Italy: first snow on the northern plains

The first coldwave of the winter has brought snow at low levels and in some areas (especially on the south of the Po valley) on the plains. Overall, temperatures have decreased towards value below the average, especially in central-northern Italy. The maximum temperatures observed on Sunday (Figure1, when most of the snow occurred) where below 5 °C in all N. Italy (except the Ligurian coast), Tuscany and Umbria. On the Alps and N. Appennines maximum temperatures were below 0°C. In central-southern Italy, the temperatures reached values between 10-13 °C and only in Sicily and Sardinia mild conditions were still observed (14-17 °C).

Figure1. Maximum temperatures on Sunday (Meteociel).

Snowfall has occurred mainly between Sunday afternoon and Monday morning due to a low-pressure system moving westward from France and colder air coming from the Balkans. However, only in few areas (mainly in Emilia Romagna) few centimeters of snow have been observed, instead elsewhere the precipitation was too weak to lead to some accumulation.

This is the first coldwave of the season, after the first 10 days of December when temperatures above the average have been observed. The next few days (until Friday, when milder air coming from the Atlantic should cause an increase in temperatures especially in central-southern Italy) will see still cold air coming from the east (though not as cold as the previous one) with minimum temperatures below 0°C especially in the areas with snow at the ground and maximum values slightly below or close to the average mostly everywhere. This will contribute to reduce the significant positive anomaly observed until last week.

N.E. Italy: A very mild autumn

Related to the analysis I made for November 2018 in Treviso (N.E. Italy), one of the warmest recorded since 1973, here I am going to analyse the temperatures of the last 3 months, that is September, October and November (‘SON’ from now onwards) in order to evaluate if Autumn 2018 has been above or below the long term average (1981-2010) in Treviso.

In Figure1 the maximum and minimum temperatures average for SON in each year are shown. Like for November 2018 analysis, we can see how the average minimum temperatures tend to increase more in the last 20 years than the maximum values and Autumn 2018 shows the highest values.

Figure1. Maximum and Minimum average temperatures for SON since 1973. The dashed lines represent the 1981-2010 average for minimum (blue) and maximum (red) temperatures in Treviso.

In fact the long-term (1981-2010) Autumn average temperatures in Treviso are (9.63/18.76 °C) and the last Autumn has recorded (13.03/21.06 °C). The magnitude of the anomaly can be seen in Figure2, where the temperatures anomalies for each year (Autumn) are shown (for both maximum and minimum average values).


Figure2. Temperature anomaly for average Maximum (left) and minimum (right) temperatures in each Autumn since 1973. Anomalies higher than 0.5 °C (red), lower than -0.5 °C (blue) and between 0.5-0.5 °C (green).

It is clear how this Autumn has observed the highest average minimum and maximum temperatures (+3.55/ +2.33 °C respectively) since 1973, with October the warmest recorded for the maximum values (anomaly +2.46 °C) and second for the minimum (anomaly +3.78 °C); also September was very mild, the 4th warmest for the maximum values (anomaly +2.78 °C) and 3rd for the minimum (+3.09 °C). 

Considering the historical analysis, though this Autumn has been the warmest since 1973, however it is not the only one observing high temperatures (especially for the minimum values). In fact, from (Figure3) we can see how the years with minimum values above the average (anomaly higher than 0.5 °C) have consequently occurred since 2008, with no Autumn recording negative anomalies in the minimum average temperatures. Instead, in the 70s and 80s, most years observed lower values. Regarding the average maximum temperatures, there is more variability, with some ‘cold’ years in the 00s and ‘hot’ years in the 80s.

Figure3. Occurrence of hot (red) and cold (blue) years (autumn). Maximum temperatures (left), minimum temperatures (right).

It would be interesting to understand why the minimum temperatures observe a higher (positive) anomaly than the maximum values and for more consecutive years; it might be due to an increase in the overcast conditions during the night which might have led to a decrease in the radiative cooling at surface.

References

https://www.meteociel.fr/climatologie/villes.php?code=16098

Italy: warm week before possible cold spell

Today and the next ones are going to be mild, with both minimum and maximum temperatures above the average, after the short cold period occurred during the last week of November. As we can see from the satellite imagery of this afternoon (Figure1), showers have occurred in central-southern Italy, especially along the Appennines, instead in the north clear skies were mostly dominant, except for some fog along the Po valley during the night/early morning. On the west (towards Spain) a high pressure system is moving eastward, bringing very mild (Tropical) air in the Mediterranean region.

Figure1. Satellite imagery of this afternoon (Sat24).

The temperatures are already well above the average (especially in central-northern Italy and in the Alps), with maximum values above 10 °C mostly everywhere and close to 20 °C in Sardinia (Figure2). Temperatures (both minimum and maximum) will increase also tomorrow and this condition should last until the weekend (when colder air is expected from Central-Europe).  The anomalies are between 2/5 °C above the average. Only in Emilia Romagna and extreme S. Italy the maximum temperatures are closer to the normal values for December.

Figure2. Temperatures in Italy this afternoon (Meteociel).

The next days the weather conditions won’t change, with still mild air coming from the mid-Atlantic and N. Africa causing only few showers (especially on the Alps and Appennines) and high temperatures (for this month) with the 0 °C level above 3000 m on the western Alps both tomorrow and Friday, and no frost at low levels is expected. Moreover, in some areas (especially in Sardinia and Sicily) the maximum temperatures might reach 20 °C, especially tomorrow.

From the weekend, colder air coming from central Europe should reach the Alps, causing unsettled weather in the north (snow above 800-1000 m), and leading to a decrease in temperatures towards the average values. 

For next week a colder air mass might reach the Mediterranean region, decreasing the temperatures towards values below the average, but it is still too early to talk about it in a detailed way.

November 2018 in N.E. Italy. Close to the record for high temperatures.

November (in Italy) has just finished with colder air coming from eastern Europe, which has caused a decrease in temperatures especially in the north.

However, these few days of temperatures below the long term averages are not enough to reduce the (significant) positive anomaly recorded in N.E. Italy during the last month. 

Related to the previous historical analysis made for Treviso, in this article I am going to analyse the temperatures observed during November 2018 and compared with the long term average (1981-2010).

To notice that the average (for both minimum and maximum temperatures) is computed on a monthly basis, thus the 10-day average is not considered (the first week of the month has a higher long term average temperature than the last one), which would have led to a more robust analysis to determine the significance of the daily temperature anomaly .

This analysis is focused on comparing November 2018 with the previous ones, considering the overall monthly average temperatures (thus between maximum and minimum values), and on computing the daily temperature anomaly in the past month in both minimum and maximum values, being conscius that its ‘significance’ cannot be quantified in a precise way because of the above mentioned limitation.

Regarding the daily analysis, in Figure1 the daily temperature anomalies (for both maximum and minimum values) are shown, considering a long term average of (4.6/12.7 °C).

Figure1. Max (top) and min (bottom) daily temperature anomaly referred to the long term maximum and minimum averages in Treviso. Positive anomaly (red), negative anomaly (blue).

It is clear how only in few days temperatures below the average occurred and the positive anomaly is observed especially in the minimum values. In fact, though we can’t exactly quantify the significance of this anomaly, the overall analysis shows how the positive difference from the monthly mean is not counterbalanced by negative anomalies. For the minimum temperatures, 13 days have recorded values 5 °C above the average, instead the same negative anomaly was never observed.

Regarding the maximum temperatures, the magnitude of the positive anomaly is lower. This might be due to the overcast conditions observed in the first week of the month, when the southerly winds caused prolonged rainfall events limiting the solar radiation and thus, an increase in the maximum temperatures.

However, November 2018 (in Treviso) has been the second warmest since 1973 (Figure2). 

Figure2. Comparison of the deviation in the average November temperatures from the long term average (1981-2010). Deviation equal or above 0.5 °C (red), between -0.5/+0.5 °C (green), below -0.5 °C (blue).

The plot shows the temperature deviation for each November and it is clear how November 2018 has the second highest anomaly (+2.79 °C). The warmest November occurred in 2014 (+3.84 °C) and the third one in 2002 (+2.74 °C). Interesting, as mentioned in a first analysis , how since the 00s there are no months observing values below the average (deviation bigger than -0.5 °C), with most years observing a significant (positive) anomaly.

November 2018 has shown as average minimum and maximum values : +8.5/ 14.4 °C, thus, respectively, (+3.9/+1.69 °C) above the long term average. This confirms what shown previously with the daily analysis, where the most significant anomalies are observed in the minimum temperatures.

We will see how December will be, though for the next few days no cold spells are expected and temperatures will increase (again) towards values above the average, even if without reaching the positive anomalies observed in November.

References

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.

References:

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.