Weekly Recap
Friday 3/27/20 - Thursday 4/2/20
This week on As the World Terns: By FAC Intern Rob Millspaugh. Video by Zach Guy.
The last week of March was the wettest in a month characterized by generally stable conditions. High pressure in the middle of the month provided us both the visibility to sight alpine objectives and the confidence to ski steep slopes. At the same time, however, this benign weather combined with cold overnight temperatures caused incremental snowfall to facet into a weak layer. Where this layer wasn’t destroyed by the sun or an ever-oscillating freezing line it was buried by more incremental snowfall. An inch or two here, some wind loading there.. While we were all busy digging out the ski crampons and skin wax, bit by bit a slab was growing in the alpine. It wasn’t until the last day of the month that things started to get spicy. A passing cold front Monday evening brought almost an inch of SWE to Flattop Snotel and on Tuesday propagating test results in the Flathead Range backed forecasters off of steep alpine terrain. Also of note was how rapidly conditions changed above the recent rain line (which varies between regions and even basins). Convective precipitation on Tuesday brought isolated intense snow showers and wind causing even greater variability. These complicated conditions surprised more than one party on Wednesday, when they accidentally triggered D2-D2.5 slab avalanches in the Flathead Range. Thankfully no one went for a ride. Since 3/24/20 Flattop has received 4.8 inches of SWE (Stahl = 3.6”, Noisy = 2.8”). That translates to some thick slabs that will only keep growing as active weather is expected to continue into next week. Although there is some uncertainty regarding the exact distribution of the underlying weak layer, it is most likely to be found on shaded aspects (Northwest through Northeast to Southeast) above the recent (and variable) rain line. What is certain is that this is not your typical spring corn cycle. We have winter problems in the alpine and they require an extra level of caution. This snowpack just pulled the Saran Wrap over your toilet seat. Don’t be the one to get fooled.
