February 29 Avalanche Advisory

Weekend Avalanche Advisory
Posted February 29th at 0700

This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for February 29- March 2nd, 2008. The avalanche danger rating expires at midnight February 29. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas. This advisory is sponsored by Becky Richards and Moonlight Basin Ski Resort in memory of Ben Richards who died in an avalanche accident on Yellow Mountain near Big Sky on March 3, 2007. Ben is sorely missed by his family and friends here and in Big Sky.

Weather and Snowpack Analysis

Mountain temperatures have been swinging through periods of thaw and freezing with just a touch of moisture earlier in the week. The Twin Lakes SNOTEL reported a high of 50 and a low of 23 on Wednesday. At other sites high temperatures were in the high 30’s / low 40’s but all nighttime temperatures dropped back into the 20’s. SNOTEL stations reported close to an inch of snow water equivalent with little or no gain in depth. Some areas are finding surface hoar and faceted crystals under a thin crust that may become reactive if they receive a big load of snow. This condition seems to be most pronounced in the area around Lolo Pass. Observers here were getting easy failures 20cm deep into the snow with obvious surface hoar at this level. This will be a layer of concern during and after the next big storm. At all other locations observers were reporting a very solid snowpack and were not getting any appreciable stability test results. The report from the Lost Trail Pass area indicated that it would take a bulldozer to get it to fail.

Today’s Avalanche Danger

With cooler temperatures at the higher elevations the snow is locked into a solid state this morning. At all mountain locations of West Central Montana the avalanche danger is LOW, natural avalanches are very unlikely and human triggered avalanches are unlikely. As temperatures rise above freezing or whenever it rains, the avalanche danger will quickly rise to a CONSIDERABLE condition where natural avalanches are possible and human triggered avalanches probable.

Weather Forecast and Avalanche Outlook

NOAA Missoula Office is forecasting a ridge of high pressure to persist through Friday afternoon when a strong front and upper trough will move into the area late Friday. Because of the warmer than normal air mass, snow levels will start out fairly high well into Saturday when cooler air aloft moves in. Snow is expected to reach the valley floors Saturday and the Bitterroot Mountain passes are expected to receive 5”-6” of snowfall.
We’ve had several days now that have helped to further stabilize the overall snowpack. There are some notable exceptions to this as always. Many areas have buried surface hoar and radiation recrystalized snow (faceted snow crystals) that is not currently a problem but may be the next time we see appreciable snowfall. Be mindful of this and treat any wind loaded slope with a good deal of suspicion until you have ruled out any threat from these upper level weaknesses. During warm sunny days, travel early then move onto the shaded aspects later in the day. If it rains, it’s probably time to go home anyway. Rain will be a big problem for us especially above 6000′.

Be safe and keep sending those great reports! They really do help. Send observations to [email protected]

The next avalanche advisory will be posted on March 7th, 2008.