Caution: fresh drifts due to snowfall+wind

 Weather review. And a peek ahead.

Last week was variable. A cold front from the northwest brought 10-15 cm of fresh snow widespread in North Tirol and northern East Tirol on Thursday and Friday, 4-5 March. Locally exceptions were startling: in the central Lechtal Alps and Venediger Massif there was 30-50 cm of fresh snow registered.

Precipitation on 4-5 March. One of the exceptions: at the high-altitude precipitation station at Rudolfshütte on the East Tirol/Salzburg border

For the rest of the week, sunshine, clouds and a bit of precipitation took turns at the helm amid lowish temperatures.

Variable… And now, winds are starkly intensifying at high altitudes. A cold front is on the way.

(Diffuse) solar radiation particularly on Thursday, 11 March, moistened the snowpack surface on sunny slopes up to at least 2500 m, on north-facing slopes up to at least 2000 m.

In the lower Kachel, air temperature (red line), thaw point (blue line) and snowpack surface temperature are depicted. Today (11 March) the snowpack surface was moist, the thawing point rising noticeably.

On Thursday, 11 March, a cold front is swiftly approaching, will move into Tirol in the evening. The focus of precipitation will be the Arlberg region, where up to 20 cm of fresh snow is anticipated. On Saturday, according to ZAMG Weather Service, an intermediate high will interrupt this before on Sunday, 14 March as of afternoon until Tuesday, 16 March, we can expect snowfall and stormy winds. The mountains are putting on their winter garb again.

Fresh snow forecast for the next 72 hours (as of 11.03 evening)

Fresh snow forecast: the first bout starts Thursday evening, the second as of Sunday, 14.03.

Concentration on weak surface layers. Main problem: fresh snowdrifts.

Last week high attentiveness was focused on fresh snowdrifts above all else. Where these were deposited atop loosely-packed powder they were prone to triggering, and the trigger sensitivity increased with ascending altitude. Most drifts were small and easily recognized, thus, only three negative avalanches were reported.

In backcountry, Ausserfern. Wind impact increased with altitude. In steep terrain, caution was imperative. (photo: 06.03.2021)

 
For the next few days the basic problem will be the same. Once again, highest attentiveness is required for freshly generated snowdrifts. More and more, the current fresh snow is the potential weak layer, although only where solar radiation has no effect before the next round of snowfall on Sunday, 14 March.

A loosely-packed snowpack surface that was still dry until yesterday, 10.3, was moistened today, 11.3, by  warm temperatures and diffuse solar radiation. North, 1950m, 35 degrees. So far, so good. But caution: potential dp.4 (cold on warm) once cold fresh snow is deposited on top of it. We’re watching it closely.

In the uppermost layers, hard and soft layers are stacked together. If fresh snow atop them is massive it could fracture inside the snowpack, particularly at high altitudes on shady slopes.

Snowpack analysis in Silvretta. The arrow poings to the layer of Sahara dust from early February. Potential problem zones inside the snowpack lie especially above that point. (photo: 09.03.2021)

Nigg Effect (surface hoar near ridgelines) on Hirzer in the western Tux Alps. Heavy wind impact on 11 March probably transported this snow and thereby destroyed this potentially weak layer (mechanically).

Similar to today (11 March) in the western Tux Alps, in coming days we will observe increasingly frequent loose-snow avalanches on extremely steep slopes. Particularly during the interims between bouts of precipitation when the snowpack surface is moistened by daytime warmth and solar radiation.