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).

First day of the week: low temperatures in central and northern Europe.

During the weekend and today temperatures have dropped in central Europe and northern Europe, with ‘freezing’ conditions in some areas.

The minimum temperatures observed this morning (Figure1) are well below 0 °C mostly everywhere in central and northern Europe, with values between -5/-10 °C especially in Germany, Alpine region and Czech republic; in some areas (central Germany and in the Alps) values between -10/-15 °C are observed.

Moving north towards Scandinavia, temperatures are well below -20 °C, with peaks of -30/-35 °C especially between Sweden and Finland.

Cold conditions are also occurring in Russia and E Europe, instead moving towards the Mediterranean region and the Atlantic coast temperatures tend to increase, with values above 0 °C mostly everywhere. The warmest conditions are observed in S Greece, Cyprus and S Spain, with minimum temperatures between 10/15 °C in some areas.

Europe minimum temperatures 21/01/2019
Figure1. Minimum temperatures observed today in Europe. (Source: Meteociel)

The afternoon has seen still very cold temperatures in central/northern Europe, with numerous cities observing maximum temperatures below 0 °C (Figure2), instead moving southward and westward temperatures have been milder, with values above 10 °C especially in Portugal, S Spain and E Mediterranean.

Europe maximum temperatures 21/01/2019
Figure2. Maximum temperatures observed today (source: Meteociel).

The low temperatures observed in central Europe are due to the combination of the cold air, moved southward last week, with a high-pressure system (Figure3) settled on central Europe since this past weekend. Thus, the cold air is ‘trapped’ at surface causing temperature inversion (temperatures in the free atmosphere around 1500 m a.s.l are ‘warmer’ than at surface) and fog/low clouds.

Looking on the Atlantic, a low pressure system is moving towards the UK, leading Polar maritime air to move south-eastward (notice the scattered clouds).

Scandinavia and Russia are reached by very cold air coming from Siberia, which is moving southward towards central Russia (see the cold front, band of clouds, from Sweden to central Russia).

Finally, in the Mediterranean region settled conditions occur on the Iberian peninsula (positioned on the warmer side of the anticylone); in Italy a weak low-pressure system is causing overcast conditions and rainfall especially along the peninsula and finally, in the E Mediterranean, showers are occurring mainly in Turkey.

Satellite imagery of Europe 21/01/2019

Figure3. Satellite image of today. I have coloured in blue the Polar maritime air, in purple the Polar continental and in yellow the high pressure. Notice the fog/low clouds in central Europe. (source: Sat24).

The next days will see colder air towards the UK and W Europe, with showers and snow at low levels especially between tomorrow and Thursday, with the high-pressure moving eastward towards Russia.

The cold air will move towards the central Mediterranean region, causing the development of a deep low pressure system between Spain and Italy which will bring rain, strong winds and snow (at low-medium levels) along the Italian peninsula and in N Spain; some snow should fall also in the Po valley and Alpine region.

Later in the week, the low pressure system will move towards Greece and the Balkans causing unsettled weather.

In Central and N Europe instead, the weather will be nice (but still cold), with some snow showers especially mainly between central Sweden and Finland.

In conclusion, it will be a very interesting week, because of the presence of different air masses on the continent and possibility of snow at low levels in numerous areas.

Europe: a beautiful satellite image of today, with very different weather conditions

If we look at the satellite imagery of today (Figure1) we can see different weather conditions across the continent.

Between the UK and Scandinavia we can see the cold air coming from the Arctic, visible for the scattered cumulus clouds on the N Sea caused by instability (ocean surface is warmer than the air mass, thus the warm air lifts up in the atmosphere and condenses forming clouds and precipitation); these are causing scattered showers (mainly snow) in Scotland and E England, as well as a decrease in temperatures in the UK, Norway and N France.

On the west (in N Atlantic) a deep low-pressure system (989 hPa) is moving towards Ireland, though it will weaken before reaching the coast (between tomorrow and Saturday), so no severe event is expected.

On the southern part of this cyclone, less clouds and calm conditions are due to a high-pressure west of Spain, which is leading to a warm air advection from mid-Atlantic towards the Iberian peninsula (maximum values of +14/+17 °C today between Portugal and S Spain).

Instead, from E Spain to the Baltic region, a long and narrow tail of clouds stands for a cold front moving southeastward (causing rainfall and snow between France, N Italy and the Alpine region ); this is simply the front of the arctic air mass which is ‘pushing’ towards central-southern Europe.

Moving eastward, we can see how between S Italy, the Balkans and Turkey the weather conditions are ‘calm’, with clear skies (notice the snow at the ground between Greece, Serbia and Romania). Here, a high-pressure is causing settled (and relatively) mild conditions (maximum temperatures between +12/+15 °C in S Greece and Turkish coast).

On the northern side of the anticyclone, we can see another band of clouds (between Ukraine and Russia) which is related to a warm front moving eastward and causing snow at ground level (due to the negative temperatures at surface).

The warm front in E Europe and the cold front in central Europe are related to a deep low-pressure system (976 hPa) positioned between the Baltic sea and S Finland. This is causing intense snowfall in the area and quite strong winds on the Baltic sea, and it is leading to very cold air from NE Russia towards central Scandinavia, where temperatures close or below -30 °C are observed.

Figure1. Satellite imagery of today. I have added the different type of air masses and weather systems. Tropical maritime air (yellow), high pressure (orange), Polar Maritime air (purple), Arctic air (blue) and Polar continental air (dark blue). (Source: Sat24).

Thus, today has been fascinating on a meteorological point of view, and also the next days will be interesting due to the fast change in the weather conditions over Europe.

UK (but not only): Beast from the east? Attention to the ‘fake’ news.

In the last few days numerous news stories have reported the possibility of a very cold spell in the UK (usually called ‘the Beast from the East’) with snow and very low temperatures which should occur from this weekend.

However, up to now, the ensemble models are not seeing any of these dramatic freezing events (not only in the UK , but also in Europe), but only cold spells which can occur during every winter. In fact, in order to be as cold as last year (when, between February and March, for more than 10 days very low temperatures and snow where observed both in southern and central Europe), the air mass should come straight from Siberia/NE Russia where the coldest temperatures are observed (usually between -30 °C/-50 °C). Thus, the type of air should be Polar Continental (that is coming from the polar region, originating on land), which is the coldest in Europe and can cause snow and low temperatures also in the Mediterranean Region (e.g. in February 2018 between 10-15 cm of snow fell in Rome).

This week instead, even if colder air will move towards the UK and Central Europe, it will mostly come from the Arctic (Figure1). Thus, it will bring snow showers (especially in Scotland and N. England between tomorrow night and Thursday), but no severe event is expected.

Figure1. Synoptic chart for Thursday afternoon. I have added (blue arrow) the direction of the air mass (from the Arctic) and the areas where snow showers are likely in the UK (blue stars). Finally, notice the 528 dam line over Scotland and E. England which shows how snow can be possible at low levels. (Source: Met Office).

It will be a normal cold spell (as usually occurs in January), with snow at low levels and frost (the latter especially in the mainland between Thursday afternoon and Friday morning).

Temperatures will be only few degrees below 0°C during the night on Thursday (-2/-4 °C in some areas), and above 0 °C during the day mostly everywhere (0/+4 °C).

In addition, already from Friday afternoon, the arctic air will be replaced by a ‘milder’ polar maritime air mass from the N Atlantic, which will cause an increase in temperatures (especially in Wales, N Ireland and W England), with values above 0 °C mostly everywhere; snow will be replaced by rain except in Scotland (mainly Highlands) and N England (Pennines).

Finally, for the next week, air from N Atlantic is still expected, with cold (and wet) conditions at times, but without causing freezing temperatures and deep snow as shouted recently. It will be simply winter.

First two weeks of the year: Was it actually colder in S. Europe than in the north?

The first two weeks of January have seen very cold air coming from Russia towards Central-Eastern Europe, with lot of snow especially in S. Germany, Austria (more than 2 metres in the Alpine region) and the Balkans. Also in Greece and S. italy snow has fallen, at sea levels at times.

In northwestern Europe instead, and especially between France, the UK and the N. Atlantic, a strong high-pressure ridge has caused settled conditions for several consecutive days, causing also very warm air towards these areas and Scandinavia.

I have analysed and plotted the daily average temperatures in 17 countries (cities) and compared the results. This analysis is simply focused on considering the main cities, thus it is not a detailed analysis for each country.

The results (Figure1) show that in S. Europe (especially between Italy and the Balkans) the daily average temperatures have been lower than in central-northern Europe.


Figure1. Daily average temperatures observed during the first two weeks of January in Europe. Notice the cities are grouped according to the geographical region.

We can see how Rome and Istanbul observed similar or lower temperatures than London, Paris and Amsterdam (to cite a few). In addition, it is impressive to see that Reykjavik has observed a very similar mean temperature to the cities in southern Europe.

Also the Balkans have observed temperatures lower than in northern Europe, with Bucharest colder than Stockholm and observing temperatures very similar to Helsinki.

However, not all southern Europe has observed low temperatures; the Iberian pensinsula has seen warmer conditions (though with temperatures close to their climatological average), with Lisbon the warmest capital so far. This is due to the frequent flow of warm air coming from the Tropics caused by the high-pressure ridge set (since December) between W Europe and the Atlantic.

In E. Europe (Warsaw, Moscow) the lowest temperatures have been observed (which is also in accordance to their climate); these areas where directly influenced by the polar continental air coming from E/NE Russia throughout the whole period.

If we look at the daily temperature anomaly (Figure2) computed respect to the climatological average (1981-2010), we can see how Reykjavik has observed the highest values (almost +5 °C; the daily mean average temperature in January for the Icelandic city is 0 °C).

Overall, all the area from France to Scandinavia and Iceland has seen warmer temperatures, instead E Europe and S/SE Europe have experienced a colder weather.

Figure2. Daily temperature anomaly (with respect to the 1981-2010 average) in Europe during the first two weeks of this year. Negative anomalies (in blue) and positive (in red). The higher is the anomaly (in terms of absolute values) the bigger is the ‘circle’.

In conclusion, we can say that the new year has brought cold weather in the southern and eastern part of the continent (with snow in numerous and unusual areas); instead, the western and nothern areas (especially the countries along the N Atlantic coast) are still experiencing mild conditions (as occurred also in December).

N. Europe: how’s the winter been so far?

This week, the lowest temperatures in Europe have been observed between Russia and the Balkans, with minimum temperatures close to -20 °C in some areas. Instead, in N. Europe, temperatures are not too low for being January, with maximum values above 0°C along the Atlantic coast of Norway up to the Arctic Circle (today maximum values between + 5-7 °C in the Lofoten Islands).

I have analysed and plotted the temperatures osberved in some cities in both Denmark, Sweden, Norway and Finland, to see if December and the first week of January have been colder compared to the climatological (long-term) average 1981-2010.

In Figure1 the main cities are shown. They are both in the mainland and along the coast.

Figure1. Cities chosen in the analysis

We can see how the lowest average temperatures (as also according to the climatological average) are observed in the mainland and far from the Atlantic Ocean, between N. Norway and Finland, where the warming effect of the Gulf stream is weaker.

The coldest city is Rovaniemi, with almost -20 °C observed in the minimum average values during the last week. The warmest, instead, is Copenhagen, with both minimum and maximum average temperatures observed during the last December above 0 °C. Also Trondheim (along the coast of Norway) is observing temperatures above 0 °C, especially during the last week.

Figure2. Maximum and minimum average temperatures observed in December 2018 (left) and first week of January 2019 (right).

The comparison with the climatological average (Figure3) shows a very clear result. Positive anomalies are osberved mostly everywhere, especially in the minimum average temperatures (both in December and January), with values up to 7 °C (e.g. Trondheim) above the long-term average.

Figure3. Maximum and minimum temperature anomaly calculated with respect to the climatological average 1981-2010. December (left), January (right).

It seems that the most significant anomalies occurr along the atlantic coast of Norway (Oslo, Trondheim, Tromso), instead moving eastward the anomalies are little (1 °C: Helsinki) or negative (Rovaniemi), though Oulu (Finland) observes significant positive anomalies (+3/+5 °C).

These ‘warm’ conditions are due to the very warm air moved northward during the past few weeks (from mid-Atlantic towards the UK and N. Atlantic); however, the areas close to Russia have been more influenced by the polar air moved towards Eastern-Central Europe, observing values closer or below the average.

The weather should be colder the next few days, especially along the Atlantic coast, due to an Arctic maritime air mass moving towards the N. Sea.

Now we need only to wait and see if the second part of January will show colder conditions (for these areas).

References:

I have collect data from: https://www.meteociel.fr/observations-meteo/temperatures.php?region=nor

UK: when is the weather going to be less ‘boring’?

Today has been mild all over the UK. Both maximum and minimum temperatures (Figure1) have been well above the average, with no values below 0 °C (except on the Highlands this afternoon).

Figure1. Maximum (left) and minimum (right) temperatures observed today. (source: Meteociel).

However, though the temperatures are pleasant, the weather is seeing overcast conditions mostly everywhere. In Figure2 we can see the satellite imagery of this afternoon. It is quite impressive: a deep low-pressure system (987 hPa) just north of Scotland and moving towards the Northern Sea is causing very strong winds (and frequent showers) in N. Scotland and N. Atlantic (gusts of 100 km/h), as well as a decrease in temperatures in this area (Figure1). Instead, the rest of the country, as well as Ireland, is seeing overcast conditions (with drizzle or light rain in some areas) with weak winds.

This is due to a high-pressure system positioned just south-west of the UK which is leading warm and humid air from mid-Atlantic (mostly Tropical maritime) towards the UK.

Figure2. Satellite imagery of today (12UTC). (source: Sat24).

The cold front, related to the low-pressure system (and which has caused showers today in Scotland) is moving southward this evening, causing a decrease in temperatures (especially from tomorrow evening when the skies will be clear mostly everywhere). Showers will be very rare, with dry conditions mostly everywhere.

However, temperatures won’t drop significantly, with values close or slightly below 0°C between Tuesday evening and Wednesday morning (especially in Scotland, N. England and Wales).

In addition, no rainfall is expected this week (until Friday) because of the semi-stationarity of the high-pressure system between Spain and the UK. Indeed, it will move northward from tomorrow, and the maximum pressure values would reach Ireland by Thursday, causing settled weather mostly everywhere.

In conclusion, this week will be still quite ‘boring’ with no interesting weather phenomenon on the country and also looking at the ensemble forecasts no significant change in temperatures or precipitation is expected for the next week.