Conditions during the formation of warm-season derechos in Central Europe during last 25 years

Derechos are large windstorms that cause significant damage to forests, vegetation, and infrastructure not only due to their scale but also their intensity. Although only a few such events typically occur each year in Central Europe, forecasting these windstorms remains a major challenge for meteorologists.

In our study, we examined the convective environment associated with derechos from 1999 to 2024 using the ERA5 reanalysis during the full life cycle of each event. The path of each derecho was refined both spatially and temporally by combining data from the European Severe Weather Database (ESWD) with weather radar observations and other sources (mainly case studies). We also analyzed vertical profiles across modelled conditions one to two hours prior to each derecho occurrence, ensuring they were not contaminated by ongoing convection. In addition, we investigated anomalies of selected variables derived from the ERA5 reanalysis using non-exceedance probability and return periods related to the derecho events.

Each derecho path was divided into segments based on storm evolution: regions of intensification, mature phase, and weakening. For each segment, we assessed environmental precursors of convection such as CAPE, vertical wind shear, or helicity, along with composite parameters, moisture characteristics, and characteristics of lapse rate.

Our findings reveal notable differences in key precursor parameters (such as CAPE, CIN, and wind shear) between the intensifying and dissipating phases of derechos. Despite low CAPE and vertical wind shear during the dissipating phase, some derechos still produced damaging winds, particularly where surface moisture and the low-level lapse rate remained favourable. In some cases, this extended the area of impact by up to 200 km.

We also analyzed vertical profiles of air temperature, humidity, and wind prior to derecho onset using median and mean Skew-T diagrams and hodographs derived from the ERA5 reanalysis. The profiles suggest that relative humidity in the lower and mid-troposphere over Central Europe is fairly uniform, in contrast to U.S. derecho environments, where low-level humidity tends to be higher and mid-level humidity lower. Hodographs show slight curvature in the 0–3 km layer, suggesting a link between derechos, bow echoes, and supercells.

Warm-season derechos in Central Europe typically develop under a positive 500 hPa geopotential height anomaly over the Balkans and a negative one over the British Isles. Between these anomalies, warm, moist air flows into Central and Eastern Europe. Results, including non-exceedance probability and return periods, will also be presented in detail in our poster.