At the beginning of August, unusually high temperatures, in some cases unprecedented, were recorded in a large area spanning much of the Iberian Peninsula, southern France, Italy, the Balkans and Hungary. For some days top temperatures rose above 40°C across southern Europe, exacerbating the impact of an extended drought and the lingering impact of a June heat wave.
Summer heat waves in Europe develop when there is a blocking of westerly winds and high-pressure weather conditions prevail. In early August 2017, the stability of the atmosphere caused high-pressure conditions to last for several days, resulting in extreme warming of the air and leading to unusually high temperatures and no wind.
ECMWF forecasts pointed to the possibility of unusually warm conditions well in advance. The monthly ensemble-based forecast showed a warm anomaly for 31 July to 6 August in many of the affected areas as early as four weeks ahead. The signal persisted at shorter lead times and the forecast starting on 27 July is in very good agreement with the analysis. The World Meteorological Organization (WMO) criterion for a heat wave is that "the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C, the normal period being 1961–1990". Using this definition, the operational analysis shows an extensive heat wave over parts of southern and southeast Europe between 1 and 10 August. In places, temperatures rose well above 5°C when compared to the ERA-Interim mean climate (1979–2016). The extent of the heat-wave conditions was very well captured by ECMWF’s high-resolution 10-day forecast (HRES) even though there was a slight overestimation of their duration in some areas. The heat-wave duration is shown for the forecast starting on 1 August as an example. However, the signal was present for a few days before. This is not surprising as temperature is one of the most predictable meteorological surface variables. For example, the high-resolution forecast for maximum two-metre temperature during the first ten days in August was in almost perfect agreement with observations from the high-resolution network of European in-situ data collected through EFAS (not shown).
Extreme heat can lead to a variety of health risks, including dehydration, hyperthermia, and even death, especially during sustained periods of high temperatures. The thermal health hazard experienced during the August 2017 heat wave can be assessed using the Universal Thermal Climate Index (UTCI). UTCI is an index representing the human body’s discomfort to thermal stress. It is elaborated as an equivalent temperature via an advanced model of human thermo-regulation that, coupled with a clothing insulation model, estimates the effect of wind speed, water vapour pressure and short- and long-wave radiant fluxes on human physiology. Maps of predicted UTCI show high values in many of the areas affected by the heat wave (not shown). Such maps are computed at the European scale using ECMWF ensemble forecasts as part of the activities of the EU-funded ANYWHERE project.
In summary, forecasts of two-metre temperature can provide valuable information ahead of and during heat-wave episodes. Impact-oriented products, such as heat-wave duration, maximum temperatures reached over a given period of time and UTCI maps, can provide supplementary information to meet the requirements of different users.