Onset of unfavorable avalanche situation in north. Receding temperatures reduce wet-snow avalanche risks.

 Difficult avalanche prognosis

We are at a point in the season where forecasting avalanches is particularly challenging, largely because even miniscule divergencies in meteorological / nivological parameters can have astoundingly big effects on avalanche danger. Specifically, this is the current rostrum of factors which interplay:

• Nocturnal skies overcast
• Clouds forming at various altitudes during the day
• Local rain showers 
• High air moisture
• (diffuse) Solar radiation
• Changing air temperatures
• Changing temperature of snowpack surfaces / thawing point
• Temperature reserves inside snowpack plus water seepage into snowpack
• Snowpack layering highly variable in different aspects and altitudes

Currently, we anticipate for Good Friday (02.04.2021) that nighttime skies will be largely overcast, local showers likely. The snowpack will not be able to cool sufficiently. Conditions will already be unfavorable in early morning. On shady slopes at high altitude it will be better. The trouble is, you can’t get there without passing through an unfavourable zone....

Further to the south, on the other hand, nighttime skies are clearer, lesser tendency to showers. But there, too, conditions are truly favorable only at high altitudes, since the melt-freeze crusts which form during the nights are breakable due to the already massive water seepage.

View from Meiler Refuge in Karwendel, looking north. Cloud cover on the increase. (photo: 01.04.2021)

View from Zugspitze, looking south. Less cloud cover moving in. (photo: 01.04.2021)

On 1 April, rise of thawing point towards 0°C, but with flattening curve of snowpack surface temperature. Both together are a sign the snowpack is becoming thoroughly wet.

Wet-snow avalanche activity.  Observations in backcountry.

Recent days lay under the spell of wet-snow avalanches and on-site snowpack analysis. We were spared a massive cycle of wet-snow avalanches, but every single day naturally triggered avalanches were reported or observed by us. Slab avalanches, for example, triggered nearly exclusively in near-surface layers (snowfall from mid-March). What stood out: the high proneness to triggering of the snowpack in steep terrain wherever it was thoroughly wet. Also conspicuous: such danger zones were very widespread in outlying terrain. These striking criteria were mirrored in the ongoing recent danger forecasts.

Naturally triggered slabs of recent days: in Weisskugel Massif (photo: 01.04.2021)

Avalanche path in Pollestal. Also visible: a glide-crack (photo: 01.04.2021)

Kaserer Winkel in western Tux Alps. Photo: 7:06 am, 1 April 2021

Kaserer Winkel in western Tux Alps. Photo: 12:14 pm, 1 April 2021. The naturally triggered wet-snow avalanche triggered at 2200m in E-NE aspect.

On-site snowpack analysis

Sunny slopes

On sunny slopes the snowpack’s heightened proneness to triggering in near-surface layers was confirmed.

Arrow points to near-surface weak layer, thoroughly wet, at 2215m, S, in Silvretta

This slab in the Weisskugel Massif was triggered by a skier in a near-surface weak layer of faceted crystals beneath a crust (photo: 01.04.2021) - very similar to the profile above.

A “crust sandwich” on a south-facing extremely steep slope at 2850m, northern Stubai Alps. The lower very thick crust initially has a stabilizing effect, but loses firmness due to water seepage.

Shady slopes

Since shady slopes were not yet thoroughly wet (and thereby weakened) during the massive avalanche cycle around 23 February, we concentrated on the approaching water seepage on shady slopes. At high altitudes there are still sufficient reserves of cold. Below about 2200m, heightened proneness to triggering is very possible in northern regions locally on shady slopes.

Important: temperature reserves. Apart from near-surface layers, the snowpack is isotherm nearly everywhere. North, 2150m. Arrow points to possible weak layers, triggerable by water seepage.

Photo for the profile above. Lukas, one of our civil service helpers, points to two potential weak layers with water seepage (photo: 01.04.2021)

Shady slope, 2740m, snowpack still cold, stable at this spot

Snow quality

Typical of the season: swiftly alternating snow quality with altitude and aspect. Everything is on offer: superb firn, still great powder, very hard surfaces, more melt-freeze crusts and, of course, a thoroughly wet snowpack in the afternoon. (photo: 30.03.2021)

Outlook

According to ZAMG Weather Service, the influence of the high-pressure weather front is diminishing. While in northern regions some rainfall is scheduled, southern regions will be more pleasant. Temperatures are gradually dropping. The wet-snow threat will recede significantly by Holy Saturday at latest. Following a very sunny Easter Sunday, and partly sunny Easter Monday,  a cold front from the northwest will reach us and bring fresh snow....

The team at Avalanche Warning Service Tirol wishes everyone a Happy Easter!

Heightened natural triggerings. Plus local old-snow problem on shady, shallow-snow, very steep slopes. Avalanche analysis: Brennerspitze.

Unusually warm: time window shrinking for backcountry tours. Naturally triggered avalanches increasing.

We are currently experiencing a phase of unseasonably warm weather. The zero-degree level today lies at 3300m, according to ZAMG Weather Service. Despite nights of clear skies and outgoing longwave radiation with resultant cooling, the snowpack rapidly softens up during the daytime. Avalanche danger thus increases correspondingly fast. Currently we are at the critical level of a “3” as of afternoon, in other words, very near to Danger Level “high.”

Yesterday (30.03) increasingly frequent naturally triggered wet-snow avalanches (mostly loose-snow and slab avalanches) were observed. This trend will intensify. That means avalanche prone locations for avalanches person-triggered and naturally-triggered will further increase during the course of the day.

Typical of current situation: on sunny slopes slab avalanches tend to fracture in near-to-surface layers, after snowfall starting on 05.03. (photo: 30.01.2021)

Temperature rises, snowpack surface becomes wet during the day, snow melts.
Muttekopfhütte station  (eastern Lechtal Alps)

A trap for experts: spots with shallow snow on shady slopes (very steep, esp. near ridgelines)

The recent avalanches involving persons which became known were by and large results of a local old-snow problem where daytime warming did not play a part.  Such avalanche prone locations continue to exist at high altitudes in shady terrain. Slab avalanches can trigger there particularly where the snow is shallow in steep-to-extremely-steep ridgeline terrain. Apart from the risks of being swept along, the dangers of falling also demand attention. The avalanche accident below Brennerspitze on 28.03.2021 fell into this category. Therefore, an analysis of that incident follows here in more detail:

Brennerspitze avalanche accident in the Stubai Alps on 28.03.2021

Yesterday (30.03) we analyzed the avalanche accident together with the Alpine Police. This was a hard slab which was about 25m wide at the point of fracture and all-in-all was 1200m long. At its widest the avalanche path was 90m. It triggered immediately when a person skied into the 45° steep north-facing slope at 2870m altitude. The person was caught by the avalanche, swept over rocky terrain and ultimately totally buried in snow. Resuscitation attempts were made, the person was flown to the hospital.

Slab avalanche Brennerspitze, one person buried in snow (photo: 30.03.2021)

Upper arrow points to the zone where the person skied into the slope. Lower arrow is the track at fracture point. The fracture depth there was about 100cm, 200cm slightly displaced, up above 20-30cm (photo: 30.03.2021)

View from ridge: here the skier jumped into the slope. Due to heavy additional loading, the initially penetration of the hard snowpack was slightly deeper. Right of the track, a fracture is visible. However, the slab subsequently triggered laterally, and somewhat lower. Burial point in the path is indicated.
(photo: 30.03.2021)

We carried out our snowpack examination in the immediate vicinity of the entry track. A major factor of this avalanche was the sequence of hard-and-very-soft layers. The uppermost part of the snowpack was massively wind-impacted and very hard. Beneath that we found a very soft layer of depth hoar and faceted crystals. Further down, other hard and soft layers alternated.

Snowpack examination at entry track (arrow). Shallow snow, hard upper layer, snow was very loose near the upright saw in photo - depth hoar and faceted crystals. (photo: 30.03.2021)

Snow profile orographically right of entry track. The block broke after being sawed off at the side and rear.

Profile orographically left of entry track. Two fractures were initiated here. Major avalanche factor: the lower fracture.

Alpine Police at the point of burial (photo: 30.03.2021)

Avalanche accident Brennerspitze in northern Stubai Alps. Caution in coming days: swift daily rise in danger.

 Avalanche accident Brennerspitze

Today 28.03, an accident occurred below the Brennerspitze in Oberbergtal, a side valley of the Stubaital. One person skiing down from the summit was caught by a slab avalanche, swept along and totally buried. The person needed to be resuscitated and flown to the clinic in Innsbruck.

The avalanche accident recalls that of 24.03 below the Giggler Spitze: the release triggered near the ridge in extremely steep shady terrain in a spot where the snow was shallow. All signs point to a small-spread old-snow problem. We shall soon undertake snowpack examination  in the vicinity of the accident with the Alpine Police. Details will be published on Tuesday evening at earliest.

Aerial view of avalanche accident Brennerspitze. Arrow points to the entry track, ellipse to where the victim was found. The avalanche triggered very near the ridge. (photo: 28.03.2021) 

Overview of avalanche fracture with entry track. In background, summit cross of Brennerspitze.
(photo: 28.03.2021)

Springtime conditions: day-by-day more rapid rise in avalanche danger

The next few days will bring us classic springtime conditions. What matters most is how well the snowpack can cool down during the nocturnal hours. The clearer the nighttime skies, the drier the air, the better it is. What requires caution: solar radiation is already quite intensive, the temperatures quite high. Thereby, the snowpack currently becomes rapidly moist or wet during the course of the day, particularly on sunny slopes. Water seepage weakens the snowpack. Especially where currently there exists a near-to-surface old-snow problem (in far-reaching parts of North Tirol and in northern East Tirol) slab avalanches can be triggered ever more easily during the course of the day. Also naturally triggered avalanches are quite possible. To start with, this will strike primarily east-facing and west-facing slopes at 2200-2700m altitude; south-facing, extremely steep terrain at 2700-3000m. Around mid-week, also north-facing slopes at 2000m could be hit.

Quintessence 

Careful planning of backcountry tours AND prudent time allowance. Those are the golden rules in coming days.

Old-snow problem still in places. Generally quite favorable conditions. Avalanche analysis Giggler Spitze.

Today, 25.03, we ventured into outlying terrain in Tirol’s highlands with the Alpine Police. We were able to closely analyze the 24.03 avalanche on Giggler Spitze; observe close-up an avalanche on Hochwanner in Kühtai; and obtain a good grasp of the overall situation from the air .

In backcountry with the Alpine Police (photo: 25.03.2021)

In a nutshell:

In the interim a huge number of backcountry tours have been undertaken, many of them in extremely steep terrain. In view of this, there do not seem to be very many danger zones left where avalanches can trigger. The highest degree of caution is necessary where there is a near-to-surface old-snow problem. The range of aspects/altitudes listed in yesterday’s blog (24.03.) has been confirmed rather well by avalanches observed. In addition, very steep ridgeline terrain where the snow is shallow also demands more caution since near nests of depth hoar slab avalanches there can still be triggered.

Last week’s weather

Last week was heavily impacted by cold air masses from the Arctic, bringing highly variable conditions. Repeated bouts of snowfall, often strong winds. It was much too cold for this juncture of the season.

The upper graph shows divergences from long-term averages at the Patcherkofel station. Significantly lower temperatures in the last 10 days are visible. A new minimum was even reached. The lower graph explains the colors used. (c) ZAMG

Variably cloudy weather persisted on Sunday, 21.03.2021 - northern Stubai Alps (photo: 21.03.2021)

Strong winds transported lots of snow, even fashioned water sculptures. This comes from the warm phase just before the beginning of March - Lasörling Massif  (photo: 21.03.2021)

At the start of the week, weather improved palpably - Paznauntal (Foto: 23.03.2021)

Initially variable with some precipitation and low temperatures, starting on 23.03 the weather improved. Temperatures climbed continually.

In East Tirol, strong winds were frequently blowing.

Snow cover

Skiers and freeriders were able to celebrate last week: apart from wind-impacted slopes, there was lots of gorgeous powder.

Powder snow dreams in the Stubai Alps (photo: 25.03.2021)

In the meantime the sun has been doing its job with bravour. Powder exists only on purely shady slopes, melt-freeze crusts tend to dominate. On steep south-facing slopes and at intermediate altitudes the snowpack, following nights of clear skies, is becoming more and more capable of bearing loads.

Reminiscent: where yellow-orange hues in the snowpack often become visible, recalling the dust blown to the Alps from the Sahara at the beginning of February.

The yellow layer obtained its color from Sahara desert dust. Silvretta (photo: 23.03.2021)

Another interesting phenomenon was also observed, fortunately irrelevant to the development of coming conditions. When on 23.03 very dry air massess penetrated at least the western regions, the fluffy, extremely light powder on the snowpack surface became moist down to a few centimetres below the surface due to intensive, short-lived solar radiation, but the snow above it remained dry. The result: skiers were surprised by abruptly sticking snow. In addition, as of late afternoon a thin melt-freeze crust formed.

“Radiation recrystallisation” is what this phenomenon is called. The snowpack surface is still loose and dry, beneath it a thin melt-freeze crust formed in late afternoon (which remained moist during the daytime). Silvretta (photo: 23.03.2021)

Visible in this snow profile is the temperature progression responsible for radiation recrystallisation: surface temperature is negative, it is 0 degrees a few centimetres below, deeper down it is lower. (Potential for short-term formation of a weak layer due to high temperature disparity.)

Avalanche accident Giggler Spitze on 24.03.2021 - a short analysis

As mentioned at the outset of this blog we examined the site of the recent avalanche together with the Alpine Police. The slab avalanche was impressively large: about 1600m long and 100m wide, fractured at about 2480m altitude on an extremely steep, NW-facing slope.

A section of the accident avalanche below Giggler Spitze in the Samnaun Massif on 24.03.2021
(photo: 25.03.2021)

Entry tracks of the two injured persons. We carried out our stability tests just above the traverse track, where there were fissures in the snowpack. The slab unleashed where the slope got steeper.
(photo: 24.03.2021)

At left, one of the entry tracks. In foreground, the traverse track. Above that, a fracture.
(photo: 25.03.2021)

Immediately conspicious, and the most probable cause of the accident, was a shallow-snow zone near the ridgeline. There, beneath the snowdrifts we found layers of loosely-packed, expansively metamorphosed (faceted) snow crystals. Stability tests there initiated a fracture which then also propagated further. We also discovered nests of depth hoar here and there. Thus, there was lots of depth hoar beneath the snowdrifts in this general vicinity and precisely there, the snowpack was particularly easy to trigger. From such a spot, the fracture was presumably initiated and further propagated to other quite stable zones.

Snow profile with stability test above the avalanche fracture, in orographically right section of the avalanche. The snow block released in a loosely-packed layer of depth hoar. To the left of the shovel, a fracture inside the snowpack is visible which was generated by the avalanche. (photo: 25.03.2021)

The snow profile of the photo above it. The arrow points to the weak layer.

Loose snow on the shovel was what composed the weak layer. (photo: 25.03.2021)

At left, the entry tracks. (photo: 25.03.2021)

The deepest fracture was about 200 cm. Fracture down to very stable zones. (photo: 25.03.2021)

View of plummet path as far as deposits in forest lanes (photo: 25.03.2021)

Other recently observed avalanches

Right off the bat, recent avalanches have one thing in common: they all released in extremely steep terrain.

Below the Hochwanner in Kühtai, one person was caught by a slab avalanche during a descent on 25.03 and swept along. The person sustained no injuries. This was an extremely steep NE-facing slope. The fracture occurred at 2350m. The avalanche was about 300m long and 50m wide.

Avalanche Hochwanner. The person skied from above left into partially rocky terrain. At bottom of photo, the rescue helicopter arriving at the site is visible. (photo: 25.03.2021) 

Snow profile of orographically left fracture zone near the entry track: beneath a thin melt-freeze crust is a faceted layer - the weak layer. On top of that, recent snowdrifts. 

Hoher Burgstall  avalanche on 23.03.2021, NE, 2500m. We assume snowpack layering is very similar to the Hochwanner avalanche. (photo: 25.03.2021)

Grünbergspitze avalanche. Presumably naturally triggered. Weakened snowpack due to solar radiation. appx. 2700m, south. (photo: 24.03.2021)

Naturally triggered avalanche near Kögele - northern Stubai Alps. This entire slope also unleashed a slab avalanche in mid-January 2021. NE, 2200m (photo: 25.03.2021)

Thialkopf avalanche - north, near timberline - similar to the one reported in Wattental, Tux Alps.
(photo: 21.03.2021)

The arrows point to small slab avalanches which triggered in the Tux Alps at the start of the week. Weak layer was presumably a wind-loaded loosely-packed powder layer, but not 100% certain, since we did not perform on-site analysis. (photo: 24.03.2021)

With intensifying solar radiation and rising temperatures over the last few days, loose-snow avalanches became more frequent. Most were relatively small, somewhat larger east of Wipptal. There on 23.03, there was more intensive diffuse radiation. Some glide-snow and isolated slab avalanches were also observed there. The slab releases were mostly on sunny slopes, near ridges in extremely steep terrain.

Deposit of a loose-snow avalanche in rear Zillertal (photo: 23.03.2021)

Recently triggered loose-snow avalanches (photo: 25.03.2021)

Bald zieht (wieder) das Frühjahr ein

Tomorrow on 26.03 it will again be sunny. On foehn-exposed summits, brisk SW winds are expected. On Saturday, 27.03, a cold front  from the northwest will swiftly move across the land. Snowfall level is expected to be between 1000 and 1500m. Generally, 5 cm of fresh snow is anticipated, 10 cm in the northern barrier cloud regions. After the front has passed, skies will clear up more rapidly in the western regions than in the east.

Clear skies following the cold front on Saturday, 27.03.2021

In wind-protected shady terrain at high altitude, small sometimes trigger-sensitive snowdrift accumulations have formed. In high-alpine regions this is possibly due to Nigg-Effect, possibly near ridgelines atop surface hoar.

Day by day starting on Sunday, 28.03, it will become increasingly sunny and milder. What matters most as of that time is the degree of moisture inside the snowpack. We expect a marked daily cycle of avalanche danger to start on Wednesday at latest.

Due to the thoroughly wet snowpack, as expected, also more frequent glide-snow avalanches are coming. (photo: 20.03.2021)