This message does not apply to developed ski areas INTRODUCTION: This is Stan Bones with the backcountry avalanche advisory issued by the U.S. Forest Service Glacier Country Avalanche Center at 6:30 AM, Friday, December 18th, 2009, for the Glacier Park and Flathead and Kootenai National Forest areas. HAZARD ANALYSIS The avalanche danger is currently being rated as CONSIDERABLE on all slopes between 5,500 and 7,500 ft. elevation. Below 5,500 ft we rate the danger LOW. These danger ratings expire at midnight, Friday, December 18th. The outlook is for the avalanche danger to inch upward this weekend as low pressure brings increased precipitation and cooling temperatures to our region. Because of the general nature of this advisory message, each backcountry party will always need to make their own time and site specific avalanche hazard evaluations. WEATHER ANALYSIS: Since last weekend the mtn ranges of NW Montana have received a significant shot of new snow. Beginning Monday this precipitation was accompanied by moderating temperatures. In the 5.5 days between Saturday and mid-day Thursday many mtn locations received over 2 inches of snow water equivalent, a couple of locations even over 3 inches. The least new snowfall appears to have been in the NW corner of our region in the N'ern Purcells where only approx 1 inch of snow water equivalent was received. In spite of the new snow accumulations most all of our mtn locations remain below normal for snow fall. The arctic cold over much of the region began to moderate on Monday. Daily average temperatures ranging from approx. zero F to the upper teens on Monday quickly rose into the low 30's by Wednesday and Thursday. Winds over the period has remained light. SNOWPACK ANALYSIS: Our backcountry observations on Wednesday on Flatiron Mtn and on Thursday on Big Creek Baldy Mtn were in the Purcell Range north of Libby on the Kootenai. On Thursday we also had observations in Noisy Basin in the Jewel Basin area of the Swan Range NE of Bigfork. In our backcountry tours and snow investigations both Wednesday and Thursday we were triggering whumphing, shooting cracks, and collapse of the snowpack. In snow pit investigations we were finding an inverted snowpack with warmer, more dense snow overlying colder, weakly bonded snow. Shear tests were producing quality one, clean, energetic failures with very little force. In the Purcells these failures were approx. 12 inches down from the surface on a layer of weakly bonded angular grains. In Noisy Basin the failure was upon the buried half inch thick Thanksgiving weekend rain crust. In Noisy we saw two slab avalanches that had released naturally, most likely either Tuesday or Wednesday, when temperatures were rapidly rising and the new snowfall was warm and dense. One was a small, steep rock rollover. The other was the western portion of Camp Misery or Crown Bowl which over most of its width had gone almost to the ground, failing on the basal melt-freeze ice from late October/early November. A portion of the viewers right flank of that slide had failed on and remained above the Thanksgiving weekend rain crust. Visibility was poor and we were unable to safely access the crown, but feel the initial failure was likely above the rain crust and then once in motion the slide was able to pull deeper down to the melt-freeze basal layer. AVALANCHE - INSTABILITY DESCRIPTION: We're currently rating the avalanche danger between the elevations of 5,500 and 7,500 ft. elevation as CONSIDERABLE. Unstable slab layers are probableon steep terrain on all aspects. Natural avalanches are possible, while human triggered avalanches are probable. Backcountry travelers should be suspicion of any steep smooth open slope lacking vegetative or terrain anchors. Be alert to whumphing and snow collapse. Confine your travel to lower angle slopes without steep terrain above. Below 5,500 ft. on all slopes, we're rating the avalanche danger as LOW. The shallow snow there is generally stable with only isolated areas of instability. Natural avalanches are very unlikely, while human triggered avalanches are unlikely. Backcountry travel is generally safe. Normal caution is advised. Like at the higher elevations travelers should be particularly cautious on any steep slope lacking vegetative or terrain anchors. WEATHER FORECAST & AVALANCHE OUTLOOK: Current Western Montana NWS Backcountry Weather Forecast The weather forecast is for a weak ridge of high pressure to build over NW Montana Friday maintaining the warming that began mid week. Friday night through Saturday a weak disturbance is expected causing a chance for shower development. A broad trough of low pressure should move into the Pacific NW later this weekend causing widespread precipitation and cooling temperatures. That disturbance could intensify even further Monday and Tuesday. Our current snowpack instability is one that will not be quickly going away. Be alert to obvious signs of instability. Until the deep instability strengthens backcountry travelers need to exercise vigilance, particularly on and beneath steep, open slopes. Because Christmas falls on Friday this year, our next regular update of this advisory message will be Thursday, December 24th, Christmas Eve. The week following Christmas we will start for the season our Tuesday advisories in addition to the Friday. We wish everyone a safe and joyous Christmas holiday.