A very mild Valentine’s day

This week a strong high pressure system has set over central western Europe, causing sunny days and mild temperatures. Today, maximum values are far above the average mostly everywhere in Europe, with temperatures reaching 20 °C in the Iberian peninsula (Figure1).

maximum-temperatures-Europe-valentine-day
Figure1. Maximum temperatures observed in Europe today (source:Meteociel).

Temperatures between 10/15 °C are observed in all the Mediterranean region, France, the UK and Germany. Moving eastward values are lower but still above the average of February. Only in Russia and north Scandinavia temperatures are below 0 °C.

The weather today (Figure2) has seen bright skies over all W Europe and central Mediterranean region, with only some local fog at the lowest levels (mainly in the morning). At the edges of the high-pressure ridge, clouds and band of rain are observed in the north Atlantic and E Mediterranean, instead over N Russia and Finland snowfall is occurring.

satellite-imagery-valentines-day-europe
Figure2. Satellite imagery of today. I have added the main weather features (rain, snow, high-pressure). (source: Sat24).

This weather condition won’t change during the next weekend, with still dry and mild conditions in central/southern Europe. Colder air will reach tomorrow the E Mediterranean, though without ‘freezing’ temperatures. From Sunday, colder and moister air from the N Atlantic will try to push the anticyclone eastward, causing unsettled and ‘colder’ weather over the UK, France and Spain. We will see next week if there is going to be a change in the weather patterns, though it seems likely that the high pressure will be still steady for the next few days.

January 2019 in Europe: cold and ‘mild’, but without extremes.

In this article I am going to analyse the temperatures observed in some cities in Europe during January 2019. A former analysis made two weeks ago had shown that the first two weeks of the year have been cold (and snowy) in Southern and Eastern Europe, with observed temperatures lower than in NW Europe.

Now that January has passed, we can see if the trend observed at the beginning of the month has occurred also throughout the last two weeks or if it has changed.

In Figure1 I have plotted the average daily temperatures observed in 17 cities in Europe.

average-temperature-January-2019-Europe
Figure1. Average daily temperatures January 2019 in some European cities. I have grouped them according to the region.

We can see that E Europe and N Europe have observed the lowest average temperatures (Moscow is the coldest: -7 °C). Moving westward and southward temperatures rise, with positive values mostly everywhere (except in the Balkans: Bucharest -1 °C). The warmest cities have been Lisbon, Athens and Instanbul.

Regarding the anomalies of the average temperatures compared with the long-term average (1981-2010), in Figure2 I have plotted the results, in order to determine where it has been colder than the average and where it hasn’t.

daily-average-temperature-anomaly-Europe-January-2019
Figure2. Daily temperature anomaly observed in January 2019 in some European cities. In blue negative anomaly and red positive; and the bigger is the circle, the more significant is the anomaly (in terms of absolute values).

We can see how the most significant anomalies (in absolute values) have been observed in Scandinavia (colder temperatures) and E Mediterranean (warmer temperatures). Warmer than the average also in Germany, Iceland and Ireland (though the anomalies are mostly between +0.2/+1 °C). In southern Europe little negative anomalies are observed.

This ‘picture’ is different than what seen two weeks ago, when significant negative anomalies were observed in SE Europe and ‘warm’ conditions were observed in the north (especially in Iceland, UK and Germany).

This change in the weather conditions was due to a different type of air masses moved towards the continent. After the new year’s eve, cold air from Russia moved towards the Balkans and the Mediterranean region on the southern part of a strong high-pressure system positioned between central Europe and the N Atlantic. Instead, after the second week of the month, the cold air was flowing from Greenland and the Arctic (thus from the N Atlantic) causing a decrease in temperatures in Iceland, the UK and central Europe, and causing warmer air to flow from N Africa towards the Balkans and E Europe.

In conclusion, January has been ‘mild’ in some places (i.e. in central/western Europe, Iceland and SE Europe) and cold (compared with the average) in Scandinavia and Mediterranean region, though in both cases, the anomalies have’t been too significant.

SE England: Was it actually cold in December and January?

The last two weeks have seen frequent snow showers in many areas in the UK, with several centimetres of snow at the higher levels. These episodes have occurred after a long period of frequent rain and mild temperatures throughout the first month of the winter season.

I have analysed and plotted the temperature anomalies observed in December 2018 and January 2019 in London Heathrow. The anomalies are computed considering the long-term averages (1981-2010), which, for London are: December (+8.3/+2.7 °C) and January (+8.07/+2.3 °C).

This analysis is simply focused on having a general overview of the observed temperatures, and on a local area; thus, it can’t be used to determine a wider analysis for the whole country. I haven’t plotted the rainfall, but data can be found in the references.

In Figure1 we can see the daily maximum and minimum temperatures anomalies observed during the last two months.

London-december-january-daily-temperature-anomaly
Figure1. Daily temperature anomaly observed in London in December 2018 (top) and January 2019 (bottom). On the left the anomaly for daily maximum temperatures, on the right the anomaly for daily minimum temperatures. Positive anomaly (red), negative anomaly (blue).

Most of December has been mild, especially in the minimum temperatures, with positive anomalies close to +10°C during the first week (when frequent rainfall has occurred). During the second week, colder and drier air has caused a decrease in temperatures (especially in the maximum values) though only for few days. Indeed, the last two weeks of December have shown mostly mild temperatures (and frequent rain) with only two days with negative anomalies (one of them it’s Christmas). No snow has been observed throughout the month.

January, instead, has shown a decrease in the positive anomaly (especially in the maximum temperatures), with colder and drier air after new year’s eve. The second week has shown settled weather, and temperatures above the average (+3/+5 °C the anomaly in the minimum values). However, from the 15th colder air (both polar maritime and arctic depending on the episode) has reached the UK, causing a strong decrease in temperatures and the first snow of the season in many areas (also in London) between the 21-23 of January and again, after few days with warmer temperatures, on the 31st and 1st of February. The last few days of January have been the coldest of the season so far, with the lowest temperatures observed before the snowfall in the morning of the 31st.

Considering the monthly analysis, thus comparing the anomaly observed in the minimum and maximum average temperatures in December 2018 and January 2019 with the historical period 1981-2019 (Figure2), we can see that December has observed a significant positive anomaly especially in the minimum values (+3.1 °C), instead January has seen values very close to the average, with a slight negative anomaly in the maximum temperatures (-0.62 °C) and positive in the minimum (+0.28 °C).

december-january-max-min-temperature-anomaly-1981-2019
Figure2. Anomaly for mean maximum (left) and mean minimum (right) temperatures for December 1981-2018 (top) and January 1981-2019 (bottom). Positive anomalies (red), negative anomalies (blue).

Overall, December 2018 is one of the hottest since 1981 (3rd for the maximum temperatures and 2nd for the minimum); instead, January has been roughly on average (27th for the maximum and 21st for the minimum temperatures).

In conclusion, even if numerous news had claimed that ‘freezing’ conditions have occurred this winter (especially in January), the actual analysis has shown that it has been a ‘warm’ winter so far, with few cold episodes mainly occurred during the last two weeks.

We will see how February will end in order to sum up the overall winter analysis.

References

https://www.meteociel.fr/cartes_obs/climato2v4.php?code=3772&mois=12&annee=2018&print=1

https://www.metoffice.gov.uk/pub/data/weather/uk/climate/stationdata/heathrowdata.txt

Europe: first week of ‘spring’

During the last two weeks wintry conditions have occurred in numerous countries, both in W Europe and E Europe. Very cold temperatures were observed in Scandinavia and Russia and snow has fallen in many areas between central Europe, the UK and the Mediterranean region.

However, these weather conditions are changing, with milder air coming from mid-Atlantic and N Africa. Indeed, already today higher temperatures are observed (Figure1) with values above 10 °C in many areas in S Europe (especially S Spain, Italy and E Mediterranean region) and above 0 °C mostly everywhere, except in the Alpine region, Scandinavia and N Russia.

Europe max temperatures 4/02/2019
Figure1. Maximum temperatures observed today in Europe. (Source: Meteociel)

The satellite imagery (Figure2) shows the main features of the weather conditions over the continent.

A high-pressure is setting on W and Central Europe, with bright conditions in Spain, S France and central/N Italy and fog/low clouds between Germany and Poland. Between France, the UK and the N Sea a (weak) low pressure system is causing showers across England and Benelux.

Moving eastward, warm air from N Africa is causing settled and mild conditions in the E Mediterranean and convection is occurring between S Italy and Greece. This is due to a (weak) low-pressure system positioned close to Sicily which is leading to a temperature contrast between the (relatively) cold air from central Europe and the mild air from N Africa. Thus, convective cells are forming on the sea, causing numerous thunderstorms in the area.

Finally, in E Europe and Scandinavia cold temperatures are still observed, with snowfall between Finland and Russia.

satellite imagery Europe 4/02/2019
Figure2. Satellite imagery of this afternoon. I have highlighted the Tropical air (yellow) and the convection (red) in the Mediterranean region; the snow
(blue) at the ground in central Europe and Alpine region, and the low-pressure system (black) in the N Sea. (Source Sat24).

The weather conditions for this week will see an increase in the strenght of the high-pressure in central/southern Europe with settled conditions and relatively mild temperatures (though fog/low clouds are possible in the valleys limiting the increase in temperatures). In fact, from Tuesday possible peaks of 20 °C are expected in S Spain, Portugal, S Turkey and Cyprus. Temperatures between 10/15 °C are expected in S France, Italy and the Balkans and close to 10 °C in the UK and Central Europe. In addition, during the night temperatures will be close or slightly below 0 °C especially in the valleys (and in case of clear sky),causing a strong temperature gradient between day and night.

Only in E Europe and Scandinavia it will be still cold, though with values above the average.

On Thursday, colder air from N Atlantic should cause a slight decrease in temperatures (with frequent showers) in Central Europe, though with no snow or ‘freezing’ conditions. However, only from next week (so in 7 days) a more significant change in the weather conditions should be observed, though it is still too early to talk about its possible effects.

UK: sunny day after the snow

Between Thursday and Friday snow has fallen in many areas, especially in England, with more than 10 cm of snow at the ground in some areas.

This was due to the cold air coming from the north Atlantic and later from the Arctic which has caused a decrease in temperatures during the previous days. Indeed, on Thursday morning (Figure1) low temperatures (the lowest of the winter so far) were observed in the country, with values between -5/-8 °C in many areas both in England and Scotland; the lowest temperatures were observed in the Highlands (-10/-14 °C). Only in Cornwall temperatures were above 0 °C, due to the influence of milder air from mid-Atlantic.

Figure1. Minimum temperatures observed in the UK on the last day of January 2019. (source: Meteociel)

The negative temperatures in the morning have allowed snow to set on the ground, especially in the mainland. Snow has fallen since mid afternoon on Thursday until yesterday evening, though with an increase in temperatures yesterday afternoon which has caused mainly rainfall/sleet at low-levels.

Today, due to the brighter conditions, we can appreciate the satellite imagery (Figure2) showing the snow still at the ground in many areas, from Scotland to S England.

Figure2. Satellite imagery (UK) of today. I have used blue arrows to point out the snow on the ground in Scotland, N Ireland, Wales and England. (Source: Sat24).

The satellite image shows also the cold arctic air on the N Sea (causing showers) and moving southward towards Central Europe. On the west instead, south of Ireland, milder air from mid-Atlantic is moving towards the UK. This will cause an increase in temperatures, especially from Monday, with positive values (both minimum and maximum) mostly everywhere, and causing frequent showers between Monday morning and afternoon, and again between Tuesday afternoon and Wednesday.

Thus, if the previous week has shown wintry conditions, with snow in many areas, the next one will see milder temperatures and frequent (rain) showers, due to the change in the type of the air mass (from polar/arctic to tropical maritime).

Central Mediterranean sea: a deep low-pressure system is causing snow, rain and strong winds.

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).

synoptic chart 24/01/2019
Figure1. Synoptic chart of today. Notice the low-pressure system in the central Mediterranean sea. (source: Met Office).

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).

Satellite imagery Europe 24/01/2019
Figure2. Satellite imagery of Mediterranean region today. I have added the main weather features (polar air, snow at ground and fog/low clouds). Source (Sat24).

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.

UK: snow updates

Today the first snow (for many) is falling and in some areas few centimetres are setting on the ground.

Last night a cold front has passed over the UK, bringing showers especially in England. Following the cold front, colder air (polar maritime) has reached the country this morning, causing a decrease in temperatures and brighter conditions. However, due to the unstable nature of the air mass, frequent showers have occurred on W England and Wales from late morning, moving eastward during the afternoon (Figure1).

Some showers, though less intense and organized, have occurred also in N Ireland and W Scotland.

Satellite imagery and synoptic chart UK 22/01/2019
Figure1. Satellite imagery of this mid-afternoon (left) and synoptic chart valid for this afternoon (right). I have added the polar maritime air (blue arrow) and notice the showers in central England due to the trough. Ahead of it, and behind the cold front, bright skies. Finally, along the cold front notice the long narrow band of clouds, which has caused rainfall last night in England. (source: Sat24, Met Office).

The decrease in temperature associated with precipitation has caused snow at low levels or ground levels in many areas across central England and Wales, especially between Bristol and London. Some snow is falling also in N Ireland since mid-afternoon.
Instead, along the coast of England and Wales rainfall is occurring due to milder temperatures (+4/+6 °C).

However, not everywhere snow is setting on the ground. This is due to the slightly positive temperatures (between +1/+3 °C in central England and London area) and to the freezing level set between 400-800 m a.s.l. These conditions represent a ‘limit’ to observe snow at the ground. Thus, some areas have seen only snowflakes, especially if the precipitation is not intense, others are instead experiencing few centimetres of snow (e.g. between Oxford and London).

In addition, the snow we are experiencing is ‘wet’, that is, the temperature in the lower levels of the air column is slightly positive, thus snowflakes tend to melt slightly when falling (in this case around 400-800 m a.s.l where positive temperatures are observed) increasing their liquid content and sticking together forming bigger snowflakes (as seen today in some areas).

Tonight showers are expected on the east of England and English channel, with some snow possible at low levels. Some showers might occur also between N Ireland, N Wales and W Scotland, though with little precipitation. Brighter conditions elsewhere.

The brighter skies will lead to a decrease in temperatures during the night, with frost in many areas (especially where snow is on the ground). Tomorrow morning temperatures will be below 0 °C mostly everywhere in the UK with values between 0/-2 °C (possible -3/-6 °C in the highlands and central/N England); milder along the coast and on W England, with values between (+2/+6 °C).