Regina's Sam Steel and Lane Zablocki each scored twice in the first period, chasing Hurricanes starting goalie Stuart Skinner from the net.
Lethbridge captain Tyler Wong scored in the second period to cut Regina's lead to 4-1 after 40 minutes.
After the Canes' Giorgio Estephan scored to make it 4-2 early in the third, the Pats poured it on with three more goals off the stick of Austin Wagner, Adam Brooks (powerplay) and Cole Sanford (empty-net). Sanford's goal came when Lethbridge coach Brent Kisio pulled his goalie with 8:00 remaining.
It was all part of a massive unravelling by Lethbridge in the third period, as Wong was given a five-minute major for charging and a 10-minute misconduct at the 7:11 mark.
Regina was 1/5 on the powerplay and 4/5 on the penalty kill as they wound up outshooting Lethbridge 35-25. The Pats' Adam Brooks had a goal and two assists to give up 12 points in the series.
Tyler Brown made 23 saves in the Pats goal while the Hurricanes' Skinner and Jayden Sittler combined to make 28 saves.
Regina advances to the second round the WHL Playoffs for the second time in as many years and now await the winner of the Red Deer-Calgary series.
REBELS 2 HITMEN 0
CALGARY - Trevor Martin made 30 saves as Red Deer blanked the Hitmen to take a commanding 3-1 series lead in its first-round playoff matchup.
Luke Philp opened the scoring in the first with Jake DeBrusk adding the other in the second for the Rebels.
Cody Porter kicked out 30-of-32 shots in defeat for Calgary.
WARRIORS 5 RAIDERS 0
PRINCE ALBERT, Sask. - Tanner Jeannot and Dryden Hunt both struck twice and Zach Sawchenko stopped all 36 shots he faced as Moose Jaw shut out the Raiders to advance to the next round of the post-season.
Brett Howden added a power-play goal for the Warriors, who won the series 4-1.
Rylan Parenteau turned aside 26-of-31 shots for Prince Albert.
ROCKETS 4 BLAZERS 0
KELOWNA, B.C. - Rodney Southam scored a hat trick and Michael Herringer made 42 saves as the Rockets shut out Kamloops in Game 5 on their first-round WHL series.
Rourke Chartier added a power-play goal for Kelowna, which took a 3-2 advantage in the series.
Conor Ingram kicked out 24 shots in goal for the Blazers.
CHIEFS 4 ROYALS 1
VICTORIA - Keanu Yamamoto had a short-handed goal and Kailer Yamamoto had a power-play goal as Spokane forced a Game 6 with a win over the Royals.
Curtis Miske and Jacob Cardiff also scored for the Chiefs, who trail 3-2 in the quarter-final series.
Jack Walker chipped in during the first period for Victoria.