Jamie Crooks, Alex Gogolev, Steven Hodges and Joe Hicketts scored for the Royals (29-19-4).
Shea Theodore had the only goal for the Thunderbirds (19-30-5).
The Royals opened the scoring when Crooks picked up a loose puck after a faceoff and snapped it past Thunderbirds goalie Brandon Glover.
Gogolev made it 2-0 with less than a minute left in the first, scoring from a perfect goalmouth feed from linemate Brandon Magee.
Elsewhere in the WHL it was: Edmonton 4, Prince Albert 3 (OT); Lethbridge 5, Regina 2; and Saskatoon 5, Moose Jaw 4.
At Victoria, Thunderbirds defenceman Theodore cut the deficit to 2-1 early in the second on the power play, blasting a rocket from the point that beat Polivka cleanly over his shoulder.
Victoria restored the two-goal lead on a 2-on-1. Glover kicked out the initial from Hodges' shot but it went in off a defender's skate.
Hicketts added an empty-netter in the final minute of the third.
Polivka made a number of key saves throughout the game, including all nine shots he faced in the third period.
Glover stopped 20-of-24 shots for Seattle.
Gogolev left the game early in the third period with a lacerated left foot.
Oil Kings 4, Raiders 3 (OT)
At Prince Albert, Sask., David Musil scored at 3:31 of overtime as Edmonton (39-11-5) got past the Raiders (30-18-5).
Hurricanes 5, Pats 2
At Regina, Ty Rimmer made 48 saves to lead Lethbridge (25-23-8) over the Pats (19-29-6).
Blades 5, Warriors 4
At Saskatoon, Brenden Walker's empty-net goal ended up counting as the Blades (28-21-3) beat Moose Jaw (18-27-9).