Skipper Daryl Mitchell led Worcestershire's chase for quick runs to set up a declaration after Gloucestershire declared 75 behind on day three of the Specsavers County Championship match at New Road.
The visitors pulled out at lunch on 364-6 after Graeme van Buurun (172 not out) and Craig Miles (60) shared an unbroken stand of 134 in 33.1 overs.
Mitchell went to a rapid half century off 57 balls with five fours before his dismissal and Worcestershire had reached 151-5 from 38 overs – an overall advantage of 226 – when rain halted play.
Gloucestershire had resumed on 226-5 – still needing 64 to avoid the possibility of being asked to follow on with the new ball due after two overs.
But Van Buuren and Miles steered the visitors past the 300 mark in relatively untroubled fashion.
Spinner Brett D'Oliveira broke through in the first over of the morning when he trapped Jack Taylor (6) lbw with only four more runs added.
Van Buuren and Craig Miles scored freely after the new ball was taken at 236-6 from 81 overs.
A lofted straight drive for four by Craig Miles off Worcestershire all-rounder Ed Barnard secured Gloucestershire their second batting point.
A square cut for four by Van Buuren off Joe Leach brought up the half century stand off 16 overs.
Worcestershire continued to probe away for a breakthrough and D'Oliveira returned to the attack.
But an on drive for four by Miles off Shantry steered Gloucestershire past the 290 follow on total and Van Buuren completed his 150 off 284 balls with his 18th boundary.
The 300 came up in the 102nd over.
Ed Barnard enjoyed a morale success when Miles on 47 edged just short of skipper Daryl Mitchell at slip before Gloucestershire declared.
When Worcestershire batted for a second time, Liam Norwell accounted for D'Oliveira lbw (3) after he had played back and knocked off Joe Clarke's leg bail before he had scored after looking to work the ball through mid wicket.
Alexei Kervezee got off the mark with a straight drive for four off Miles but was dropped twice, on five at backward fine leg and on 15 at short mid wicket.
Mitchell scored at an aggressive rate in completing his second Championship half century of the campaign.
But after making 57, the 32-year-old departed when flicking Miles off his legs into the hands of Hamish Marshall at square leg.
First innings century-maker Tom Kohler-Cadmore (18) shouldered arms and was bowled by Norwell on his return to the attack after tea.
Kervezee made 39 before lofting spinner Jack Taylor into the hands of Michael Klinger at mid on.
Spin was employed at both ends in murky light and there was time for Ross Whiteley (20 not out) to hit sixes off Van Buuren and Taylor before rain halted proceedings.