The result ensures that the six-team championship will be decided after the final round robin encounters on Saturday.
Scores: Jamaica 170 for 8 from 50 overs; Leeward Islands 81 all out in 30 overs.
Sent in to bat after the Leewards won the toss, the Jamaicans were set back early and struggling at 29 for three after 12 overs. However, a fourth-wicket partnership of 116 in 32 overs by Donza Hyatt (79) and the classy Durale Forrest (52) not only repaired the early damage but restored the innings to respectability.
But once Forrest went stumped to Austin Richards and Hyatt top edged a return catch to Anthony Martin after smashing the pacer for a six and a four in successive deliveries, the Jamaicans could only muster a mere nine runs from the last four overs for the loss of three more quick wickets.
An expected Leeward Islands challenge failed to materialise has the spin duo of Andre O'Sullivan and Frew exposed their vulnerability to slow bowling.
In his very first over Frew, had top scorer Austin Richards (13) bowled playing back before choking Mali Richards, son of West Indies batting great Viv Richards, with a googly. When wicketkeeper Peter Caballero had captain Chaka Hodge brilliantly stumped for 10 as the batsman attempted to leg glance Frew, the writing was on the wall.
At Kaiser: A brilliant 113 off 128 balls in 158 minutes by opener Garvin Roberts carried the Windward Islands to a massive 169-run win over Trinidad and joint leadership in competition.
Scores: Windward Island 254 for 6 off 50 overs, Trinidad 85 off 31.2 overs.
Windward Islands openers Roberts and skipper Sergio Fedee flayed the tame Trinidad bowling and posted 135, before Fedee was run out for 52.
Trinidad were then bundled out for a paltry 85 as spinner Alston Bobb's 4-9 off 10 overs spun a web around the batsmen as only Johann Marin (16), Syed Ragoonanan (13) and Nicholas Ramjass (11) entered double figures.
At Folly: Guyana's Narsingh Deonarine made 113 not out as his team defeated outgoing champions Barbados by seven wickets.
Batting first, Barbados were bowled out for 183, one ball short of 50 overs. Dwayne Smith top scored with 71 from 120 balls, Rohan Nurse made 37, Ryan Nurse 27, and Kayode Pantophlet 23.
For Guyana, Roopnarine Ramgobin took four for 32 from nine overs and Suraj Paltoo two for 30.