Classic "Nord Stau" Storm

We have finally gotten a taste of winter here in Innsbruck with a storm that has lasted for a bit over 36 hours in the mountains and has provided a steady snowfall even on the Inn Valley floor.  I had a beautiful walk to the office this morning following the walking path from our flat above town and then across the Inn River.  

Explosions from avalanche mitigation work on the Nordkette ridge above town are shaking the windows of my office currently.  Skies are still overcast, so I can't see the results.  I've been here when you can watch the avalanches from town.  

The snow was produced by a cold-frontal passage Wednesday evening followed by a period of northerly post-frontal flow, or what is sometimes referred to as "Nord Stau" in which northerly flow impinges on the Alps and experiences blocking and orographic lift.  This is the eastern Alps equivalent of northwesterly flow in the Cottonwoods, although the snowfall is much more expansive in area.  Below is the ECMWF HRES analysis for 0000 UTC Thursday 26 March during the Nord Stau period of the storm showing a deep upper-level trough over Germany, Switzerland, and Austria at 500 mb (hence cold and unstable) with moist north-northwesterly flow impinging on the Alps at 700 mb.  

The European radar composite from 1630 UTC late yesterday afternoon was a classic with convective cells upstrea of the Alps over Germany and France.  These were moving south-southeastward toward the Alps where the convection became more continuous over then northern Alpine Rim of Switzerland, German, and Austria.  Note the lack of echoes over Italy, which was in the lee of the Alps.   This is all happening in the wake of a low-level cyclone (i.e., low pressure system) that was producing broader, stratiform precipitation over eastern Europe.  

The "Euroregion" comprised of the Austrian state of Tirol and Italian states of South Tyrol and Trentino collaborate extensively on snow measurements and avalanche warnings.  There is a dense array of automated snow depth sensors in this Euroregion operated by many groups.  These measure total depth and one can access the 48-hour difference in snow height from these sites athttps://avalanche.report/weather/stations.  While not strictly equivalent to new snow amounts, in this case they provide a reasonable estimate of snowfall amounts given the well-consolidated snowpack before the start of this case. 

As is typically the case in Nord Stau, snowfall from this storm was greatest in what is known as the Northern Alpine Rim, which in western Austria is north of the Inn Valley and includes the snowy region near and around the Arlberg Pass (simply called the Arlberg).  There are a number of stations with 48-hour increases in snow height over 50 cm (20 inches).    

48-hour difference in snow height for the period ending ~0730 UTC 27 March. Source: https://avalanche.report/weather/stations. 

Snowfall decreases as one moves southward into the Inner Alps of Tyrol.  For example, in the Alps around the Paznaun valley, snowfall amounts are genreally less than 45 cm and in some areas less than 20 cm.  One area of the Inner Alps with greater snowfall is in the Tux Alps just west of the Ziller Valley, which I've circled in the right of the diagram.  Not shown here are areas farther east in the Kitzbuhel, Zell am See, and Hohen Tauern that also saw more than 50 cm where storm penetration into the inner Alps is often a bit stronger than in the Innsbruck area.  

The Austrian-Italian border (and farther west the Swiss-Italian border) represents the Main Divide of the Alps, which is the hydrologic divide.  Although the highest peaks near the Main Divide are quite high, snowfall is more limited.  There are a couple of sites just north of the main divide with > 30 cm but many other sites have lesser precipitation.  This is not uncommon as Nord Stau storms tend to dump their loads on the Northern Alpine Rim with less snowfall over the Inner Alps.  

Once south of the Main Divide, snowfall decreases further into the lee and South Tyrol.  There are some light amounts in the Dolomites, but those were actually produced during the passage of the cyclone rather than in the post-frontal Nord Stau period.  In situations like this, you have your pick of powder on the northern Alpine rim or dry weather for hiking or biking in South Tyrol.  Just take the regional train from Brenner Pass on the Alpine Rim until you find the weather that you want.  

During this an many other storms, there can be a good deal of variability in the change in snow depth in any Alpine region.  In part, this reflects they large contrasts in the elevations of stations, but also the fact that there is a good deal of snow redistribution by the wind above timberline in the Alps, both prior to deposition on the ground and after.  I'm still trying to learn what sites are generally representative of the immediate surrounding area.  

The good news here is that after a break later today and tomorrow (Saturday), there's another storm coming in.  Unlike what has happened in Utah, this should enable many resorts to make it to their target closing dates.