Generally favorable avalanche conditions – Little snow in many places – Danger Pattern “cold on warm” frequently developing

 Summing it up

Since yesterday, 01.03.2023, the avalanche danger level in Tirol has been low. We assume there are only few danger zones where avalanches can be triggered, if at all, then on shady slopes at high altitudes. Nevertheless, we are currently observing the development of a potential weak layer due to Danger Pattern “cold on warm” (dp.4). In isolated cases this layer could become a problem during the next few days. We are training a sharp eye and lots of research on just where this might happen. In this blog you can read about the latest stage of our knowledge regarding dp.4.

Finally snow - esp. in the northern and eastern regions

Last week bore the stamp of initial snowfall, striking mainly in the northern and eastern regions of the land where generally 30-50cm of fresh snow was registered. Further south the amounts of fresh fallen snow were noticeably less. Due to low temperatures the snow was quite loose. Winds generated snow transport. We were often confronted with a new-snow problem (where danger zones are not visible to the naked eye). In the regions with least precipitation, on the other hand, a snowdrift problem dominated (wind-loaded masses recognizable in open terrain, could be circumvented). During the last few days, beautiful weather prevailed, while southern regions were overcast. There was some snow in the south on 01-02 March, mostly 5-10cm.

72-hr fresh snow between Friday 24.02 and Monday 27.02.2023

Some fresh snow in the south: 24-hr snowfall between 01.03. and 02.03.2023

A gorgeous winter landscape following the snowfall from 24.02.2023. Nevertheless, still danger of rocks on the descent. Central Lechtal Alps. (photo: 28.02.2023)

Superb powder in western Kitzbühel Alps (photo: 28.02.2023)

Following the snowfall, increasingly frequent loose-snow avalanches were observed in extremely steep terrain. Slab avalanches were generally small, fractured beneath the fresh fallen snow (cold, loose snow blanketed by freshly generated drifts). In addition, the snow tended to slide over steep grass-covered slopes.

Loose-snow avalanche in western Lechtal Alps (photo: 01.03.2023)

Small naturally triggered slab avalanche, presumably on 26.02. Grieskogel Massif (photo: 02.03.2023)

Glide-snow slide in eastern Lechtal Alps (photo: 28.02.2023)

Danger Pattern "cold on warm" (dp.4)

Danger Pattern “cold on warm” occurs whenever an initially moist or wet snowpack surface gets blanketed by cold snow. Due to the great temperature disparity over small spaces, an expansively metamorphosed (faceted) layer can develop with a bit of delay. Currently we see such a development taking place. This process is important in the northern and eastern regions which recently had heavy snowfall. There, in addition to the formation of a potential weak layer, there is also the crucial slab. Today, 02.03, two reports about settling noises arrived at headquarters which can in all likelihood be attributed to this process (regions Kalkkögel and Sellrain). Dp.4 may also be the cause of an avalanche in the Kalkkögeln, presumably on 28.02 in the afternoon or 01.03 in the morning.

Prerequisites for “cold on warm:” ellipse denotes moist surface; then snowfall plus dropping temperatures.

Current estimate for pinpointing “cold on warm”

At the presenet moment there are relatively few reports of a potential “cold on warm” problem. We base this not only on reports, but also on our own open terrain explorations and snow profiles.

The forming of a threatening weak layer is currently most possible, in our opinion, in the following zones or is already the case there:

• North, altitudes 2100m-2300m
• East / Northeast and West and Northwest: 2100m-2500m
• South 2500m and above
  

Please assist us

We have assigned this project to all our observers: be attentive to this development over the next few days and inform us. The more comprehensive our information is, the clearer the overall picture becomes. For that reason, we turn to those of you who like to have a look inside the snowpack. What interests us most is whether faceted crystals have formed near the melt-freeze crust which formed last week. Of course, a stability test would be even better, e.g. a quickly executed ECT. We look forward to lots of reports from you, mail them to  lawine@tirol.gv.at  

A glance into the snowpack helps us to assess the development of dp.4 - Venediger Massif
(photo: 01.03.2023)

Slab avalanche with entry track of a snowboarder (upper fracture zone between rocks) at 2300m North (photo: 01.03.2023)

Photo for comparison: on 28.02 in the afternoon, only one traverse track could be seen. The avalanche had not yet triggered. (photo: 28.02.2023)

Arrow points to a thin faceted layer beneath a thin melt-freeze crust. Stability test showed stable conditions; however this could be due to the looseness of the snow. North, 1940m

At 2720m NW in Venediger Massif there are also faceted crystals beneath a thin melt-freeze crust, but these are probably not from dp.4. Far more likely in this case is the previous “knobby powder.”

In this spot with little snow, faceted crystals probably formed quite a while earlier. Additional effect of dp.4 cannot be ruled out. Nonetheless: only partial fractures. Notable: no sufficient slab on top of it.

Partial fractures above superficial melt-freeze crust. West, about 1750m

Assorted tid-bits

There are also other interesting observations to report. Over the last few days we have had quite good conditions for forming surface hoar. On the one hand, the fog (especially in the southern regions). On the other, due to thermic air currents (“Nigg Effect”).

Warmer air masses rising. Clouds brush the summits. Good prerequisites for the “Nigg” Effect.
Venediger Massif (photo: 01.03.2023)

Surface hoar adjacent to a ridgeline (“Nigg” Effect) in the Samnaun Massif at about 2500m
(photo: 02.03.2023)

Persistent lack of snow

GeoSphere Weather Service (formerly ZAMG) informed us in one report that this winter has been very mild. In the lowlands it was the sixth warmest winter in (measurement) history. In the mountains, it lay in 12th place. Furthermore, western Austria was 15-60% drier than average. Both phenomena, mild weather and low precipitation, are the causes of the below-average amounts of snow.

Not unusual on ski tours this winter: at least for short passages, you have to carry your skis.
(photo: 02.03.2023)

Above snow situation is not at all typical for this juncture of the season. Central Stubai Alps
(photo: 24.02.2023)

What can we expect?

We are keeping a sharp eye on the development of dp.4, since it could mean increasing avalanche danger in the regions which recently had the most snow.