Shaun White Snowboarding

You need to gain 3500 points and obtain the "avalanche" bonus TWICE while in a single avalanche run.

PAUSE the game
Scroll down to "DESTINATION"
Choose "ALASKA" (if not there already)
Open the Map by pressing "BACK"
Tap "LEFT TRIGGER" till the map stops moving
Select the highest Helicopter point on the map

*REMEMBER* You can hold DOWN on the d-pad and press X to make a marker to warp back to if you mess up. Hold DOWN and press B to warp back to the marker

Once there, you will want to head down the mountain staying as far to the LEFT as possible.
You can check the map to see how close you are to the long avalanche zone on the top right of the map.
There is a jump just before the avalanche zone. Go off that jump doing a backflip. When you land the flip, the avalanche starts coming for you.

I found it easiest to stop and let the avalanche catch up till the "danger" meter was in the yellow. Then take off as fast as I could, paying attention to the meter, slowing when necessary. make sure to stay in the RED area of the meter. You will recieve the "avalanche" bonus for being in the danger zone.

The 3500 points has to be current points. Not the overall session points.
The achievement will pop while you are still in the avalanche.

The board I was using is available right from the beginning. It is called "stomper" and is a freeride type of board.

Here's a video to further help with achieving

Go to Alaska, go to the top and go to the danger zone to the left.
Wait until and avalanche starts and slow down until you're in the red danger zone. Keep that position until the "avalanche" logo which gives you 10k points pops up 3 times and the achievement is unlocked.