Unstable snow pack

Attention! Slabs have already been triggered today due to wind transporting snow (wind-loading) and the weak layer of snow that has been in place for almost a month now. These slabs have been triggered naturally and at distances of 50 metres or more.

 

Weather Forecast (as reported by Météo-France Savoie on 24th December 2013)

for Savoie & Northern French Alps
A very pronouced low-pressure system north of the British Isles is causing us to have strong, often violent winds, and we are expecting some heavy snowfall in mountain areas on Christmas Day.
 
Tuesday 24th: 
Mild and windy! Strong winds throughout the day. Stormy gusts (sometimes over 100 km/hr) of Foehn and Lombard winds at areas along the French/Italian border, as well as in the Foothills (Pre-Alps) and at altitudes above 2500 m. Wind at 2500 m: SW to SE, 30 - 110 km/hr (up to 140 km/hr tonight). Wind at 4000 m: SW to S, 100 to 140 km/hr. Mild temperatures: 0° at 3000 m (1800 m tonight in the Haute Maurienne), -10° at 4500 m (over 3400 m tonight). Bands of cloud coming through throughout the day, building up in areas along the French/Italian border, especially in the afternoon, and some very light snow in the Haute-Maurienne (Lanslebourg, Val Cénis).
 
Tonight: Clouds thickening up. Stormy winds along the French/Italian border, Pre-Alps and above 2200 m. Heavy snowfall from the Aiguilles d'Arves to Mont Blanc, and into neighbouring mountain areas too. Temperatures +2° at 1000 m -2° at 2000 m and -7° at 3000 m.
 
Wednesday 25th:
"Violent" winds throughout the morning, slackening off slightly as the day goes on. A lot of snow over the Foothills (PreAlps) during the afternoon, with more modest amounts elsewhere. Rain snow limit 1200/2200 m, down to 100/1500 m overnight. 50 cm snow expected in the Bauges (between Chambery & Albertville), 35 cm in the Beaufortain (Les Saisies, Areches) and 10 to 40 cm elsewhere.
 
Thursday 26th: 
Wind turning NW and lessening in strength. Cloudy morning with a few light snow showers in mountain areas. Becoming dry and feeling colder.
 
Friday 27th:
Moderate NW winds. Feeling warmer, particularly at altitude, with sun and light cloud.
 
Saturday 28th & Sunday 29th:
Saturday: Snow across Savoie in areas above 1500 m, and down into the valleys overnight. Sunday: widespread snow showers in the morning, brightening up as the day goes on.

Monday 30th: 
Sunny with a few cloudy spells. Temperatures normal for the time of season.

Tuesday 31st: 
Variable then snowy
 
Weds/Thurs 1/2 Jan: More snow!

 

Off Piste Snow Conditions

On steep SE to SW slopes, there has been a lot of snowmelt, especially below 2800 m. Sunshine and mild temperatures have dampened the thin layer of fresh snow, although it still remains light and fluffy above this altitude. The layer beneath this recent snow is crusty - up to very high altitude. On NE to NW slopes, rain below 1500 m has made the snowpack humid to wet.
 
Higher up, a thin layer of less than 5 cm of fresh snow is lying on top of a layer of around 20 cm of recent snow, or on top of areas of deeper accumulation and windslab formed by the strong SW/S wind at high altitude (and Foehn near the French/Italian border). These areas are not bonding well with the cohesionless sub-layers underneath. Slabs have already been triggered today and natural releases have been observed today, December 24th (see photos).

Snow Stability 

Strong SW to SE winds, together with Foehn and Lombard winds at high altitude, are funnelling around more sheltered areas too at very high winds speeds, leaving very few places untouched. Recently-fallen light snow and older sugary snow is being blown and building-up in certain areas, and this wind transport of snow (known as wind-loading) is creating hard and soft slabs. These slabs will be very unstable, particularly on sheltered NE to NW slopes. Some areas of slab could release naturally, leading to avalanches, or could be triggered by the weight of just one person (see photos).

With the mild, sunny weather, we could also see some slides/avalanches of humid snow or naturally releasing glide cracks. There was a tragic reminder of this in Alpe d'Huez, Isère, a few days ago, when a skier was killed by a wet snow slide on a SE low-angled slope - see the account on the PisteHors.com.  
 
Check out the regular N. French Alps avalanche bulletins by Météo France .
 
 

Tip of the Week

Go for the easy stuff on low angles slopes to start with!
First hang out in areas where the slopes are not steeper than 25°- 30° (about average black run steepness to steep red-run steepness) or less. Hold back and think before you go onto steeper slopes with fresh snow- even if there are several dozen tracks on steeper slopes… 

Slopes facing North'ish will be critically unstable with the new snow and wind-loading, but remember, with any warming (especially if/when temps go above 0°), the South facing slopes will become unstable too.

..if there’s a lot of recent avalanche activity we all will be staying in areas where there are no slopes more than 30° steepness.  

 
Ride Hard, Ride Safe and happy holidays to all!
 
At HAT we'll be sharing what we find with you again each week here. We'll be updating our blog as much as possible, especially if conditions start to look unstable (or if we have some nice photos from a great ski!). Don't forget you can follow us on www.henrysavalanchetalk.com/blog, on Twitter @HenryOff_Piste and on Google+ and Facebook

 

< back