50 equals major league average, 55 is above average, 60 is plus, 45 is fringy or below average and so on.

Now, on to the rankings.