N Italy: difference of 20°C in 200km

Yesterday, a significant temperature gradient was observed along the Po Valley in N Italy. The strong pressure gradient between central Europe and the Mediterranean has caused a significant foehn effect across the Alpine region, leading to a sharp increase in temperatures in the afternoon over SE France, NW Italy and S Switzerland.

Looking at the temperatures (Figure 1) we can see that most of the areas downwind the NW’ly flow observed values above 20°C.

Figure 1. Maximum temperatures observed in the Alpine region on 03/02/2020. Source: Meteociel

Some temperatures have broken the all time records, like in Turin (27°C, which is 20°C above the climatological average).

These extremely high temperatures were caused by unseasonably mild air coming from the mid-Atlantic/N Africa combined with the catabatic wind (i.e. foehn) flowing down from the Alps towards W Po Valley, Ticino and SE France. Thus, being the temperature at 850hPa (1500m a.s.l.) very mild (around 12/15°C), the warming effect of the air flowing down the mountains slope (1°C every 100m) has caused temperatures to reach 20/25°C in these areas, as well as clear conditions.

On the central/E part of the Po Valley the NW’ly wind wasn’t powerful enough to ‘clear’ the layer of fog/low clouds lying on the area for several days (Figure 2). Thus, in cities like Venice and Trieste, maximum temperatures barely reached 10°C (though being still 2/3°C above the climatological average).

Figure 2. Sat Image of Alpine region on 03/02/2020. Notice the low clouds over NE Italy and clear skies over the NW areas. Source: Sat 24

Alpine region: strong winds, snow and spring on the same day.

A high pressure system is positioned on W. Europe (Figure1) causing settled and mild conditions between Spain, France and the UK. Instead, on the eastern side of the anticyclone, colder air from Scandinavia is moving southward towards central Europe and the Balkans.

The pressure gradient between France and Italy is causing an increase in the wind speed especially on the Alpine region, with northerly winds blowing on the eastern side of the anticyclone. The gusts, especially at high altitudes (above 2000-2500 m) and on the southern side of the Alps are pretty strong, reaching 80-100 km/h (between Trentino Alto Adige and Veneto).

Hopefully the winds should not cause damages to the forests already hit by the strong winds in November.

Figure1. Synoptic chart valid for today (12 UTC). Notice the quick change in the pressure values between central Europe and N. Italy (from 1032 to 1024 hPa).
(source: Met Office)

The northerly winds are also causing the Stau effect. That is, overcast conditions with showers are observed in the northern side of the Alpine region (manly between Austria and Switzerland), with snow above 800-1000 m. Some clouds and showers have also reached N. Italy (especially between Lombardia, Trentino Alto Adige and Veneto). However, sunny and mild conditions are observed on the pre-Alps and in the Po valley due to the fohn descending from the Alps (Figure2).

Figure2. Satellite imagery of today (12 UTC). I have coloured in blue the wall of clouds (stau effect) and in orange the fohn blowing on the southern side of the Alps. (source: Sat24).

The fohn is causing a strong temperature gradient between N. Italy (especially N.W. Italy) and Switzerland-Austria. Indeed, there is a difference of more than 10 °C between the southern and northern side of the Alps (Figure3), with values between 10-15 °C in N.W. Italy (peaks of 20 °C in Liguria) and only 3-5 °C observed in Switzerland and Austria.

Figure3. Temperatures observed this afternoon in the Alpine region. (source: Meteociel).

This weather condition should last until Wednesday, when a cold air mass from Russia should move south-westward towards central Europe and the Mediterranean region, causing a strong decrease in temperatures (especially in the areas where mild conditions are observed today).

However, there are still uncertainties on the correct direction of the cold air and on its possible effects (in terms of temperatures and precipitation) especially in the Mediterranean region and W. Europe; thus, it would be better to wait Monday/Tuesday to check the output of the weather prediction models.