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.
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).
The cold air moved towards central Europe the last few days has reached the Mediterranean region, causing the development of a deep low-pressure system (981 hPa) between Spain and Italy (Figure1). This is causing strong winds especially southwest of Sardinia, with gusts up to 100 km/h.
In addition, since yesterday rainfall is observed along the Italian peninsula (especially in Liguria and S Italy); last night and this morning, with the colder air approaching the Mediterranean region, snow has fallen on the plains in N Italy (especially E Romagna and Piemonte), but snow showers have been observed also in Genova on the coast of Liguria.
Moving south, the snow limit increases (between 300-1000 m a.s.l in central Italy and 1000-1500 m a.s.l in the south).
The satellite imagery of today (Figure2) shows clearly the different weather conditions on the Mediterranean region, with the low-pressure system on the Tyrrhenian sea, the colder air entering the Mediterranean region from France and the low clouds in central Europe (where very cold temperatures are still observed).
Tomorrow the low-pressure system will move eastward towards Greece, though it will start weakening. Thus, winds won’t be as strong as today and the weather conditions should see brighter skies by evening, with some precipitation still possible in Sicily and SE Italy (with snow on the Appennines above 700-1000 m a.s.l. due to the colder air).
Finally, temperatures will decrease mostly everywhere, with negative values tomorrow morning in N Italy (especially in the area with snow on the ground) and close to 0 °C along the coast in Central Italy. In the south possible +8/+10 °C only in extreme S Italy and Sicily. The maximum values will be between +4/+8 °C in the north, +5/+10 °C in central Italy and between +8/+13 °C in the south.
However, December has not seen any relevant episode, except before Christmas, when some snow fell in the Po valley (especially in Emilia Romagna).
I have extracted data from the main cities in Italy and plotted the results in order to have a general overview of the maximum and minimum temperatures observed in December and compared with the long-term average 1971-2000 (there is no data for Aosta in N.W. Italy).
Regarding the average maximum and minimum temperatures observed during the last month (Figure1), we can see how the highest temperatures are observed in the south (mainly Sicily and Sardegna) with average temperatures between 14-16 °C. Instead, in the north, except for Liguria, the maximum values were below 10 °C on average.
Regarding the average minimum temperatures, only in few cities (regions) the values are below 0°C, with Bolzano the coldest one.
Considering now the comparison of the observed temperatures with the climatological average (1971-2000), we can see how southern Italy (mainly the regions along the Adriatic coast) have observed negative anomalies in both the minimum and maximum values (Figure2). Instead, in N.W. Italy the most significant positive anomalies are observed especially in the maximum values (more than +2 °C in Lombardia), instead in N.E. Italy the anomaly is less (between +0.5/ +1 °C).
The positive anomalies in N.W. Italy are mainly due to the frequent episodes of fohn, caused by a strong flow of northerly winds towards the Alpine region, which have caused sunny and dry conditions in the southern part of the Alps and mild temperatures (the last episode occurred few days ago). Instead, in southern Italy more rainfall and overcast conditions (as well as the easterly winds) have contribute to the (little) negative anomalies.
Since Wednesday cold air from E. Europe has caused a decrease in temperatures along the peninsula, with values below the average (especially in central-southern Italy). The cold air, moving above the Adriatic sea, is causing snow on the eastern side of Italy, with more than 1 metre of snow in some areas on the Appennines (e.g. Capracotta in Molise). Some snow has reached also the coast (especially between Abruzzo and Puglia), with few centimetres on the beaches. Snow has also fallen in Sicily at low levels.
Meanwhile, in the north, bright conditions are occurring, with strong winds especially on the Alps, which have caused some wildfires between Veneto and Lombardia due to the very dry conditions (also some were caused by human activities).
Today has been the coldest of the year so far (and since the start of the winter in the south). In fact, both minimum and maximum temperatures are below the average mostly everywhere (Figure1). Negative values have been observed in the mainland along the peninsula, with peaks of -5/-8 °C on the Po Valley and Tuscany, and below -10 °C on the Alps. The maximum temperatures have been below 10 °C mostly everywhere, except in Liguria and Sardinia.
The interesting thing has been, as said previously, the snow along the coast of the Adriatic sea. This event is quite unusual (though it can happen every few years) and it is due to the Adriatic sea effect: very cold air (from the Balkans) moves above the warmer sea (picking up moisture), and pushes against the Appennines condensing and leading to precipitation. If the column of air is cold enough (like today), snow can fall also along the coast.
For this reason the snow observed today was significant, because the continuous flow of air from N.E. has led to precipitation on the same areas.
The situation is clearly visible from the satellite image (Figure2).
The cold air should move eastward tomorrow and by Sunday there will be an increase in temperatures especially in the south. Next week cold air from Central Europe should reach the peninsula, but it is still too early to describe the possible effects. In the meantime, we can enjoy this unusual weather in the south.
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.
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).
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.
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.
Christmas day in Italy has shown both a wintry and mild aspect. Looking at the minimum and maximum temperatures (Figure1), we can see how values below 0°C were observed in the morning in the Po valley and Alpine region, with frost especially in the countryside. Values close to 0 °C were recorded also in central-southern Italy on the Appennines, instead mild values (10-15 °C) were observed mainly in Sicily and Sardinia.
The night and morning has been sunny mostly everywhere; only in Calabria and Sicily some showers have occurred and foggy conditions were observed especially on the plains in N.E. Italy.
The afternoon has been settled mostly everywhere, with sunny conditions along the peninsula. However, in N.E. Italy, especially close to the Adriatic coast (Figure2), foggy conditions have lasted during the whole day, causing cold temperatures in the plains.
Indeed, the maximum temperatures (Figure1) were just above 0 °C in the area between Venice, Verona and Ferrara; instead, in N.W. Italy, were bright conditions have occurred most of the day, maximum temperatures reached 10-11 °C.
Also in central-southern Italy this Christmas has not been cold (though with temperatures close to the average), with maximum values between 10-15 °C mostly everywhere (lower values along the Adriatic coast due to the colder air moving towards the Balkans). Finally, the warmest temperatures were observed in Sardinia and Sicily, with peaks of 16-18 °C in some areas.
The very different temperatures observed were due to both local orography/climate and larger scale weather patterns.
The foggy (and cold) conditions on the plains in N.E. Italy were due to the absence of winds at surface in the area and to anticyclonic conditions in the western Mediterranean region. Moreover, being the area surrounded by mountains (Alps and Appennines), the cold air is ‘trapped’ at surface along with humidity and pollutants (which contribute to the formation of the fog).
Instead, in N.W. Italy, the winds were ‘stronger’ and coming from the Alps, leading to a decrease in the humidity and to an increase in temperatures during the day.
Along the peninsula, the colder air on the Balkans has caused a decrease in temperatures on the Adriatic coast with strong northerly winds; instead, the Tyrrhenian side, ‘protected’ by the Appennines, has seen a little decrease in temperatures.
The minimum and maximum temperatures observed in the main italian cities on Christmas day, compared with the 1971-2000 average, are shown in Table1.
We can see how Venice has observed low temperatures (-3 °C of anomaly in both minimum and maximum values), instead the cities in N.W. Italy (especially Torino) have shown maximum values well above the average (+7 °C of anomaly in Torino). Also the cities on the Adriatic coast (Ancona, Bari) have shown temperatures below the average, though only for the maximum values, instead Roma and Palermo (Sicily) were warmer than the average.
These weather conditions will last until Saturday, with the high-pressure still positioned on W. Europe bringing mild and settled weather on the peninsula (though foggy conditions are still possible in the Po valley with temperature inversion at surface). Then, from Sunday, a colder air mass coming from Scandinavia should cause a decrease in temperatures especially in northern-central Italy. However, it is too early to talk about it in details, especially regarding the likelihood of precipitation (e.g. snow).
In this article I am going to analyse the temperatures observed in Autumn 2018. I have extracted temperature data (minimum and maximum) for each main city in every region in Italy, and plotted the results. To notice that this is only a general overview, which doesn’t consider differences between mountainous regions and the plains or the local climate.
The anomaly is calculated with respect to the long term average 1971-2000 for each city.
It is clear how all the three months have observed temperatures anomalies well above the average (for both max and min temperatures) mostly everywhere. Only in southern Italy negative anomalies (-0.5/-1.0 °C) have been observed (especially in September); instead in the north (in particular in the N.W. Alps), the anomalies have reached +5 °C especially in September and November. This is strictly related with what found in the Autumn analysis in N.E. Italy.
In summary, the last Autumn has been very mild especially in the north, with continuos significant positive anomaly in both maximum and minimum temperatures due to the presence of the high-pressure positioned in the western Mediterranean (in September) and to the southerly winds (in October and November) which have brought lot of rain but very mild temperatures.