Summertime characteristics of a thunderstorm in April? Environmental causes and numerical investigation of a severe thunderstorm in Northern Greece using the WRF model

In the early afternoon hours of April 17, 2024, a severe thunderstorm hit northern Greece and more precisely the administrative area of Central Macedonia. The storm developed extreme characteristics, unexpected for an early spring storm. Its maximum height reached almost 11 kilometers and the maximum reflectivity was about 62dBZ. Seeding operations for the prevention of hail were performed in the context of the National Hail Suppression Program (NHSP) of Hellenic Agricultural Insurance Organization (ELGA). Even after the operations, hail was recorded in some of the hail pads along the storm’s 50 kilometers path.

Radar data from the C-band weather radar located at Filyro (30 kilometers east - southeast of the storm) are used to present the storm’s evolution while the synoptic environment of the onset and the following development of the storm is examined. The unusual high temperatures (+10°C above the climatic mean) in the days that preceded the storm seem to have played an important role in its final intensity.

The non-hydrostatic Weather Research and Forecasting model with the Advanced Research dynamic solver (WRF-ARW 4.3.3) is used to perform multiple simulations to explore the model’s sensitivity to different parametrizations for this -out of its season- extreme storm. Three model domains with grid-spacings of 15km, 5km and 1.67km were used with the finer ones covering Greece and Northern Greece respectively. The parameterization used for operational forecast in the NHSP (inherently lighter due to limited computational power) proved good enough, giving almost the exact position and intensity but lagging for about a couple of hours in the time of the storm. The temporal evolution of the storm was better captured using more complex parameterizations.