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 06/05/2010 7:36 AM by Scott Toepfer 2 2

Highlights

It is June 2, 2010. This is our last regularly scheduled update for the 2009-2010 avalanche season. Forecasts will resume on or about November 1, 2010 as conditions warrant.

If you have observations on avalanche incidents or large and unusual events please feel free to send us the details. We really appreciate your observations.

Weather Discussion

If you're headed into the high country use our Weather Stations by Zone page to check the overnight low temperatures for the last few days. You can also check stream flows ( Colorado DWR, USGS) to see how much water is draining through the snowpack and into the rivers and creeks. Remember there is a lag between melt and measured runoff.

You can use our Weather Stations by Zone page to monitor past and present weather conditions too.

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 generally infrequent, and we can let our guard down a little bit, but not drop it completely. The majority of these spring avalanches are wet, when there is enough water in the snowpack for it to lose strength.

In the spring the snowpack transitions from a cold, multi-layered snowpack to one that is isothermal (one temperature) and with uniform grain types throughout. At the higher elevations, especially northerly aspects, this isothermal snowpack may not be quite there until mid-June or later. 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.

The transition to a little 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 by Zone will help you determine the overnight temperatures.

o Recent snow can become weak after one or two nights without a freeze.

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

o 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.

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

You can determine how strong the surface snow is by how deep your boots penetrate into the snowpack. It is boot penetration into the snow more than the type of equipment you are riding, you need to see how far your boots sink into the snow.

o 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.

o Boots sink to your ankles means it is time to descend. The surface snow is starting loose strength.

o 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.

o 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 hucking a cornice, 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. In general cornices are dangerous structures and should be avoided.

It helps to know the micro-topography of routes before you ascend or descend. 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 away and feeds into 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 there-for weaker than the snow you are on. The snow can quickly switch from supportable, strong corn to weak slush.

From all of us here at the CAIC, thanks for your support, and your observations. Have a safe and enjoyable summer and we'll be back next fall.

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