The National Weather Service is forecasting another wallop of snow to land in the mountains of Northwest Montana beginning tonight (Thursday) and continuing into Friday. This additional rapid loading of snow will cause an increase in sensitivity of any existing weak layers within the snowpack.
BOTTOM LINE: Backcountry travellers should exercise caution by choosing low angle terrain until this new snow has had time to settle and strengthen. There will be a high risk of natural and human triggered avalanches throughout the weekend due to this rapid loading of storm snow.
The snowpack of the Purcell Range (i.e. Buckhorn/Yaak/NW Peaks) is still harboring some persistant weaknesses that have gained strength since last weak but still remain an issue for backcountry travellers.
1. Februrary 16th rain crust: Due to the colder temperatures of the Purcell Range this rain crust layer is not likely to be found above 6,500' in elevation. Below this altitude the snow that sits on this crust layer is very reactive and likely to propagate in steep open terrain.
BOTTOM LINE: The stability of the Purcell snowpack has improved considerably during the past week; however, with the incoming weather predicted I would still urge caution in terrain and travel selection until the new storm snow has had time to settle.
Check out the forecast for the Idaho Panhandle by clicking the link below!
Today we travelled back into the Pete Creek drainage to check on the progress of the weak layers we found in last weeks stability tests. The buried surface hoar layer was found in 2 of 3 pits but showed no reactivity in stability tests. The new snow is still bonded poorly to the rain crusts and is quite reactive in pits below 6,000'. The base of the Purcell snowpack consists of facets and depth hoar that developed during the early part of the winter when the snow was thin and the temps were cold. This weakness is likely to stick around through the winter and is unlikely to be a problem to riders unless a thin weak spot is found. It would be prudent to avoid areas where the snow is thin and scoured as this would be the most likely place to trigger and avalanche on this weak layer. Avoid areas where you see rock sticking out of the snow. Remember, thin snow is weak snow!
Backcountry Forecast from NWS Missoula issued:
220 PM MST Thu Mar 9 2017
DISCUSSION: Widespread light snow down to 1500 feet this morning
will slowly rise throughout today, particularly for all ranges
south of I-90.
Significant heavy snow remains in the forecast for northwest
Montana ranges, particularly in the vicinity of Glacier Park, this
evening through Friday. An arctic airmass will move west of the
Continental Divide today and stall along the Canada border today.
The stalled edge of the arctic air will be a focal point for
periods of intense snowfall. Up to 2 feet accumulation is expected
along the Continental Divide. The arctic air will slowly retreat
east late tonight, with snow levels and snow density increasing
over time (i.e., upside down snow pack). Wind will also be a
factor right along the Continental Divide, where east winds
gusting to 30 mph will transport fresh snow to west facing slopes.
Large fluctuations in temperatures will add to dangerously
variable backcountry travel conditions.
Conditions for backcountry travel improve this weekend with high
pressure overhead, however it is not necessarily a dry forecast.
Snow levels will increase substantially through the weekend,
reaching 5000 feet at times. Additional storm systems will bring
more snow, mostly above 5000 feet, through the next week. Mountain
snow pack will continue to steadily increase.
--------------------------- 5000-7000 FT ----------------------------
Tonight Fri Fri Night Sat
Cloud Cover 95% 90% 75% 80%
Hi/Lo Temps 21 to 30 34 to 39 22 to 27 32 to 38
Winds(mph) SW 11G31 SW 21G47 SW 13G33 S 7
Precip Chc 100 100 30 40
Precip Type snow snow snow snow
Liquid Amt 0.77 0.49 0.01 0.05
Snow Amt(in) 9-11 5-8 0 0-1
Snow Level 1000 3000 4500 3000
This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.