Eddie Byrom ton puts Glamorgan on top against leaders Nottinghamshire
Billy Root added a half-century for the Welsh county, whose nine-over blast at the end of the day saw Michael Hogan bowl first-innings half-centurion Haseeb Hameed for one with a ball that cut back.
Ben Slater and Matt Montgomery survived to the close.
Nottinghamshire face an uphill struggle on the fourth and final day to hold their rivals at bay, closing on 19 for one on a wicket offering some assistance to the bowlers.
Worcestershire’s win over second-placed Middlesex may have pushed Glamorgan down to fourth for now, but victory would put them second and close the gap on Nottinghamshire to just five points.
Glamorgan had started the day with a useful 86-run lead and Byrom and captain David Lloyd put on a 172-run opening partnership, the former racing to three figures in the morning session.
Nottinghamshire quick Brett Hutton resorted to short-ball tactics, but Byrom avoided the leg-side trap by hitting the bouncers like a swinging tennis serve to the straight midwicket boundary until the experiment was abandoned.
Just as it looked like the perfect morning session for Glamorgan, Lloyd fell for 60, caught at first slip by Montgomery off Hutton, the last ball before lunch.
Byrom and Colin Ingram smoothly added a fifty stand before the latter was undone on 33 by a ball from Liam Patterson-White carrying on with the arm, the outside edge pouched by wicketkeeper Joe Clarke.
An attempted reverse sweep saw Byrom depart leg-before midway through the afternoon, Patterson-White gaining some revenge. It meant Byrom fell short of his 176 against Sussex earlier in the season.
Kiran Carlson went for just two, caught behind by Clarke trying to steer the medium-pace swing of Steven Mullaney to third man.
If the rapid fall of three wickets brought any nerves to Glamorgan, they were eased by Northeast and Root seeing them safely through to tea.
Captain Mullaney and spinner Patterson-White took on the donkey work overs in the hottest part of the day, while Northeast and Root went on the offensive.
After a steady start, former England Lion Northeast motored to and past his fifty as the lead went beyond 400 and Nottinghamshire went to damage limitation mode, fielders spread and wide bowling.
Northeast passed his previous highest score for Glamorgan of 85, then Root made 50 before Northeast notched his century and the declaration came slightly after Root’s tame departure to a Luke Fletcher long-hop.
Support our Nation today
For the price of a cup of coffee a month you can help us create an independent, not-for-profit, national news service for the people of Wales, by the people of Wales.