A westward travelling ULL induced the rapid reversal of a pressure gradient across the Alpine range in mid-March 2016.
05 May 2023
15 March 2016
By HansPeter Roesli
On 14 March an upper-level low (ULL) developed at the eastern border of a robust high-pressure area, which was centred over northern Europe.
The evolution of the ULL and the MSLP field can be seen on the composite of WV6.2 imagery from Meteosat-10 and MSLP analyses from the Global Forecast System (GFS) model (Figure 1).
The water vapour whirl of the ULL was initially collocated with a small low-level depression over the tri-border point of Poland-Ukraine-Belarus.
Between 15 and 17 March the ULL travelled south-westward and then westward, dragging along the low-level depression that passed just south of the Alps on 16 March.
The animated sequence of the Airmass RGB spanning from 14 to 17 March (Figure 2), gives more details of the ULL trajectory, marked by the red blotch that developed on 14 March over the borders between the Baltic states and Russia.
High pressure systems centred over northern Europe typically create a rather strong pressure gradient in the boundary layer across the Alps. The gradient gives rise to channelled easterly wind along their northern flanks (called bise on the Swiss plateau) and to a moderate north-foehn tendency south of them.
In this case, the pressure gradient got temporarily reversed during the transition of the ULL on 16 March, bringing a few hours of a barrage-type weather, including sleet and snow, along parts of the alpine south side.
HRV imagery from Meteosat-10 and MSLP analyses from the GFS model for 15/16 March (Figures 3 & 4) illustrates this rapid gradient reversal.
The image of 15 March 12:00 UTC (Figure 3) shows the classical positive north-south pressure gradient and the so-called foehn gap along the southern flanks of the alpine main range, slightly disguised by high cloud.
Figure 4 shows the situation 24 hours later — an inverted north-south pressure gradient, foehn gaps now north of the Alps and thick barrage clouds over Piedmont-Ticino, already starting to dissolve though.
A few hours later, with the ULL now over France, the normal MSLP pressure situation had been re-established.
Snow from trough over eastern Mediterranean Sea
The Airmass RGB shows the deep upper level trough over the eastern Mediterranean Sea.
Upper-level low off coast of South Africa & Namibia
In January 2014 an unseasonal upper-level low brought heavy rains to South Africa.