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 and the high-resolution Central-European ALADIN 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.

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. Apart from that, we also studied the convective environment across whole Central Europe in relation to derechos, using both the ERA5 and the ALADIN reanalysis datasets.

Our findings revealed notable differences in key precursor parameters (such as CAPE, CIN, and wind shear) between the intensifying and dissipating phases of derechos. Interestingly, even with low CAPE and vertical wind shear values ​​during the dissipating phases, some derechos were still capable of producing damaging winds, especially where surface moisture and temperature gradients remained favourable. In some cases, this fact prolonged the area of the derecho's impact by up to 200 kilometers. We also analyzed vertical profiles of air temperature, humidity, and wind before the derecho arrived, using median and mean Skew-T diagrams and hodographs derived from the ERA5 reanalysis. Profiles showed that relative humidity in the lower and mid-troposphere in Central Europe is, on average, quite homogeneous unlike in the derecho environments in the USA, where the relative humidity at low levels is higher and at mid-levels is lower than in Central Europe. We found out that hodographs were slightly curved from 0 to 3 km in height which suggests the connection between derechos, bow echoes and supercells.