Convection in the Carpathian Basin: Short summary

Storm impact:

From available observations, the thunderstorm's impact was most severe over the Hungarian-Slovak border (cities Komárno and Komárom) and in central Slovakia (Kremnické Bane). There were no synoptic stations within the thunderstorm-affected areas, but some daily precipitation observations were available. At the station Hadovce (SHMÚ) near Komárno the amount of precipitation reached 79 mm in a single day. In the period discussed here, the precipitation intensity was 47 mm/1 h. It should be noted that the station is situated about 2 km north of the center of the highest radar reflectivity as identified in radar imagery, and of the reported flooding. The station at Kremnické Bane (central Slovakia) measured 122 mm with a peak intensity of 72 mm/1 h (www.shmu.sk). Large (2-3 cm) hail, strong wind and fallen trees were also observed in Komárno. The streets of both Slovak and Hungarian cities were flooded, causing extensive damage.

Summary:

Both synoptic and mesosynoptic conditions were favorable for intense thunderstorm development, particularly in northwestern Hungary. Thunderstorm development likely took place elsewhere as well, but with less intensity and probability. The instability and low-level moisture content were high and deep convection could develop along or in the vicinity of stationary low-level convergence zones. Later a convective system developed and new, even heavier cells could develop along its outflow boundary. This could be attributed to the presence of dry mid-level air and large negative buoyancy, which enabled the development of a strong and large cold pool. The spreading of the cold pool triggered new cells in its highly unstable environment, although gravity waves could also have formed and supported the development of new convective cells farther away. Although deep-layer wind shear was fairly low, long-lived storms could also develop. The reasons for that and the dynamics behind them would need further study but it is probable that the orientation of the in- and outflows, which allowed back-building, could have played an important role. In combination with the cells' low speed of propagation this resulted in locally high amounts of precipitation and flooding.

In other parts of Hungary, such as the southeast, there was probably no sufficient lift to elevate the air parcels to their LFC. Large scale upward motion was absent in the 700 hPa vertical velocity field in much of southern and southeastern Hungary, possibly even weak downward motion (forecast by the ECMWF model) and elevated capping inversion levels could be generated. The southerly advection of drier air started earlier in these regions and inhibited, which was also the reason why thunderstorms did not form here although the convection temperature had been reached and some other parameters (such as CAPE) were high.

The aforementioned processes were only partially forecast by mesoscale NWP models although their initial state seemed to be very similar to observations. The situation was relatively difficult from the forecasting point of view. Although the convective environment was clearly favorable for heavy thunderstorms and even flash floods, it was not easy to predict where exactly the most intense cells would form and which areas would remain free of convective activity. The most important signs were the sudden formation of high radar reflectivity and the rapid cooling of thunderstorm tops in satellite infrared imagery. This happened in a region where high TPW, CAPE and low-level convergence were indicated by large-scale model outputs. The sudden change of the storm motion vector's orientation and the decrease in its propagation speed could signify that the evolution of the storm would differ locally from other cells over Hungary and Slovakia. However, the maximum lead-time of the flood warning could hardly exceed 30-45 minutes, which would probably still be too short for taking broader measures. Information could still be distributed to the populace in order to reduce their outdoor activities and have them seek shelter.

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