Somerset boosted their hopes of defending their title in September by seeing off South Group leaders Surrey by six wickets in Taunton with an over to spare.
A century stand in 68 balls between Rory Burns and Sam Curran helped Surrey recover from 24-3 in the fourth over to post 182-5.
Red-ball skipper Burns hit eight fours in a 44-ball 62 before top-edging back to bowler Jake Ball while Curran made 59 from 42 deliveries before holing out to Craig Overton, whose brother Jamie smashed an unbeaten 30 off 13 balls to finish the innings.
It was not enough, however, as the hosts produced a near-perfect chase, smashing 19 fours and five sixes.
Three of those maximums came from Tom Kohler-Cadmore who top-scored with 51 from 32 deliveries after 20-year-old opener George Thomas blasted 40 from 25 balls.
Howell stole the show for Hampshire as they scored 31 from the final two overs to beat visiting Gloucestershire with a boundary off the last ball.
Miles Hammond made 66 and put on 92 with Cameron Bancroft (34) inside 11 overs, but the visitors stumbled from 125-1 in the 15th over to post 176-9.
Only Ben Charlesworth (39 from 19 balls with three sixes) was able to advance the score before falling to Brad Wheal, who picked-up 4-35 as the hosts lost five wickets in the final three overs.
It still looked like it might be enough as the Hawks were four down and needing 101 runs from the final 8.2 overs.
However, with some able support from James Fuller (20no), Howell smashed four sixes in his incredible 62 from 27 deliveries and brought up a five-wicket win with his fifth four from the final ball, an audacious ramp off David Payne.
Sussex strengthened their grip on a knockout place with an 88-run demolition of Kent at Canterbury.
Tom Alsop bludgeoned seven sixes in a blistering unbeaten 87 from 41 balls as the Sharks posted 203-4. After coming together on 64-4, Alsop and John Simpson (53) amassed an unbroken fifth-wicket stand of 139 from 12.1 overs.
James Coles was the star with the ball, claiming 4-12, including Spitfires opener Tawanda Muyeye (45 from 25 balls) – more than double any of his team-mates – as the hosts lost their last seven wickets for 39 runs to crumble to 115 all out with 22 balls remaining.
Luke Hollman took four wickets in two overs as Middlesex picked up another win to dent Glamorgan’s quarter-final hopes in soggy Cardiff.
The match was reduced to 10 overs a side by the elements but it was the hosts’ No.10, Mason Crane, who top scored with 18 not out as they posted 88-9 – their unbeaten last-wicket stand of 18 being their highest partnership.
Hollman ousted skipper Kiran Carlson, Tom Bevan, Chris Cooke and Marnus Labuschagne to finish with 4-13 from two overs.
His side looked to be cruising home but three wickets fell in nine balls, only for Joe Cracknell to find the ropes to secure a six-wicket win with three balls in hand.