While booting up Y Couloir as a party of 2, partner 1 breaking trail looked up the remainder of the chute to see dry loose snow picking up speed & momentum from an unseen trigger above. Partner 1 called "slide" immediately and both skiers were able to move to the edge of the slide and out of risk of being carried. The slide continued to pick up in amplitude and moved dry loose snow down the majority of the couloir, passing one other skier lower on the ascent that also had time to move out of the way. Both skiers in the party transitioned immediately after the slide and skied the debris out of the couloir. While the snow in the couloir was dry & sugary, much of the overhead terrain was rapidly warming in the sun and shedding new storm snow.