Colorado Avalanche Bulletins

This bulletin is from the CAIC, which is an agency of the State of Colorado.

Colorado Avalanche Information Center (CAIC) Advisory

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Issued 05/24/2009 8:41 PM by Spencer Logan

Weather Discussion

It is May 24, 2008. This is our last update for the winter of 2008-09.

Forecasts will resume in November 2009 as conditions warrant.

Snowpack & Avalanche Discussion

Spring and Summer Avalanche Safety

There have been avalanche fatalities and accidents in Colorado every month of the year. Spring and summer avalanches are infrequent, and we can let our guard down but not drop it completely.

The majority of these avalanches are wet, when there is enough water in the snowpack for it to loose strenght.

In the spring the snowpack transitioning from a cold, multi-layered snowpack to one that is isothermal (one temperature) and uniform throughout. Lower elevations have turned isothermal and melted out. The upper elevations, especially northerly aspects, may not be quite there until mid-June. As the snow turns isothermal, it turns into rounded clusters of little ice pellets. These snow grains resemble crushed ice in a snow cone more than they resemble the angular crystals of mid-winter.

Free water is one of the primary keys to stability in an isothermal snowpack. Free water is liquid water either hanging between the grains or pooling in the snowpack. A little bit of water makes the grains stick together, just like damp sand that builds a strong sand castle.

Too much water pushes the snow grains apart and turns them to a weak slurry, just like wet sand that will not form turrets.

The transition from a little to too much water is subtle and can happen rapidly. Fortunately, there are several signs to help us recognize the transition.

One is overnight temperatures. Cold temperatures allow the snowpack to re-freeze. It will be strong in the early mornings, allowing for fast, safe travel. Climb when it's cold, and descend as the snow softens. Our list of weather stations will help you determine the overnight temperatures.

* Recent snow or low elevations can become weak after one or two nights without a freeze.

* Summer snow can become weak after two or three or more nights without a re-freeze.

* Late summer snow may not need a re-freeze, because any free water drains out rather than sticking between the grains.

Another factor is high day time temperatures. Shallow snow around rocks, the edges of snowfields, and overhanging cornices or recent snow will warm up faster than deep isothermal snowfields. These are likely places to trigger an avalanche. Several hot days in a row, with warm overnight temperatures, will increase the potential for avalanches.

Again, the list of weather stations will help you determine the daytime temperatures. These weak spots are greatly reduced in the summer, when the snowpack drains out easily.

You can determine how strong the surface snow is by how deep your boots penetrate into the snowpack. It is boot penetration, and it does not matter what you are riding, you need to see how far your boots sink into the snow.

• Boots on the surface and the snow is frozen or just corning up. Keep climbing or wait just a bit for wonderful, silky corn turns.
• Boots sink to your ankles means it is time to descend. The surface snow is starting loose strength.
• Boots sink to your calves and you are getting red flags from the snowpack. Choose a route that avoids descending steep slopes, and avoid crossing under them.
• Boots sink to your knees and you are getting flashing sirens. It is past time to get off of and out from under steep slopes. Bare ridges and shallow slopes are your safest options for routes.

Overhanging cornices should be suspect any time the snow is soft. They can give way with little warning, and might be sufficient to trigger avalanches on the snow fields below. If you are climbing routes with overhanging cornices or contemplating hucks, make sure you are there while the core of the cornice is still cold and strong. Remember that cornices may see the sun much earlier than the snowfields below them.

It helps to know the micro-topography of routes before you ascend. A shaded couloir may have frozen, strong snow, but could have avalanches running down it from sunny snowfields above. Careful perusal of maps and guidebooks can be of great benefit.

Most free water drains away smoothly once the snowpack is completely isothermal and uniform. There will be little build-up of water during the day, instead it drains way and feeds rivulets and streams. Most snowfields that last into mid-June will be draining well. Avalanches become a slight concern, but still happen. Several of the fatal summer avalanches have been triggered by late season cornice-fall. Again, watch for routes that have overhanging cornices.

Constantly monitor the condition of the snow surface. Remember that slopes above you can be much sunnier, and therefor weaker than the snow you are on. The snow can quickly switch from supportable, strong corn to weak slush.

Thanks for the good winter. Enjoy the summer snow and other pursuits.

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