Solo 3:30 am start with 5.5 mile approach to the base of the east face of tinemaha. Skinned up the main bowl to approx 11,000ft. Boot packed from there to 30ft below the summit ridge. Final 200was much deeper and felt wind loaded. I accessed a rock/snow spine for the final 100ft. Just before the summit, while traveling across a spine ridge, i triggered a small avalanche on a ENE aspect. 18 inch crown. 10-15 feet wide. Time was 11am. I decided to put my board together and ride down the start of the debris field before making wide arcing turns down the center of the upper bowl. As I exited that bowl into the more east facing gully, i traversed more northerly into a separate NE facing bowl that had no signs of having slid recently. I was making wide arcing turns at approximately 30mph when i was hit from behind by an avalanche traveling approximately 50mph. Luckily i was on my heel edge, and was traveling down slope on my back. I was submerged for approximately 500 ft, went over a small ledge, and popped out the skiiers left side of the avalanche. It continued downslope another 500 feet or so. From the debris field, the crown looked 15-25 inches deep, 200ft wide. A second, lower bowl was sympathetically triggered, and the crown also looked 15-25in deep, 100ft wide. I was able to ride down to the flats, and hike most of the way to my car before hitching a ride back to my car. I feel lucky to be alive.