Tomorrow, Wednesday, we will face a dangerous air mass constellation, especially in the center and south of Germany.
We expect widespread freezing rain from Saarland across parts of Rhineland-Palatinate, Baden-Württemberg, the southern half of Hesse and large parts of Bavaria, which will cause a significant risk of black ice and massive disruptions to road, rail and air traffic. From the Eifel via Saarland, northern Hesse, Thuringia and the south of Saxony-Anhalt to Saxony, 5 to 30 cm of new snow will accumulate and also cause disruption.
But how exactly does this explosive weather situation come about and what is freezing rain?
After a very cold night with freezing temperatures between -2 and -11 degrees, a small low pressure area called GERTRUD will reach us from France tomorrow morning. It brings significantly milder, but also very humid air from subtropical regions. The milder air of the low slides onto the cold air and is lifted. This creates a wide precipitation area. Initially, in the second half of the night, it moves across the southwest of Baden-Württemberg and spreads towards the northeast.
At the beginning there is still snow falling. However, the frosty nighttime air in the southwest warms up very quickly due to the incoming mild air mass, which means that the snowflakes are already melting in the air. The ground is still frozen due to the cold night and is not thawing as quickly. When the drops hit the earth’s surface, they freeze within a short time and form a layer of ice as smooth as glass. This phenomenon is called freezing rain. Caution! There is a widespread risk of black ice! The ice layer can grow from several millimeters to a few centimeters thick. However, it is not just the ground that freezes, but all surfaces whose temperatures are far enough below the freezing point. For example, trees, bushes, cars, street lights or power lines. This will massively affect road, rail and air traffic. In addition, trees and power lines often cannot withstand the ice masses and risk breaking. So much for the theory.
The exact timing and local boundaries between snowfall, sleet and freezing rain are fluid and often cannot be predicted to within a few kilometers. The weather models are still not entirely in agreement as to how far the precipitation area advances north and, above all, where exactly the boundary between snowfall and freezing rain lies. In addition, it is not yet entirely clear how long freezing rain will fall in the affected areas. With the mild air mass, the ground gradually warms up. However, how quickly the whole thing happens will vary from place to place.
The precipitation area will spread into the Rhineland via Hesse, Thuringia, the south of Saxony-Anhalt and Saxony until the early afternoon. North of a line between the Eifel and Lusatia, the cold air will remain predominant. That means snow will fall there consistently. 5 to 30 cm of fresh snow is possible here over time. The most snowfall is expected particularly along the Eifel and Westerwald via the Rhön to the Thuringian Forest. South of there, the snow turns relatively quickly into sleet and ultimately freezing rain in the morning. In Breisgau and along the Alps, the ground could already be sufficiently warmed up in the morning so that freezing rain is no longer a problem. In the other regions, the freezing rain sometimes lasts until the evening and locally even into the night.
On Thursday night the cold air will shift southeast again. The precipitation also moves to the southeast. The rain then gradually turns back into snow. On Thursday morning snow is still falling mainly in Baden-Württemberg and parts of Bavaria and initially as rain in the south. In the middle of Germany the weather has calmed down significantly again and the sun is shining more and more often between the clouds. In some areas a few flakes are still falling from denser clouds, but the quantities are very manageable. The snowfall continues to decrease from the northwest and moves completely over the Alps in the second half of the night.