Koe will face Brad Gushue of St. John's in the final on Sunday afternoon.
Howard, who won the Grand Slam of Curling Masters in Abbotsford, B.C., two weeks ago, scored two points in the eighth end to tie the game 3-3. But Koe managed a single point in the extra end to seal the victory.
Earlier in the day, Koe earned his semifinal berth by downing Rob Rumfeldt's Guelph, Ont., rink 7-4 in the quarter-finals.
In the first semi match of the day, Gushue scored single points in the first, third, fifth, and eighth ends to defeat Winnipeg's Jeff Stoughton 4-2.
Gushue began his day with a 7-4 quarter-final victory over Edmonton's Kevin Martin.
Howard beat Toronto's John Epping 6-3 in a morning quarter-final match to set up his berth in the semifinal, while Stoughton edged Toronto's Mark Kean 6-5 before losing to Gushue.