Nordstau

Perhaps the most well known of the Stau Effects, the Nordstau, is characterized by cold, moist air masses colliding with the northern side of the Alps, leading to persistent and heavy precipitation.

Regions on the northern side of the Alps can receive substantial snowfall due to this phenomenon, and these events can last for several days, especially when a strong and persistent northerly flow sets up.

The areas below are favored during the Nordstau.

When the jet stream has a more meridional (north-south) pattern, which is common during the negative NAO phase, the chances of Nordstaus increase.

Below is an example of the jet stream and mid level pattern during a Nordstau. 

You can see the counterclockwise flow around the low pressure system bringing northerly winds to the Alps.

When the jet stream is taking a north to south path through the Alps, as shown above, it usually brings cold polar air, resulting in low snow levels. 

Some areas that see significant snow during a Nordstau include the Glarus Alps (Elm, Braunwald) and Nidwalde region (Engelberg) in Switzerland, the Bregenzerwald (Damuls-Mellau, Warth-Schrocken)), Arlberg (St. Anton, Lech) and Karwendel (Pertisau am Achensee) regions in Austria, and the Chablais (Morzine), Aravis (La Clusaz), Chartreuse (St. Pierre), and Vercors (Villard-de-Lans) regions in France. Generally, the areas on the northern edge of the northern Alps do best. 

For this pattern to set up, look for an area of high pressure to the west of the United Kingdom, turning the flow to the north. This will typically coincide with a low pressure system east of the Alps, allowing a northerly flow through the Alps. 

Another historical Nordstau is shown below.

Again, you can see the northerly winds resulting from the counterclockwise flow around the low pressure system.

If the jet stream is too far to the east, the high pressure sets up too close to the United Kingdom, and the heaviest snow will miss the Alps, falling in the Balkans instread.

The Nordstau is one of the largest snow producers for the northern Alps, while skiers and snowboarders rejoice when this type of storm is mentioned. This storm type has a major influence on the region's climate during the winter months, providing some of the biggest snowstorms every year.

Check out some more shots from the Nordstau in January 2018.

Luke Stone
Forecaster, OpenSnow