Heatwaves across Western Europe

By Ivan Smiljanic (Exostaff), Natasa Strelec Mahovic, Federico Fierli and Julien Chimot (EUMETSAT), Miguel-Angel Martinez (AEMET)

Heatwaves

Following several heatwaves in parts of southern, western and central Europe in June and the first half of July, a powerful heatwave built over western Europe during mid-July, under the influence of a wide and persistent synoptic ridge, centred roughly over north-west Africa, the western Mediterranean and towards France and Germany.

The high pressure ridge was responsible for mostly cloud-free conditions, enabling the sun to additionally heat up already warm air masses. No low pressure systems managed to break the ridge and exchange air masses on a synoptic scale. Also a relatively strong and persistent cut-off low system resided for many days off the coast of Portugal, pulling even hotter air from north-west Africa towards the north. In Portugal and Spain, temperatures in the low to high 40s were recorded from 7 July, with the maximum temperature of 47°C recorded in Pinhão, Santa Bárbara, Portugal on 14 July and 45.7°C recorded in Almonte, Huelva, Spain on 13 July.

The Meteosat-11 Airmass RGB animation from the beginning of July, reveals the very hot air mass over the wider Iberian peninsula, depicted by the pulsating (with the diurnal heating) red to dark red shades (Figure 1).

The combined Natural Colour RGB (day) and Dust RGB (night) imagery (Figure 2) shows that, except for a few short-lived convective episodes, there was not much cloud cover across western Europe. Light blue to pink shades in the Dust RGB reveal the low content of low-level moisture, thus missing one of key ingredients for convection.

The journey of the very warm air pushed across western and central Europe by the persistent cut-off low is also seen by Figure 3.

The ECMWF temperature forecast fields in Figure 4, show that in the afternoon hours of 18 July the temperature was expected to exceed 40°C in parts of Spain and France, with temperature higher than 35°C expected also in UK and northern Italy.

On 18 July Spain recorded the maximum temperature of 41.4°C (Pamplona Aeropuerto), France recorded 41.5°C (La Roche Sur Yon, Pays de la Loire) and the UK recorded 38.1°C (Santon Downham, Suffolk).

ECMWF's Extreme forecast index (EFI) illustrates how extreme this situation is. The animation (Figure 5) contains EFI from Monday 18 to Friday 22 July. Values between 0.5 and 0.8 can be generally regarded as signifying that unusual weather is likely, whereas values above 0.8 signify that very unusual or extreme weather is likely to occur. In this case, for 19 July over UK values higher than 2 occurred, both the Met Office and Météo France issued red warnings for exceptional heat for 18 and 19 July (see Met Office warning in Figure 6).

Maximum temperatures measured at Synop stations* on 18 July exceeded 40°C in western and southern France and central Spain, reaching around 42°C at several stations. In the UK the hottest day was 19 July when several stations in central and eastern England measured record-breaking maximum temperature of 40 deg (provisional maximum of 40.3 deg measured at Coningsby, Lincolnshire) with at least 34 stations beating the previous record of 38.7 from 2019, according to Met Office. *Note: these are the maximum temperatures, rounded to the whole number, measured by maximum thermometer from 18:00 UTC on 17 July to 18:00 UTC on 18 July.

Droughts and fires

With stable and hot weather prevailing over much of the continent the drought, already very severe in some areas, has continued. The drought code, a product from the Copernicus Emergency Service EFFIS, shows very severe drought conditions in all Spain and the majority of Italy, and, also, parts of UK, Germany, Hungary and the Croatian coastline (Figure 7), which result in very high and extreme fire risks (Figure 8).

Multiple forest fires occurred in Spain and Portugal, and the smoke from these fires was visible across the Iberian peninsula, seen in light blue/cyan shades, best in the morning or evening in the Meteosat-11 Natural Colour RGB animation (Figure 9 and also Figure 2).

Sentinel-3 observations of Fire Radiative Power (Collection 2.0 - from OFRaP-CS3 processor) shows the development of intense fires in northern Portugal and western France (visible as darker spots) from 12-16 July in (Figure 10).

On the Sentinel-3 imagery (Figure 11) the widespread smoke is visible on 15 July in central and northern Portugal and western France.

The hottest soil evolution in the last weeks can be seen in the Meteosat-11 RGB based on 10.8µm plus the RGB composite for weekly brightness temperature maxima (Figure 12). Colour intensity shows how high temperatures were at 12:00 UTC for every pixel. For instance, south-west areas were hotter than in eastern Europe. Colour hues indicate the date of the heat peak in the period. For instance, the most intense heat in Portugal was around 7-11 July. Some colour artifacts are due to persistent cloud, as in Hungary, this is a limitation of this product.

For assessment of fire extent and the burnt areas during the related period, this slide deck uses NASA Worldview to look at high resolution imagery from instruments onboard polar-orbiting satellites, featuring both the MODIS Band 7-2-1- RGB product and the VIIRS DNB channel (see example in Figure 13).

Additional content

A milestone in UK climate history (Met Office)
Heat wave kills more than 1,700 people in Spain and Portugal (AXIOS)
Record high temperatures registered around France as fires rage in southwest (France24)
Heatwave: More evacuations as Mediterranean wildfires spread (BBC News)