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Weather Wise: Avalanche Outlook

Weather Wise: Avalance Outlook
Posted at 6:05 PM, Nov 15, 2021
and last updated 2021-11-16 11:12:17-05

When it snows in September and October, skiers and riders start rejoicing for the winter sports season to come.

However, winter usually does not lock itself in this early. Instead, the skies eventually clear and fall returns for a few weeks or even a couple of months.

With warming temperatures or no new precipitation falling on this early season snow, the snow characteristics begin to change. The snow melts and refreezes, or it begins to facet. Faceted snow crystals break down and form weak bonds with neighboring snow grains. This is often referred to as sugary snow.

If early season snow falls and sits around, the mountains are forming a weak foundation for the season's snowfall. When winter finally shows up in earnest, the snow falls on a weak base structure, similar to building a house on a foundation of sand. As this persistent weak layer gets buried, a persistent slab avalanche problem can develop. This early season weak layer will take time to strengthen or bond to new snow, remaining a problem well into winter.

This is exactly the situation that created very large avalanches in Montana in the last few years, especially around Cook City where avalanche crowns were nearly 15 feet tall, breaking down to the early season layer.

Some naturally occurring avalanches have already slid in the mountains this year, and there will inevitably be active avalanche cycles this winter. But the potential for mega-avalanches at this point is lower than the last few years.