The persistent southerly air current with precipitation along the Main Alpine Ridge and in East Tirol has been replaced by a NW air current. The ZAMG Weather Service warns of snowfall and rainfall between 22.12 6:00 pm and 25.12.2019 6:00 pm, most of which is expected to fall in the Silvretta, Arlberg and Allgäu/western Lechtal Alps: 100-150 cm, more from place to place. From the eastern Lechtal Alps over the Karwendel to the Wilder Kaiser, 70-100 cm is expected, with local deviations above that. In the rest of North Tirol and northern East Tirol, amounts between 30 and 80 cm are expected.
On Hydro Online, it is easy to recognize the 24-hr precipitation in Tirol (cutoff point: 23.12.2019 at 12:30 pm):
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| Most of the precipitation is expected in the northern and northwestern regions. Here the example of Muttekopfhütte Refuge, where 50 cm of snowfall has already been registered. Stong winds, the shift of wind direction and the descending temperatures are easily recognizable. |
What matters most for the avalanche situation are not the great amounts of fresh snow, but the temperature fluctuations. After a cold front on 23.12 (snowfall level 800-900 m) there followed a warm front on 24.12 (snowfall level: about 1700 m) which was subsequently replaced by a cold front (snowfall level at 1000 m). This precipitation was accompanied by storm winds in the mountains.
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| The stripes in dark green: forecast of precipitation and temperature fluctuations in Kaiser and Ausserfern (“Heute” refers to 23.12.2019). |
Rising avalanche danger
All these factors combined lead to rising avalanche danger. We expect the highest naturally triggered avalanche activity with the warm front passing through on 24.12 . Rainfall will weaken the snowpack at low and intermediate altitudes. Subsequently, numerous glide-snow avalanches will be observed on steep, grassy slopes, also wet, loose-snow avalanches in rocky terrain in the areas where snowfall has been heaviest. At higher altitudes where there has been snowfall, more frequent slab avalanches will trigger naturally.
Thoughts about snowpack structure
As regards avalanche magnitude, we have the following considerations. Possible weak layers for slab avalanches are currently found inside the old snowpack in E/S/W aspects beginning at about 2300-2500 m and upwards. These are faceted layers near melt-freeze crusts which have been generated quite near to the upper surface of the old snowpack. Such areas are not area-wide. Fractures which are generated due to the burdens on the snowpack are not expected to propagate over widespread areas of the terrain.
An additional possible weak layer inside the mass of fresh fallen snow: the new fallen snow at cold temperatures in wind-free zones. When the warm front passed through, the higher temperatures formed a superficial “slab” which can easily be disturbed atop the loose and cold snow. The slab will become deeper through additional snowfall. Higher proneness to triggering thus persists, even while the cold front passes through and at least until the following day, 26.12.
In addition, one more development inside the snowpack occupies us. This is Danger Pattern 4 (cold on warm). The moist old snowpack surface until 20.12 at low and intermediate altitudes and on sunny slopes up to at least 2500 m was covered by cold snow on 20.12. Due to temperature variations on the borderline surface of these two layers, a faceted weak layer could form, with some delay. Currently there is no information which indicates that this layer is sufficiently pronounced to generate avalanches, this is expected only in altitudes above the timberline.
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| The red perpendicular line and the arrow symbolize the possible start of danger pattern 4 formation (cold on warm). |
Okay, to try to complete the circle: avalanches will be generally of medium size, but in the major areas of precipitation often large-sized (very large avalanches are expected to be the exception. If at all, then in the regions where snowfall has been heaviest in leeward, high-altitude and ridgeline starting zones, i.e. below rock walls).
Interim high pressure influence on Thursday, 26.12. Restraint is called for!
According to ZAMG Weather Service forecasts, conditions on 26.12 should improve in an interim high. The first day of beautiful weather after a period of storm winds and snowfall is especially prone to accidents. For that reason we recommend a great deal of restraint in outlying terrain. Avalanche prone locations are widespread in the areas where snowfall has been heavy.
