Mostly moderate avalanche danger – Rain + warmth weakening the snowpack

 In a nutshell...

April weather conditions persist. A warm front today and tomorrow (28-29.04) is bringing rain up to 2300 m, most of which (30mm) will be north of the Inn. At high altitudes, strong westerly winds will be blowing. On the weekend, dispersed clouds, more pleasant weather before on 1 May slight foehn-impact and during the daytime more rainfall can be expected. Rain and warmth will push the wet-snow problem to the forefront next week.

Precipitation is coming, will spread throughout Tirol today, 28 April.

Rain + warmth weakening the snowpack

The snowpack was thoroughly moistened several times this winter: on steep sunny slopes up to high alpine zones, on shady slopes up to 2500 m. This is decisive in assessing avalanche danger resulting from rainfall. If the rainfall level is at 2300 m (as forecast), then the near-surface layers (of recently fallen fresh snow) will be weakened.

Fresh snowfall from 24-25 April. Water seepage can weaken the snow (which had settled) in many places.

Near-surface loose-snow avalanches will become more frequent due to rain. (photo: 25.04.2023)

If the rainfall level is further up, heightened proneness to triggering is also indicated on shady slopes. Both rainfall intensity and nocturnal outgoing longwave radiation play a role in the snowpack from 27-28.04. Wherever nighttime skies were clear for longer, the water seepage is delayed due to a thin melt-freeze crust forming.

Initially clear nighttime skies...

In early morning: overcast.

Above all else, naturally triggered slab avalanches cannot be ruled out on shady slopes if the snowpack is thoroughly wet. Fractures in near-surface weak layers (formed since the end of March) can lead to fractures of more deeply embedded layers in the snowpack.

Short review of the last week

The week was marked by the month: April. Following very warm days last weekend (22-23.04) there was a cooler phase. In the meantime, temperatures have risen again. A warm front is now bringing rain.

High moisture in the atmosphere and warm temperatures on 23.04 - Golzentipp - southern East Tirol

Deteriorating conditions on 23.04.2023

Snowfall in the Weisskugel Massif (photo: 25.04.2023)

Quite good conditions in the Silvretta (photo: 27.04.2023)

Week review: initially warm, then snowfall and dropping temperatures, the snowpack stabilizes, then again highly variable conditions, and again it turns warm. At high altitudes, quite windy. The snowpack surface cools somewhat during the night.

Cornices are a threat to the snowpack with large additional loading. Silvretta (photo: 27.04.2023)

Even if the crevices were covered in April, they remain a big threat. (photo: 25.04.2023)

Good tour planning and time organisation

...both help. Choose a suitable touring goal. If it is not too high, the conditions on ski pistes are currently quite good. 

At high altitudes the danger zones are mostly on very steep shady slopes due to near-surface weak layers (persistent weak layer due to cold-on-warm since the end of March). Particularly between 2600 and 3000m this is the case. In high alpine regions (above 3000m) isolated spots also on sunny slopes.

After snowfall, loose-snow avalanches in extremely steep terrain, in high alpine regions esp. near ridgelines, high attentiveness to fresh snow is required.

24-hr fresh snow forecast. Up to intermediate altitudes, rainfall (not visible in this graph).

Highly variable April weather leads to quickly shifting avalanche dangers

 In a nutshell...

April is quite justly famous for its caprices with the weather. This is having rapid effects on avalanche dangers. Due to an expected surge in temperatures tomorrow (Saturday, 22.04) the danger will increase during the course of the day. More frequent naturally triggered avalanches will be the result. Savvy tour planning and exercising personal restraint are crucial.

Following snowfall, increasingly wet and weak snowpack

Following a striking springtime injection at the end of March and the coldish phase which came in its wake, we now adjust our sights to the snowpack moistening or becoming thoroughly wet. Current snowpack analysis shows temperature reserves inside the snow cover to be nearly used up, even on shady slopes below 2700m. That means, the snow temperature is already 0° or pretty close to it. Warm temperatures, high air moisture (convective cloud build-up during the day) and (diffuse) solar radiation lead to the snowpack swiftly becoming thoroughly wet. Water seepage weakens it further. As of then, likelihood of slab avalanches being triggered by winter sports enthusiasts rises, as does likelihood of naturally triggered avalanches (slab, loose-snow, glide-snow).

The red line shows snow temperature. On 19 April at 2650m it lay at nearly 0° on a 28°-steep NE-facing slope. Potential weak layers for slab avalanches occur near the melt-freeze crust which formed at the end of March or in loose layers near ground level.

In this snow profile taken yesterday (20.04) at 2120m on a 35°-steep north-facing slope in the northern Zillertal Alps, the snowpack is thoroughly isotherm.

The increasing wetness of the snowpack is visible in exemplary fashion in this graph from Mosesgipfel weather station in the eastern Deferegger Alps. In the graph second from the top, snowpack surface temperature (gray line) flattens out more each day. Also, the thawing point (blue line) gets closer to the 0°-line. Currently, skies on-site are cloudy. Moisture of the air is 100%. Higher temperatures and increasing meltage are notable, alongside lessening snow depths.

Heightened avalanche activity - for a short time, large-sized avalanches are possible

As mentioned, we currently expect heightened avalanche activity. In the areas where recent snowfall has been heaviest (near the border to South Tirol in the Main Alpine Ridge) numerous loose-snow avalanches can be expected today (21.04). Due to the additional loading from loose-snow avalanches, in their wake slab avalanches could also be triggered near persistent weak layers at mid-level in the snow cover and at ground level. Thereby, large-sized releases could be generated.

48-hr fresh snow forecast from 20.04.2023

24-hr precipitation distribution (cut-off date: 21.04.2023 7:20 am)

Impressions from last week

View from Hochstein above Lienz, in the Lienz basin (photo: 14.04.2023)

A slab avalanche triggered by a winter sports enthusiast near Flimjoch, bordering on the Samnaun Massif in Switzerland; north 2700m 35° (photo: 18.04.2023) 

Graupel - often accompanying snowfall in the last few weeks (photo: 19.04.2023)

Visible on the right side of the persons is a recently triggered loose-snow avalanche. Gurgler Massif
(photo: 19.04.2023)

What’s on the horizon?

On Sunday, 23.04, a cold front will again bring precipitation. Weather conditions will remain instable. The forecasts are still uncertain. Precipitation in showers is anticipated, which means we’ll have heightened potential of graupel accompanying the snowfall. Wherever snowfall is intensive graupel can quickly form a weak layer for slab avalanches to trigger from. The situation demands meticulous backcountry tour planning. Apart from frequently poor snow quality, it is also necessary to keep an eye on heightened avalanche risks in open terrain. The only place where conditions tend to be better is at very high altitudes where the warmth has less of an effect.

Considerable avalanche danger due to heavy snowfall in places

 In a nutshell...

A cold front is delivering fresh snow down to intermediate altitudes. At elevated altitudes conditions for a short time will be like in deep winter. Simultaneously, avalanche danger in many places has increased to considerable. We expect higher likelihood of slab avalanches getting triggered by winter sports enthusiasts, and also increasingly frequent natural triggerings.

Fresh snow plus wind:  ratcheting up avalanche danger

Currently (13.04.2023) it is snowing in far-reaching parts of Tirol. Up to now there has been up to 40 cm of fresh snow registered from place to place, most in the western and eastern regions of North Tirol, also in southern and furthermost northern East Tirol.

Map by Hydro Online shows overall 48-hr snowfall (time: 4:00 pm on 13.04.2023)

48-hr fresh snow forecast 13.04.-14.04.2023

Precipitation / wind forecast for a heavy-precipitation region. Winds will intensify tomorrow, Friday, 14.04.

Slab avalanches, loose-snow avalanches, glide-snow avalanches

Lots of fresh snowfall in late spring always means that increased avalanche activity can be expected, at latest, as of the first bright intervals, in other words, when diffuse radiation begins. These will be primarily loose-snow avalanches in extremely steep terrain. In the regions where snowfall has been heaviest they can be of medium size. Besides that, heightened likelihood of slab avalanches can also be expected. Slab avalanches can currently be unleashed by several different causes:

• The winds (expected to intensify tomorrow, 14.04) transports the fresh snow, generates fresh snowdrift accumulations. The frequency and proneness to triggering of fresh drifts increase with ascending altitude.
• In regions where precipitation is heavy but winds are light, near-surface loose snow can be bonded by diffuse radiation. Thereby, a slab forms near the surface. The still loose, cold snow beneath it serves as the necessary weak layer for the slab, active for a brief spell.
• In regions where snowfall has been especially heavy, slab avalanches can break through the snowpack in isolated cases, likeliest at mid-level. This will occur most frequently above 2600m. (Weak layer due to cold on warm, see  Blog. Fractures down to ground level can’t be ruled out.

This photo dating from 06.04.2023 shows a large-sized avalanche on an extremely steep E/NE facing slope at about 3000m in the Kühtai-Geigenkamm region. In regions where snowfall has been heaviest, similar naturally triggered avalanches are possible in isolated cases.

In addition, glide-snow slides and glide-snow avalanches will also be observed, particularly in regions where snowfall has been heaviest. This is most likely to occur on steep, grass-covered slopes where prior to this bout of precipitation the ground was bare.

For a short time: treacherous conditions. Then a rapidly improving situation in view. Thereafter, future developments still uncertain.

We assume a highly treacherous situation for winter sports enthusiasts for a brief spell. In southern East Tirol where weather conditions will improve more swiftly, the fact is that on the first day of fine weather (even if only a short spell of it) the risks in backcountry are especially high. It is also characteristic of the season that the near-surface snowpack rapidly improves and stabilizes. Due to April weather caprices, we still need to await coming conditions before we say anything more.

Mostly favorable avalanche situation – Near-surface weak layer esp. on extremely steep slopes above 2600m

 In a nutshell...

Till now the Easter holidays have been much too cool for this juncture of the season (Easter has been colder than Christmas). On the other hand, that meant that the snow which fell this week was able to remain good powder for a long time. The near-surface weak layer problem generated by dp.4 (cold on warm) currently seems most threatening above 2600m. Danger zones are rare. It occurs mostly extremely steep N/NE/E facing slopes.

Mostly favorable avalanche situation

Reports from winter sports enthusiasts, snowpack analysis and information about avalanches which have released reveal a mostly favorable avalanche situation.

Last week had lots of great powder. Carnic Alps (photo: 06.04.2023) 

Another snapshot of terrific powder in the Sellrain mountains. (photo: 06.04.2023)

Near-surface weak layers found increasingly on N/NE/E facing slopes at 2700-3100m

We were able to observe the danger pattern described in our last blog  (cold on warm) in many places, but stability tests mostly showed only partial fractures in the weak layer. That fits with the fact that most of the dp.4-relevant avalanches last week occurred in extremely steep terrain. What was striking, was how many of them were on N/NE/E facing slopes at 2700m-3100m (Wasserkar-Weikugelgruppe / Müncher Abfahrt-Weißkugelgruppe / Zirmkogel-Gurgler Gruppe / Wildgratscharte-Alpeiner Berge /  Stöcklengrube-Alpeiner Berge / Federbettkees-Zentrale Zillertaler Alpen / Kleiner Happ-Venedigergruppe / Wagenstein-Östliche Deferegger Alpen).

Typical snow profile: at top, snowfall from 25.03. The smooth surface beneath that shows the partial fracture of a thin faceted layer atop a pronounced melt-freeze crust. Beneath that, a frozen water canal from the warm-weather period before 25.03 is visible.

Minor snowdrift problem

The snowfall from the beginning of this week occurred at low temperatures and was transported at high altitudes. When snowdrifts are deposited on cold, loose fresh snow (even snow-fluff was observed) there is a heightened likelihood of slab avalanches triggering. With sufficient experience, such danger zones are easily recognized and can be circumvented. The danger zones currently (07.04.2023) occur especially on very steep ridgeline shady slopes.

 Snowdrift accumulations adjacent to a ridgeline in the Allgau Alps (photo: 04.04.2023)

Caution: cornices

Cornices have become quite pronounced this spring. Keep an eye on the risks of taking a fall. In addition, additional loading caused by a breaking cornice can trigger an avalanche.

Cornices near the ridgeline: a potential alpine danger (photo: 06.04.2023)

Avalanche due to a cornice breaking on Gufelseejoch (photo: 04.04.2023)

Coupled to alpine dangers are the glaciers: increased risk of falling into crevices due to lack of snow.

Short review of this last week

A quick glimpse at the weather is easiest through an automated weather station.

At month’s end it was still warm, the snowpack surface was moist. Then some snowfall, initially accompanied by wind. Thereafter, temperatures dropped. Conditions improved rapidly.

Fresh snowfall at the beginning of the week (02.04-03.04), most of which fell in eastern North Tirol and in northern East Tirol.

A cold week by and large...ice hanging from a piste vehicle on Stubai Glacier. (photo: 04.04.2023)

Often observable: just beneath the snowpack surface, a thin moist layer develops due to low temperatures, very loose powder, dry air and intensive solar radiation. This layer then freezes into a thin melt-freeze crust (radiation recrystallisation is the name of the process which leads to a weak layer forming for the next snowfall). At the moment, based on current weather forecasts, this does not seem to be a threat.

What’s next?

April weather variability will continue, with some snowfall over the Easter holidays. No significant change is expected in the avalanche situation.

48-hr fresh snow forecast from today, Good Friday, 07.04.2023

Until Easter Monday, repeated bouts of precipitation. Only light winds. Cold.

HAPPY  EASTER!

We wish you Happy Easter and a period which is both high in enjoyments and low in accidents out in backcountry terrain. Yours truly, the team of the Avalanche Warning Service Tirol.