Winter returned with a roar over the Easter holidays. Hurricane Niklas was followed by further disturbances embedded in a strong NW current and snow and cold down to the valleys. Easter Monday seemed more like a successful St. Stephen's Day. This impression was disturbed at most by the occasional rumble of thunder.
Winter thunderstorms are rare and the WeatherBlog sometimes flinches when it suddenly flashes and thunders while skiing. In English, winter thunderstorms are called thunder snow, a more poetic neologism in my opinion. The thunder snow is often interspersed with sleet and, as with summer thunderstorms, the intensity of the precipitation can be very high. Basically, thunderstorms in winter develop in the same way as thunderstorms in summer, but as it is colder and the atmosphere is generally more stable, the violent lifting processes required for a thunderstorm rarely occur. While the ground heats up more and more in summer during typical heat thunderstorms due to solar radiation (this creates the necessary vertical temperature gradient), the upper layers of air generally cool down during winter thunderstorms, for example due to the inflow of cold polar air. A temperature difference of around 30 degrees between approx. 1500 m and 5000 m altitude would be a good prerequisite. In addition to a strong temperature gradient, we also need a trigger that ensures that the warm air can be lifted at ground level. In coastal regions or on large lakes, the temperature difference between the warm water surface and the cold air at altitude is sometimes enough to trigger convection (the so-called "lake effect"), but even there a thunderstorm usually only develops with the help of frontal uplift. As is so often the case, the topography can also play a role or significantly intensify the uplift processes, so that the thunderstorms are more severe in congested areas. Of course, it is most entertaining when warm water, a strong front and mountains come together. Despite the considerable amounts of precipitation at the end of the month, March (like February) was unusually dry compared to the long-term average. Better late than never and in some congested areas, so much precipitation fell in the last week that the rainfall balance was evened out at the last minute.
The next few days will be largely sunny across the entire Alpine region. Precipitation is not in sight until Monday at the earliest and at the moment it doesn't look like there will be much. Even in the medium term, the signs are pointing to quite friendly, quite dry and generally quite spring-like. The large-scale trough of the Easter days will disappear to the east and the Alps will slide into the high pressure of an Omega-like weather pattern.
