It's been snowing heavily on the northern slopes of the Alps recently. Our colleague Orakel has sounded the powder alarm twice in four days and that's saying something. We'll leave the specific details to him and focus on the bigger picture that makes it all possible.
Downstream Development
So let's first take a look at the current situation in the northern hemisphere last Friday (Map 1). Europe was in a relatively warm westerly flow. We remember rain up to high altitudes! The low pressure system in the area of the North American Atlantic coast was decisive for the further development. It deepened increasingly and shoveled more and more warm air to the north, causing a wedge to form over the Atlantic (Map 2). Downstream, the current turned to the north, with cold air being tapped over the British Isles and transported southwards (Map 3). We remember a marked drop in temperature on Saturday, followed by increasingly fluffy snow! In just under three days, the general weather situation in Central Europe had completely changed: A few thousand kilometers to the west, i.e. upstream, something happened that triggered a development downstream: this is known as downstream development, a.k.a. DD.
Planetary waves
So once again we see that somehow everything is connected. The jet lies like a sloppily tied loop around the northern hemisphere. If it throws a new wave somewhere, sooner or later it will influence our weather. A certain Mr. Rossby came up with the theory behind this back in 1939. There are various equations that describe the behavior of liquids on rotating spheres. (After all, who isn't interested in liquids on rotating spheres?) If you throw a few of them into the blender, you get a second-order harmonic differential equation that gave Mr. Rossby no peace until he discovered that its solution must have the form of a periodic wave. Until then, the existence of planetary waves had been suspected, but had not been proven on the basis of the meagre measurement data. Rossby also realized that the phase velocity of such waves depends, among other things, on the square of the wavelength. Larger waves move more slowly than smaller ones and can even shift retrograde (to the east) under certain circumstances. For us, this increases the chance that the general flow pattern will remain intact for a while. For next week, we'll be on the lookout for double Ds (a prankster who thinks evil!) and for the weekend we're waiting for news from the oracle.
