Three days deep under the deceptively charming spell of the Boston Globe Jazz and Blues Festival, an afternoon downpour gave way to a power-packed night of killer improv, good feelings, and otherworldly vibrations on Day Four. While a seemingly arduous journey, especially for the uninitiated (1 hours of Robert Randolph's gospel magic as a primer to the Sco/MMW cosmic workshop), it was a journey well-taken.

Robert Randolph was his usual self, ticking the audience pink with soulful grooves and his ability to make any venue a makeshift gospel revival meeting. Although marred initially by sound problems (if there's one drawback to the Globe fest, it's that it always chooses performers who are best heard in small indoor venues), Randolph and crew sailed through exhausting, crackling jams all spearheaded, in usual fashion, by his mind-blowing pedal steel work.

If there's one drawback, however, that's slowly but surely throwing the brakes on Randolph's inspirational party train, its a creeping repetitiveness in two areas: Randolph's own chosen licks and setlist composition. Having seen Randolph on more than ten very different occasions, ranging from opening slots to full on club gigs to guest appearances with the likes of Gov't Mule and others, he's developed a habit of using hooks from the band's signature "I Don't Know What You Come to Do" when he runs out of improvisational places to go during a solo. Either that, or he tickles the high register, which is such a fun effect it's hard to criticize, but not after ten times in the same song. All great improvisational soloists hit this wall at least once in their careers, and thus this will set up an even more fascinating progression of Robert's playing: how he gets around it.

Chosen sets are another matter, and one that RRFB is ably dealing with, moving some songs around while knocking dead some newer covers (their versions of "Don't Worry, Be Happy," and "Purple Haze," which made an appearance today out of an earth-shattering "Voodoo Chile," are dynamite). But the newer original material (with the exception of the kickass "Going In the Right Direction") leaves much to be desired, and often seems little more than simple hooks worked up as an excuse to jam.

Still, RRFB maintain the same seismic punch that first pushed them onto the scene's A-list. Most encouraging of all is that the other members of the band (including recent addition Jay Caver on rhythm guitar) are given more time to shine in the long jams and in doing so reveal that, as much as their featured player is the star, RRFB remains a full-band affair.

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Riding high on RRFB's good vibrations, at least half of the crowd hopped the T downtown and grabbed the shuttle to the FleetBoston Pavilion (formerly Harborlights), where John Scofield and Co. were already heating an otherwise brisk night on the water. Good ol' Sco's already got two of the year's best discs already under his belt: Up All Night, a tighter and even groovier sequel to his last, Uberjam, which features the crew he brought to the stage with him tonight; and Oh!, from the superband Scolohofo (which features Scofield, Joe Lovano, Dave Holland and Al Foster).

This incarnation of the Scofield band, which features Lettuce drummer Adam Deitch, guitarist Avi Bortnick, and bassist Andy Hess, is his tightest unit in years. All accomplished players in their own right, they work well as a collective, able to follow Scofield as far out as he wants to go, but able to bring him back to earth when he gets a little too spacey. Using many of the tunes from Up All Night as playgrounds, Scofield deftly commanded a windswept outdoor venuenot an easy task for a band that harnesses much of its power from subtle nuances. Their many highlights included a set-closing grooveathon on Joe Lovano's "I Brake 4 Monster Booty," a playful and infectious tune that was enough to make 45 minutes until MMW bearable.

There are so many elementsamong them, tireless musicianship, boundless creativity, discerning earsthat go into maintaining Medeski Martin & Wood's status as kings of the rapidly expanding jazz-fusion arm of jam culture, it's difficult to break them down into separate categories, yet it seems easy to go into each one in passionate detail. Similarly, it's difficult, especially during their grooviest highs, to lock on to just one of the band members' playing, yet it also seems easy to describe one independent of the other two.

The end result is that Medeski Martin & Wood are the ultimate embodiment of that trite old expression "a whole being greater than the sum of its parts," and if there is another outfit of their ilk that contains so much individual expression yet operates so functionally as one unit, it's news to this journalist.

Too many so-called "groove musicians" are falsely praised for boring, meandering fluff that often reveals spectacular musical skill yet can't for the life of it produce something cohesive. The best of all these types of bands understand that being tight and locked into one another is directly proportional to playing loose, not the other way around. At its most basic level, MMW listen to each other; it is no great, cosmic mystery why they are able to get so unbelievably abstract at times yet captivate and engage an audience.

Granted, there are always faith-testing moments at any top-notch MMW performance: if the band didn't push the creative envelope night after night, what the hell would be the point? The opening run of an improv jam that led to "Rise Up" was a bit too amorphous for the tired crowd, but MMW quickly fired things back up by landing in "Beeah," an older favorite from the mid-90s that preceded the inevitable Scofield guest appearance.

The crowd perked up considerably as Scofield hopped aboard, stage right, next to John Medeski, whose island of keyboards had thus far dominated the jams. "Coconut Boogaloo" is always nice to hear, and augmented considerably by Scofield, who, like DJ Logic, speaks enough of the same language that he could easily be the fourth member of the band. But the four really hit the proverbial note on "Your Name Is Snake Anthony," which on MMW's latest Univisible is a bit subdued by Col. Bruce Hampton's narrative voice-overs, but here brought some of the most expressive highs of the two hour set.

Scofield's all-too-quick departure gave way to more earnest surprises, as the band funked through Peter Tosh's "Legalize It" and then "Pappy Check," which gave way to Chris Wood's now-standard, but no less captivating solo punishment of his giant standing bass. Medeski and Martin returned to segue into Univisible's "Ten Dollar High," and brought with them percussionist Brahim Fribgane, a virtuoso of African and Middle Eastern percussion styles and frequent collaborator with Boston's Club D'Elf. Fribgane's contributions weren't as forthright as Sco's and were a bit lost trying to compete with Billy Martin's table o' toys, but he colored the ensuing "Ten Dollar," and then later Mongo Santamaria's "Afro Blue," with simple rhythmic shades. Billy Martin's solo drums breakdown, a fascinating exercise in coordination and percussive trickery, soon paved the way for "Uninvisible," and the richly-rewarded crowd had no choice but to bring them back for a lone "Swamp Road" encore.

The fourth day of the Boston Globe's increasingly popular festival managed to ward off the rain and richly satisfy casual and trained ears alike with performances by three of the scene's most rewarding live performers. Though they each take the slippery concept of "groove" in different directions, they all manage to speak the same language when it counts.

(FYI: if you find yourself in the Boston area on Wednesday, July 2, haul ass down to the Museum of Fine Arts courtyard to catch both Medeski and Fribgane, among others, joining in with Club D’Elf)