Blue skies over W/NW Europe chilly and rainy in the Mediterranean

In the last few days a strong High has set over the UK, moving E over the North Sea today. This High is causing dry and very warm temperatures across the British Isles, France, Spain and S Scandinavia.

On the other side of the continent, a Low over Ukraine is bringing unsettled and relatively cold weather across E Europe whilst another Low over the Gulf of Genoa is causing unsettled weather across the Central Mediterranean, with thunderstorms and strong winds observed in the last 24h over Northern Italy.

In Figure1 the current weather situation is shown.

Figure 1. Satellite image of this morning. Warm air and clear skies across W/NW Europe (yellow arrow) and colder air with showers/thunderstorms across E Europe/Central Mediterranean (blue arrow). (Source image Sat24)

Because of this synoptic situation, temperatures are above the climatological average across most of W/NW Europe (values already above 20°C this morning), whilst are below the average (temperatures below 20°C) over the E/SE areas (Figure 2).

Figure 2. Temperature observed today at 10UTC across Europe. (Souce Meteociel)

This situation is going to persist until mid-next week when the High will start to weaken and colder air from the Arctic seems to start pushing south towars the UK/Scandinavia.

Weather Europe: Cold in the north, hot and dusty in the south

Today’s weather conditions vary significantly across the continent. Indeed, looking at the minimum temperatures (Figure 1) we can see values below zero across Scandinavia, Belarus, W Ukraine and Tatra mountains, whilst the UK, N France, Germany and most of E Europe has seen temperatures between 0/+5°C. These values are well below the climatology (up to 10°C in some areas) and are due to the strong cold air flow which pushed south from the Arctic the previous weekend.

The cold air is moving E across Russia (see strong temperature gradient just E of Moscow) with snow occurring at low levels in some areas. On the southern part of the continent instead, very warm (sometimes hot) conditions are observed (see temperatures already above 20°C in Sardinia, Sicily and Tunisia). This is due to a significant hot air advection from the Sahara which will reach the peak between today and tomorrow across the central Mediterranean, before moving E over the Balkans during the weekend.

Figure1. Minimum temperatures observed in Europe 13/05/20 (Source: Meteociel)

This will be the first heatwave of the season for most of the central Mediterranean with risk of record-breaking temperatures in Sicily (expected 35/38°C). Meanwhile, between the cold and hot air masses, heavy rain and thunderstorms are occurring across the Alpine region, N Italy, S France and Spain (Figure2) due to a series of lows developing between the Balearics/Sardinia and S France.

Figure2. Europe sat image 13/05/20. Notice Saharan dust (yellow arrow) moving NE. The two lows (L), one south of France and one between Estonia and S Finland. Cold air advection (blue arrows) over central Europe. (Source of image Sat24).

Along the occluded front of this low, the strong S’ly flow is causing a dust advection across the Italian peninsula (Figure3), which will then move E over the Balkans into tomorrow.

Figure3. Dust load across Europe for today. (Source SKIRON).

In summary, it has been a interesting week, with very different weather patterns across the continent and possible record-breakings temperatures.

From Summer to Winter across the UK

In 24h we have experienced a drastic change in the weather conditions. Until Saturday very warm air from continental Europe was causing settled and relatively hot conditions (for the time of the year). Indeed, temperatures have reached 24/25°C in many areas of Southern England, with 20°C observed also in Wales and S Scotland.

Overnight into Sunday the synoptic situation saw a broad high just W of Ireland, and very cold Arctic air flowing south along its E flank towards Scotland.

I have plotted the temperatures observed during the day in some cities across the country (Figure1: Edinburgh, Newcastle, Manchester, London and Dover) to show the drop during the day.

On Sunday morning temperatures were still relatively mild in central/S England and Wales (13/18°C), whilst N England and Scotland were seeing values well below 10°C as the cold front approached.

Later in the morning, central England was experiencing a drop in temperature with Manchester observing values below 10°C around midday. Finally, just after midday London was reached by the cold air, with a drop of 8°C in three hours. Dover was reached only one hour later.

Figure1. Hourly temperatures observed in different cities across the UK on 10/05/2020. The different colours are used to differentiate the cities.

The evening has been very cold, with values just above zero in Scotland and N England and between 5/10°C elsewhere.

It is nice to see the cold advection across the country on Sunday morning on the Satellite image (Figure 2) with the Arctic Air over Scotland and N England (scattered clouds and wintry showers over hills) pushing south. Indeed, the cold front is well visible across central England with rain in some areas. Finally, further south settled and warm conditions were still in place.

Figure2. Sat image of the UK on 10/05 morning. Arctic air (blue arrows) and cold front (black arrows) are well visible.

In summary, it has been a very interesting weekend with a drastic change in the weather conditions. The main absent is still the rain, a trend that seems persisting since April in many areas.

Interesting Bora effect across the Po Valley

On April 14th an interesting weather event occured across the Po Valley. Until this day a strong ridge extending across central Europe was causing warm and sunny conditions over the region. Then, a drastic change in the synoptic situation has seen a cold air mass moving S from Scandinavia and reaching the Alpine region/Balkans on this day.

The contrast between the pre-existent warm and dry air over N Italy and the cold air on the other side of the Alps triggered the development of showers and thunderstorms between Austria, Slovenia and the extreme NE Italy during late morning. Around midday the cold air (as Bora wind) reached Istria/Trieste flowing then across the N Adriatic Sea. The combination of moisture with the warm/cold air contrast enhanced the showers/thunderstorms across NE Italy, which are well visibile from the satellite (Figure1, see line of convection across Veneto).

Figure1. Bora entering NE Italy at 13UTC (source Sat24).

As the cold air flows over the Po Valley, it starts losing its moisture content becoming dry. Thus, by early afternoon most of the showers/thunderstorms ease and don’t move further W (Figure 2). Instead, the cold air still continue flowing at the lower level (below 850hPa, 1500m asl) reaching Lombardy by mid-afternoon whilst carrying all the dust picked up across the valley. Indeed, the previous weeks have not seen rain over N Italy, thus the strong winds have picked up all the dust accumulated previously and caused a sort of dust-advection across the W plans (Figure 3).

Figure2. Convection eases as cold air dries across the Po Valley (15UTC, source Sat24).
Figure3. Cold air flow (between red arrows) is completely dry and pushes all the dust across the dry Po Valley towards Piedmont and Lombardy (16UTC, source Sat24).

It is interesting noticing how the temperature observed in the main cities (Venice, Milan) has changed during the afternoon (see attached plots).

As the cold air reaches Venice during late morning, the wind speed increases significantly up to 30kt (60km/h), before dropping once the cold front passed. Temperature drops from 18/19°C to 11°C in one hour.

Further west, Milan sees very warm conditions until mid-afternoon. As the cold (though dry) air reaches the city, the wind suddenly veers becoming E-ESE with peak of 20-22kt (40/45km/h). The temperature drops by 10-12°C in one hour.

In summary, this has been a very interesting event (as this “cold dust-advection type” is very unusual over N Italy) especially after two weeks of settled weather over the region.

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.

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.