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Andrés Muñoz pitched a 1-2-3 ninth, striking out two to earn his 20th save this season.
The Bells have already clinched a postseason berth, with their first home playoff game coming on Saturday.
Seranthony Domínguez allowed four runs while only recording one out in his Mariners debut.
The M’s made a flurry of roster moves Tuesday to make room for their trade deadline additions.
Flau’jae Johnson scored 25 points, while Natisha Hiedeman and Dominque Malonga each added 17 for Seattle (6-26).
The Mariners had an eventful MLB trade deadline, dealing a longtime starter while adding the AL-leader in walks.
Reliever Andres Munoz (5-4) picked up the win despite giving up the game-tying homer on his second pitch of the ninth.
The Bells are entering a crucial series that could bring extra playoff baseball to Bellingham.
Bryan Woo gave up five runs and five hits in five innings while striking out eight.
Storm co-owner Celeste Keaton will miss their next five home games after an explicit interaction with two young fans.