The persistence of strong snowfalls in both Central and Southern Italy between December 2016 and January 2017 has been the most dramatic effect of a particular quasi-steady meteorological situation known as Ω block. This situation has concerned a large European area as well. It is worth to stress that this meteorological pattern is usually associated with length of time larger than that typical of meteorological weather. In the present paper we will: 1) show that, in the context of the non-linear quasi-geostrophic dynamics of the atmosphere at the synoptic scale, the Ω block is the result of an interplay between Rossby waves and zonal flows; 2) provide, in accordance with the statistics of the above blocks, the standard duration of the corresponding meteorological configuration which, in any case, is candidate to the break up via dynamical instability.
