Italy 28/11/2018, cold or simply back to the average temperatures?

A colder air mass coming from the Balkans is causing a decrease in temperatures in Italy (especially along the Adriatic coast). As shown in Figure1, the temperatures observed this late afternoon are below 10 °C in Northern and Central Italy (except in the coast of Liguria), with values below 0°C in the Alpine region. In the south, milder temperatures are observed especially in Sicily and southern Calabria (temperatures above 15°C).

Compared with yesterday afternoon (Figure1), a decrease in temperatures between 2-5°C has been observed in the Alps and Po valley, as well as in central Italy. However, this colder air (which will continue, especially in the north, throughout this week) is simply bringing the temperatures towards the average of the last week of November; thus, the real anomaly have been the very mild temperatures (especially the minimum) observed during the whole month (except for few ‘cold’ days: see Europe: First cold wave of the season ) in the Mediterranean region and the current month might become one of the warmest ever recorded in some areas.

Figure1. Comparison temperatures in Italy between today (left) and yesterday (right). (Meteociel)

Regarding the weather conditions (Figure2), clear skies have been dominant in the north, Sardinia and Tyrrenian coast. Typical of this weather condition is the stau effect, visible from the satellite imagery, from Marche to Puglia, caused by the easterly winds which blows above the Adriatic sea (picking up moisture) and, once raising on the Appennines barrier, contribute to form clouds and precipitation on the Adriatic coast, instead clear skies and milder temperatures are observed on the western side of the Appennines (i.e. Tuscany, Lazio, Campania).

Figure2. Satellite image at 11 UTC (Sat24).

Tomorrow clear skies will be mostly everywhere, with still some rainfall possible on the Adriatic coast (with snow on the Appennines above 1000-1500 m). On Friday overcast conditions will be in northern-central Italy (and later in the day also in the south) with showers expected from Friday morning on N.W. Italy (mainly on the Alps with snow above 500-800 m), in the afternoon between Umbria, Lazio and Tuscany and during the evening in south/western Sardinia.

The temperatures will decrease (especially on Friday), with minimum values below 0°C in the north (Alpine region and Po valley) and in the mainland in central Italy. The maximum temperatures will be below 10 °C in the north and Appennines (both tomorrow and Friday), between (10-12°C) along the Adriatic coast, (10-14°C) along the Tyrrenian coast and (14-16°C) in Sicily.

Thus, the colder air will simply cause a decrease in temperatures towards values closer to the normal conditions of this time of the year, breaking the anomaly lasted for several weeks with osberved temperatures close to 20 °C (as happened in central-southern Italy) and no cold nights (temperatures below 0°C) in the north.

November historical analysis in N.E. Italy

November 2018 is showing very mild temperatures in the N.E. of Italy, with only few days observing temperatures below average
 (see Italy: First cold spell of the season). It might be possible that the current November will be one of the warmest ever recorded (especially regarding the minimum temperatures), but before claiming that we need to wait few days still.

However, in the past (also quite recently) November has observed coldwaves with snow also in the Po valley (e.g. 2005, 2008). I have considered the temperature data from 1973 to 2017 from Treviso S. Angelo in Veneto. Unfortunately, some daily data are missed in few years, thus the monthly average is not completely exhaustive (especially for the years 2005 -2012 where I have used data from Treviso Istrana, which is only few miles from S. Angelo). The idea is to give a general overview of the historical temperatures in November in the Veneto plains.

In Figure1 is shown the monthly maximum and minimum temperatures average.

Figure1. Monthly average of max (red) and minimum (blue) temperatures in November. The dashed lines show the max and min averages in the period 1981-2010.

It is clear how the monthly average temperatures in the 00s have been only few times below the long term average 1981-2010 (4.6/12.7 °C). The warmest November occurred in 2014 , instead the coldest one in 1988 for both minimum and maximum values. The minimum temperatures, especially, tend to be higher than the long term average in several years during the 00s, with only 3 years observing lower values (2001, 2007 and 2011) (Figure2).

Figure2. Occurrence of years observing max (red) and min (blue) temperatures above the long term average (equal or above +0.5 °C from the long term average).

It is interesting to notice how the frequency of mild temperatures in November is increasing from the end of the 90s, instead in the 70s and 80s only few years observed mild temperatures in November.

If we consider the top and bottom 10% of both the minimum and maximum temperature distribution (thus the coldest and warmest November), the analysis has shown that the coldest November (the lower 10% of the distribution) occurred mainly in the 80s, and no one is observed after 1993. Instead the top 10%, thus the mildest November, is observed only in the 00s, which explain what found in Figure2. In conclusion, it seems that there is a (positive) shift in both maximum and minimum temperatures in November, with the last two decades observing the highest values.


Italy weather condition

After the first cold spell of the season, temperatures have increased during the week, especially in central-southern Italy. This afternoon (Figure1) very warm temperatures were observed especially in Sicily, Calabria and Sardinia (locally above 20°C) instead in the north temperatures were between (7-13°C) moving from Piemonte to the Adriatic coast, thus around the climatological average.

Figure1. Maximum temperatures on Friday (Meteociel).

This difference is mainly due to two different type of weather systems over the Mediterranean region (Figure2).  In the south, the high pressure positioned between Tunisia and Sicily brings warm, dry air from N. Africa towards the south of the peninsula. In the north instead, a low pressure system positioned north of the Alps leads moist and colder air towards  the south-east from France, causing overcast conditions and rainfall especially on the Alpine region and N.W. Italy, with snow on the Alps (above 1200-1600 m).

Figure2. Satellite image of this afternoon (Sat24).

This situation will last also tomorrow, with humid and chilly conditions in the north, and milder elsewhere, though with showers moving southward especially on the Tyrrhenian coast.

October 2018 N.E. Italy analysis

I have considered the data from Treviso airport (N.E. Italy) to sum up the monthly analysis (Figure1) in the N.E. of Italy.

The mean minimum and maximum temperatures (11.84/21.17 °C) have been considerably above the 1971-2000 average (8.8/18.9 °C). This is due to the continuous mild conditions especially between the first and second decade, with no cold spells during the whole month (Figure1) and numerous days with maximum temperatures above 20 °C and minimum above 10°C.

Figure1. Max and min temperatures Treviso October 2018.

In fact, the number of days observing maximum temperatures above 20°C is 22, with 15 consecutive days (in the first two decades) when the maximum values never fall below 20°C. Moreover, during this period, for 3 consecutive days the maximum values reached 24°C, almost 6°C above the climatological average, and, during the whole month, this values has been reached 6 times.

During the long period of very mild maximum temperatures, also the minimum values were well above the average, with 18 consecutive days observing minimum temperatures above 10°C, 5 consecutive days above 13 °C (almost 6°C above the climatological average). To notice that, even if the maximum values decreased during the intense rainfall event occurred in the last week of October, the minimum values stayed high due to the strong southerly winds blowing from the Adriatic Sea which caused still mild conditions and due to the continuous overcast conditions which have limited the radiative cooling at surface during the night.

The observed rainfall (135 mm) ,instead, has been above the average (92 mm). This is due to the intense event occured in Italy during the last week of October,which has caused floods in several areas. Thus, though the total observed rainfall is above the climatological average, it was not evenly distributed during the month, with 20 consecutive days (between the 6th and 26th) with no rain (anticyclonic conditions), and only 9 with observed rainfall, 6 of them consecutively during the last week of the month.

Figure2. Daily rainfall Treviso October 2018.


Italy: First cold spell of the season

Finally, the first cold spell of the season is reaching the Italian Peninsula.  As we can see in Figure1, easterly winds (Bora, Grecale) coming from the Balkans are blowing over the Adriatic sea, reducing the humidity and causing clear skies in the Po valley. However, these winds are causing the stau effect in Piemonte and Emilia Romagna, where overcast conditions occur. This is happening also along the peninsula, with more clouds on the east coast and bright conditions on the west.

Figure1. Satellite image of this late morning (Sat24).

The cold spell is causing (and will cause) a decrease in both minimum and maximum temperatures, with snow showers especially on the Appennines (low-medium height) and bright (but chilly) conditions elsewhere. The temperatures will plummet especially in the North, after weeks of mild conditions, with minimum values probably observed between next Monday and Tuesday. In Figure2 is shown the comparison between the actual temperatures and the situation observed 24 hours ago (Friday).

It is clear how the main variation is observed in the Alps, Adriatic coast and Sardinia (decrease in 4-6 °C in 24 hours).

Figure2. Comparison temperatures recorded at 12 UTC on Saturday and Friday (Meteociel).