Italy: December 2018 analysis

The first snow of the season in central-southern Italy is occurring these days, with snow also in unusual places.

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.

Figure1. Average maximum (top) and minimum (bottom) temperatures observed in December 2018 in the main italian cities. Each color is related to a single city.

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).

Figure2. Temperature anomaly of the maximum (left) and minimum(right) temperatures observed in Italy in December 2018 calculated with respect to the climatological average (1971-2000).

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.

Italy: Dry and cold in the north, snow in the south

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.



Figure1. Maximum (top) and minimum (bottom) temperatures observed today in Italy. (Source: Meteociel).

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).

Figure2. Satellite imagery of today (12 UTC). I have coloured in blue the area along the Adriatic coast interested by snow (source: Sat24).

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.

UK: Where is winter?

If we look at the temperatures observed today, we can see how the values (especially maximum) have decreased since last week. In fact, the maximum temperatures (Figure1), are between 3-8 °C in the UK, with negative values on the Pennines. Instead, in Ireland, temperatures are higher, between 6-10 °C. The decrease in temperatures, especially in S. England, is due to a cold air mass moved southward towards Central-Eastern Europe.

Figure1. Maximum temperatures observed this afternoon (source: Meteociel).

However, the cold air has reached mainly only the eastern part of the UK (and not at all Ireland), causing a decrease in temperatures towards value close to the average or slightly below. Until Sunday, minimum values (especially in England) will be close or below zero (possible -3/-4 °C in some areas) and maximum temperatures will remain between 3-7 °C mostly everywhere.

Thus, the significant element is the positive anomaly seen during the first month of the winter season, when there have been only few cold events.

Indeed, if we look at the temperatures observed in some cities in the UK in December, we will see how the average minimum and maximum temperatures have observed values above the long-term average. I have analysed data of some cities in the UK and plotted their anomaly referring to the long-term average 1981-2010 (Figure2).

Notice that this is a general analysis (it doesn’t consider all the weather stations and local orography in detail), but I think it’s interesting to notice the overall view of December.

Figure2. Maximum (left) and minimum (right) temperatures anomalies observed in some cities in the UK during December 2018 compared with the 1981-2010 average. The dots increase their size according to the temperature scale, that is, the bigger is the dot the more is the anomaly.

We can see how all the UK has observed values well above the average, especially the minimum temperatures (close to + 5 °C in Wales). Only in eastern England, along the coast of the N. Sea, negative anomalies are observed (Norwich -0.2 °C anomaly in the minimum temperature).

Thus, we can see that up to now in the UK winter has shown very few episodes with cold weather, and the temperatures observed these days are nothing exceptional. Next week, colder air should move towards the UK from Greenland and N. Atlantic, causing a decrease in temperatures compared with the next weekend, but it’s still to early to describe the event in details. Thus, we will see how January will be, maybe changing the mild trend observed during the last month.

References: 

I have used the data from both Meteociel and Met Office

First week of 2019 in Europe: between cold and mild conditions

The first week of the year is going to see two different weather patterns over the continent. On W. Europe, a strong high-pressure system will be positioned between the UK and N. Atlantic, bringing mild and settled weather in Spain, Portugal, Ireland (and western part of the UK).

Instead, on the eastern side of the anticyclone, a deep low pressure system, positioned between Sweden and Finland, will lead very cold air (polar continental with values between -12/-14 °C at 850 hPa (1500 m a.s.l)) to move southward during the week (Figure1) towards central-eastern Europe. In addition, the cold air will cause the formation of a low-pressure system in the Mediterranean Sea (between S. Italy and Greece) which will lead to snow at low levels and (very likely, due to the cold temperatures) along the coast of the Adriatic Sea.

Figure1. Synoptic chart valid for 00 UTC on Thursday night. I have coloured in orange the mild air moving towards W. Europe and in blue the cold air (polar continental, PC) coming from Scandinavia. Notice the 528 dam line reaching S. Italy and Greece (which means snow at sea level is possible). (source: Met Office).

The cold wave should reach the maximum intensity between Thursday and Friday, instead Saturday and Sunday milder air from the Atlantic should move eastward, pushing the cold air towards E. Europe and Russia.

Thus, during the week, the continent will be ‘divided’ in two parts, with temperatures well below 0°C (also during the day) in Scandinavia, Balkans and Central Europe, with snow expected especially between Poland/E. Germany and Czech Republic, and between Serbia and central/northern Greece and central-S. Italy.

Instead, warm temperatures on the west (possible maximum values between 8-11 °C in Ireland and between 16-18 °C in S. Spain/Portugal), with settled conditions in both Spain, France, Ireland and the UK (except for some low clouds close to the coastal areas and valleys).

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.

Christmas 2018 in Italy: very different weather conditions

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.

Figure1. Minimum (left) and maximum (right) temperatures in Italy on Christmas day. (source: Meteociel).
Figure2. Satellite imagery of Christmas afternoon (13 UTC). (source: sat24).

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.

Table1. Minimum and maximum temperatures obseved this Christmas in Italy and compared with the long term (1971-2000) average. Values below the average (blue), values above the average (red).

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).

Christmas in Europe: where could it be ‘white’?

This morning (Figure1) the weather condition hasn’t changed from the weekend, with a strong temperature gradient between northern- eastern Europe and western-southern Europe. Values well below 0 °C are observed in Scandinavia, Russia and between Romania and the Baltic region, with peaks below -20 °C between Finland and Russia.

Meanwhile, in western Europe and in the Mediterranean region (where a high-pressure ridge is setting from Spain towards N. Atlantic) temperatures are above 10 °C, with peaks of 14-16 °C in Sardinia, southern France and Cyprus. To notice that the high-pressure ridge is causing mild conditions also in Iceland, with observed values well above 0 °C.

Regarding the precipitation, snow at low-levels/plains is observed between Norway and Sweden (mainly in the mainland) and between Poland and the Baltic countries. Some snow (showers) are observed also in the Alpine region and between Czech Republic and Romania (on the Tatra and Carpathians mountains).

The unsettled weather between the Alpine region and E. Europe is due to a cold front related to a low-pressure system moving southward from the Baltic region towards the Balkans. The pressure difference between central Europe and the Mediterranean region is causing strong winds in the Alpine region leading to the fohn effect in N.W. Italy (Figure2); this is leading to drier and milder conditions compared with the previous days.

Figure1. Minimum temperatures observed this morning in Europe (source: Meteociel).
Figure2. Satellite imagery of this morning (10.00 UTC). I have coloured in orange Tropical Maritime air (TM), in blue polar continental (PC) and in red the fohn effect in the Alpine region (notice the ‘wall’ of clouds in N.W. Italy). (source: Sat24).

Regarding the weather for Christmas day, the high-pressure ridge will set between France and the UK, leading the cold air mass (from Scandinavia and the Baltic region) to move southwards towards the Balkans and the E. Mediterranean region.

Thus, tomorrow the weather conditions will see warm temperatures in western Europe (especially between Spain and France and in Ireland), with maximum values above 10 °C (peaks of 15-18 °C are expected in S. Spain). Only between N. France, English Channel and Belgium (where fog/low clouds are possible) lower temperatures are expected.

Instead, moving eastward, the cold air will cause a decrease in temperatures in the Alpine region, in Italy (especially along the Adriatic coast) and E. Europe. However, the lowest temperatures are expected mainly in the Balkans, with maximum values close or below 0 °C between Romania, Bulgaria and Serbia. Also in Greece will be cold, with values below 10 °C mostly everywhere (except in Crete and Peloponnesus).

Most of the precipitation will occur this evening, when the cold front will be more active (and temperatures will still be slightly above 0 °C at low-levels), thus snow is expected mainly on the mountainous areas in E. Europe. However, during the night and tomorrow morning, the decrease in temperatures might lead to snow at low-levels/plains in the mainland between Romania, Bulgaria, Hungary and Serbia.

Finally, in Scandinavia, Baltic regions and Russia the weather conditions will still see low temperatures and some snow at low-levels/plains is expected between Poland, Belarus and Ukraine, as well as between Finland and N. Russia and in W. Norway.