Our conditions here in Girdwood are ripe now for roofalanches. It has been very cold for very long, with about a two foot compacted snowload on the roof. Our temperatures in December have averaged 10.7F below normal, and we've only been above freezing for a few hours, and that wasn't enough to shed the snowload.
Roofalanches can occur in any conditions just like any avalanche, but are especially prone to happen when temperatures rise above freezing for the first time in a long time. Not all roofs have enough of a slope to avalanche, but in Girdwood it snows so much that you need a sloped metal roof (or else you need to shovel it). Our roof is metal and sloped, but not sloped enough to shed the snow after each storm, so we need the above freezing temperatures for it to shed.
Above freezing temperatures are on the way over the next few days, along with some rain. Roofalanche warning! Our worst roofalanches came our first winter here, the winter of '06-'07, when roughly 4-5 ft of snowpack roofalanched. As the snow freefell about 10 ft off of the roof and onto the awning out back, it snapped some of the beams and we had to have the awning replaced, and with stronger beams this time. That could be your bones snapping! The whole house shakes when we get a roofalanche, probably about like during a 4.0 earthquake. It freaks the animals out (and us too), and wakes me up from sleep every time.
Not only is there the risk of trauma from the freefalling snow, but you can easily be buried (just like in a real avalanche) and die. An avalanche forecaster at the Chugach National Forest Avalanche Center, Jeff Nissman, was killed by a roofalanche just down the road in Portage back in 2004. What can you do to protect yourself from roofalanches? Basically, look upward at your surroundings when around a structure. When we're under the threat of roofalanches, I don't walk near the side of the house. It happens fast and you won't have time to avoid it!