spring – weather-meteo

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.

A very dry April in Scotland

The second month of the Spring is going to end soon but will be certainly remembered as one of the driest and mildest in many areas of the UK. Focusing on Scotland, the eastern side has seen very dry conditions for several weeks, with numerous sunny and mild days.

Indeed, if we consider the maximum and minimum temperatures observed in Edinburgh (Figure 1), many days have experienced mild conditions (Max temp>15°C) instead only two days have seen frost (14th, 19th, whilst the long term average sees 4days). The average temperature up to April 26th is 8.97°C (+1.1°C above the 1981-2010 average of 7.8°C).

Figure 1. Max and min temperatures observed in April 2020. Note last day is 26th. (Data source: Meteociel).

But the most significant aspect of the last few weeks has been the lack of rainfall in many areas, with several days of sunshine. Figure 2 shows the daily hours of sunshine and the daily rainfall amount observed in Edinburgh during the last month.

Figure 1. Daily rainfall (blue) and hours of sunshine (yellow) observed in April 2020. Note last day is 26th. (Data source: Meteociel).

We can see that no significant rainfall has been oberved, with less that 1mm of cumulative rain up to April 26th. And if we consider the World Meteorological Organisation (WMO) definition of rain day (i.e. cumulative daily rainfall must be >=1.0mm), then no rain day has occurred yet. On the other hand, long sunny days have been observed, with a total of 176h of sunshine in 4 weeks. The climatology sees 136h, thus it is clear that this has been a very unusual April.

These dry conditions have been caused by an almost stationary high pressure close to the UK which has caused a NE drifts of most lows and troughs towards Iceland and Scandinavia (causing very dry conditions also across central Europe and the Mediterranean).

The next few days will see a cooler trend, with temperatures close or slightly below the average, but still in a mostly dry condition with only few showers at times. Thus, it is likely this April will be one of the driest ever recorded in the area!

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.

Early Spring over Europe

Today’s satellite image (Figure 1) shows widespread clouds over the continent. This is caused by a Tropical Maritime air advection (TM) along the N flank of a High over Spain. All the moisture and warm air is pushed over the Mediterranean region and central Europe. Meanwhile, a deep low over the Atlantic brings unsettled and windy weather across the British Isles. On the E side of the continent cold air flows S from Siberia, though mitigated as it reaches E Europe/Russia.

The temperatures observed at 12UTC today (Figure 2) are significantly above the climatological average for this time of the year across most of the continent.

Figure 2. Temperature observed in Europe at 12UTC 31/01/2020

Indeed, temperatures between 10/15°C are observed over the UK, France, Germany, Italy and Iberian Peninsula (here with peaks of 20/22°C along the E coast). Also in the Balkans and Greece temperature are very mild (between 15/20°C in S Greece).

Only in Russia and Central/N Scandinavia wintry conditions are observed, with values below freezing. However, these temperatures are still ‘mild’ compared with the climatology (e.g. in Moscow 0°C, but it should be -9/-4°C).

The next few days will see a further increase in temperature over all W Europe, with risk of some record-breaking values in France/Spain/Italy (possible peaks above 20°C) in some areas. This is caused by a northward shift of the Jet Stream over W Europe combined with the strengthen of the High over Iberia. Only from next week, the Jet Stream will push S towards central Europe causing a drop in temperatures (but there is still some uncertainty about its effects on a local scale).