When we used Statcast™ to introduce Catch Probability before the season started, we knew it could have some really interesting applications toward reimagining how to quantify outfield defense. We used it to show why the Rays were happy to pay Kevin Kiermaier, which teams had the most highlight plays last year and which outfielders were just the best overall.
That was all looking back to 2016, however. This year is the first time we've had it available as games are going on, and with the first month of the season in the books, it's time to take a look at '17's most interesting catches so far. Remember, here, that "best" has a very specific meaning, because Catch Probability is measured based on "How far did the fielder need to go?" and "How much time did he have to get there from the pitcher's release?", and based on those two inputs you can say how often a similar opportunity was converted into an out since '15. (A play with a 99 percent Catch Probability is virtually always made; a play with a one percent Catch Probability is virtually never made.)
That makes this a very speed-based metric, and that's a good thing -- of course guys who can run like Billy Hamilton, Kiermaier, Ender Inciarte and Mookie Betts should be expected to rank very well. But what this doesn't account for right now are home run robberies, so don't expect to see great wall-leaping grabs by Christian Yelich and Albert Almora Jr., here. We can all but guarantee this warning will not actually prevent people from asking about them.
On to April's hottest catches!
BEST CATCH, OVERALL
6 percent -- Max Kepler, April 14 (34 feet in 2.7 seconds)

Remember back in the spring when we said that despite the fact the Twins' outfield defense was poor last year, it looked likely to be a strength in 2017? So far, so good, as the gloves have been for real, even though Byron Buxton has not lived up to expectations with the bat. Yet while Buxton has had some wonderful moments, like when he robbedAlex Gordon on April 3, he's not the Twin who had the top catch of April. It was Kepler.
Watching the video, you can immediately see that there are different types of amazing catches. Sometimes, there are catches that require great speed over a great deal of distance, like when Hamilton covered 123 feet to steal a hit from Carlos Beltran last year. And sometimes, as we see here, it's about quick movements and good reflexes. Kepler had to cover only 34 feet to get to Kevan Smith's sinking liner, but he also only had 2.7 seconds to do it. More than 90 percent of the time outfielders have had similar chances, they haven't made it.
"Raindrops did fall in the Twins' outfield [tonight]," said the Twins' broadcast, "but that's about it." That's a good line.
BEST CATCH THAT DIDN'T NEED TO LOOK DIFFICULT
14 percent -- Odubel Herrera, April 22 (88 feet in 4.6 seconds)

What's particularly fun about this one is that it's a very good example of how some outfielders can do spectacular things in ways that don't necessarily look like unbelievable highlights. 
That's not to say that Herrera didn't make a great play to rob Dansby Swanson's 104.8 mph rocket, because surely he did, and that he ran into the wall after was the cherry on top. It's just that he got there so quickly that he didn't even need to leave his feet to make a catch on a ball that falls for a hit more than 85 percent of the time, and too often, we tend to define "great plays" based on dives. (See, for example, Curtis Grandersonlaying out to make a nice-looking catch in New York, but receiving just a 75 percent Catch Probability since a faster outfielder gets there more easily.)
"The shortest distance between two points is a straight line," said the Phillies' broadcast, "and he was on a beeline for that one." Herrera began the play 87.7 feet away from the ball's projected landing point, and he covered 88.6 feet to actually get there. Nailed it, Phillies booth.
Herrera, for what it's worth, leads baseball with three Five-Star catches (those with Catch Probability of 25 percent or less) in April, including a fantastic play to rob Yasiel Puig on Saturday night:

BEST CATCH, LONG DISTANCE DIVISION
24 percent -- Kiermaier, April 6 (125 feet in 5.9 seconds)

The greatness of Kiermaier, as it so often does, has changed the game. We wanted to show clips for both the greatest distance covered and the highest speed achieved, but Kiermaier managed to do both of these on the same play, this long haul in early April to slice between his teammates and grab a Josh Donaldson popup. So for this exercise, we'll focus on just how far Kiermaier had to go, and put speed aside for a second.
When you're watching that video, your first thought is "Well, that was a popup with nearly six seconds of hang time, Kiermaier had practically an eternity to get there," and that's true. But since he was shaded well toward right field against Donaldson, just look at the distance he had to travel to get to where the ball came down:

At the start of the play, Kiermaier was 125 feet from the ball's landing point. Left fielder Mallex Smith was 104 feet away. Shortstop Timothy Beckham was 106 feet away and second baseman Brad Miller, shifted to the left side of second base, was only 86 feet away. There were three Rays who were closer to the ball than Kiermaier was, but it was still him, using his elite speed, who made a catch on a play that doesn't get made by similar outfielders 75 percent of the time.
BEST SPEED, NON-KIERMAIER DIVISION

30 percent -- Hamilton, April 12 (99 feet in 5.1 seconds)
You're shocked that we're using Hamilton as our example of great speed, right? The Pirates' announcers weren't, exclaiming on this play that "it's extraordinary the amount of ground Hamilton covers … it's just incredible."
As we introduced recently, "Sprint Speed" is our latest way to track running speed. It measures "feet per second" rather than "miles per hour," since feet and seconds make far more sense in the confines of a baseball field and game, and it looks at feet per second over a player's fastest one-second window. When we introduced it, Hamilton and Buxton were on top, which made perfect sense.
As we noted, 27 feet per second is roughly what an average fielder can get to when making a catch, and the range between "poor" and "elite" is approximately 23 feet per second and 30 feet per second. (Which makes sense, because six feet per second over several seconds would be a lot, if a fielder were going full speed the entire time.) Hamilton, on this play to rob Francisco Cervelli, made it up to 29.8 feet per second. This is an opportunity that's not caught 70 percent of the time, and he made it look easy. Hamilton is just the best.
COOLEST CATCH, WALL DIVISION
Lorenzo Cain, April 7

As we said, Catch Probability doesn't account for walls yet. It's based on "how far" and "how long," and by that measure, the 100 feet in 7.3 seconds Cain had to travel to rob George Springer wouldn't rate as standing out. (As we showed above, Kiermaier's catch was 125 feet in only 5.9 seconds, and it's clear on the video Cain isn't running at full speed.) Without the wall there, nearly every outfielder makes that catch.
Of course, the wall was there. Cain did time his jump absolutely perfectly, and he did flash his million-dollar smile at Springer, who tipped his cap to the outfielder who'd just taken away a nearly-sure hit. Similarly to Adam Jones's catch in the World Baseball Classic, what made this one memorable isn't that no one else could have made it, because that's probably not true. What made this one great is that the opportunity was there, and Cain made it look great. How could we not respect how cool that is?