Fresh snow + wind heightening avalanche danger over the holidays

Intensifying snowdrift problem at higher altitudes

Since Monday (21.12), quite strong westerly winds have been blowing at high altitudes, generating new snow transport. The fresh snowdrift accumulations are currently (23.12) prone to triggering particularly above about 2400 m. This borderline will descend when a cold front strikes on 24-25 December.

Snow plumes at about 2900 m in the Ötztal Alps (photo: 22.12.2020)

The background:

Act I: during the period of fine weather and mild temperatures (Thursday 17.12 – Saturday 20.12) surface hoar formed during the clear nights. In addition, the snowpack surface became more loosely packed, especially at high altitudes and on shady slopes. Both those things together created a weak layer for all snowdrifts deposited on top of it.

Surface hoar in the Tux Alps (photo. 18.12.2020)

Act II: a warm front (Monday 21.12 – Tuesday 22.12) unleashed precipitation throughout Tirol (focal point: western regions). Wherever it rained, the loosely-packed surface and surface hoar became moist, thus destroying the potential weak layer. That’s the reason snowdrifts currently can be triggered only at higher altitudes, i.e. where both the loosely-packed surface and the surface hoar are still intact.

Snowpack simulation Axamer Lizum: the magenta line at the surface (19 - 22.12) shows a thin layer of surface hoar which was destroyed by rainfall on 22.12 (altitude: 2103 m, Axamer Lizum)

Distribution of precipitation 21-22 December. Most fell in the west, very little in the south.

Rainfall limits of warm front 21-22 December. Most at 2400 m; in East Tirol most at 2000 m.

The proneness to fresh snowdrifts triggering is evident from fracture cracks, on the one hand, and from small artificially triggered (in isolated cases naturally triggered) slab avalanches, on the other.

Fracture cracks when stepping on a fresh drift at 2900 m. The weak layer consisted of loose decomposed crystals and surface hoar. (southern Ötztal Alps, 22.12.2020)

Avalanche release, Mittagsköpfe, Sellraintal. Visible just below the rocks: a shallow fracture; below that, a group of people the avalanche struck. No injuries. 2900 m, SE (photo: 22.12.2020)

The advantage: with some experience in avalanche assessment on-site, the fresh snowdrifts are easily recognized. However, starting on 25 December they will be blanketed by fresh snow, making it harder to spot them.

Old-snow problem in NW regions of North Tirol

Last week the frequency of avalanche prone locations (where triggering is possible in old snow) decreased also in the regions where there is much less snow. Nevertheless, stability tests still show that weak ground-level layers fracture from the weight of skiers and slabs can be triggered. We know of two avalanches last week which involved persons (both on 19 December), which fractured in the expansively metamorphosed ground-level layers, one on Schlantekopf in the Glockturm Massif, one on Larsengrat in Muttekopf region near Imst. No injuries.

Snow profile in Tux Alps near Nafingalm. A ground-level loose layer is visible, potentially a weak layer for slab avalanches. (photo: 19.12.2020)

Avalanche Larsengrat; 2530 m, NE. Encircled: three persons who were struck. (photo: 19.12.2020)

Old snow problem will intensify due to fresh snow + wind

As a result of precipitation and strong winds forecast for 24-25 December (predicted initially as rainfall up to 2000 m by ZAMG Weather Service, subsequently as snowfall down to low lying areas) the old snow problem in the regions with little snow will intensify. This is due to the wide-ranging snowdrift masses. Avalanches can thus grow to larger size; moreover, fracture propagation is enhanced. North-facing slopes will be at increased risk, also east and west-facing slopes at higher altitudes.

Snow forecast for 72 hours (23.12 7:00 am – 26.12 7:00 am) most of it in furthermost western regions.

Unfavourable: clearly alternating soft and hard layers. Region in Lechtal Alps, Arlberg region

Outlook


Increasing avalanche danger

Avalanche danger levels will increase due to a cold front which will bring fresh snow and strong winds over the holidays. “Considerable” danger will be reached above the timberline.

Starting next week, a Mediterranean low will bring snowfall to the southern regions. Current forecasts are for 50 cm of fresh snow, more from place to place. This, more than anything else, will intensify the snowdrift problem.

The south, already with ample snow, will get a new round of fresh snow next week.

Potentially, danger pattern 4 (cold on warm)

For avalanche forecasters, the anticipated danger pattern 4 (cold on warm) is of interest. Currently, the snowpack surface is frequently moist up to about 2400 m (see rain map above). Starting on 25 December, cold snow will be deposited on top of this. Temperatures will remain low. Due to marked temperature gradients on the surface, loose faceted crystals can form inside the snowpack within a very short time, potentially a weak layer for slab avalanches. We will study this development very closely, together with our observers, and are grateful for any reports (send to lawine@tirol.gv.at).

The moistened snowpack surface from rain + mild temperatures generates lumps, about 2000 m in Obergurgl (photo: 22.12.2020)

The uppermost 5 cm of the snowpack is moist, with a temperature of 0°C (profile at 2040 m near Obergurgl on 22.12.2020).

The week in review

Generally favorable avalanche situation

In regions where snowfall was heaviest, favorable conditions reigned by and large. Those who love steep slopes had plenty to enjoy.

In the Venediger Massif in extremely steep terrain, on a thoroughly stable snowpack (photo: 19.12.2020)

Glide-snow activity greatly diminished

Glide-snow activity has receded enormously in the interim, although a certain risk still prevails.

Photo in Schmirntal. In foreground, ski tracks through the powder; in background, grassy slopes nearly bare of snow where numerous glide-snow avalanches have already released. (photo: 18.12.2020)

Glide-snow fracture in Sillian (photo: 20.12.2020)

Backcountry is full of skiers

We currently observe a veritable boom in backcountry ski touring. Until today, many also went skiing on the prepared ski slopes while the ski areas remained closed.

Full parking lot in Praxmar im Sellraintal, 20 December

Backcountry skiers on a prepared ski slope in Kühtai (photo: 17.12.2020)

Weather review

A mild week with temperatures rising another notch due to a warm front (21-22 December) that brought a bit of precipitation. Intensifying winds from varying directions. Galzig, Arlberg region