Abstract. This paper presents three indices for evaluation of hydrometeorological extremes, considering them as areal precipitation events and trans-basin floods. In contrast to common precipitation indices, the weather extremity index (WEI) reflects not only the highest precipitation amounts at individual gauges but also the rarity of the amounts, the size of the affected area, and the duration of the event. Furthermore, the aspect of precipitation seasonality was considered when defining the weather abnormality index (WAI), which enables the detection of precipitation extremes throughout the year. The precipitation indices are complemented with the flood extremity index (FEI) employing peak discharge data. A unified design of the three indices, based on return periods of station data, enables one to compare easily interannual and seasonal distributions of precipitation extremes and large floods.The indices were employed in evaluation of 50 hydrometeorological extremes of each type (extreme precipitation events, seasonally abnormal precipitation events, and large floods) during the period 1961-2010 in the Czech Republic. A preliminary study of discrepancies among historic values of the indices indicated that variations in the frequency and/or magnitude of floods can generally be due not only to variations in the magnitude of precipitation events but also to variations in their seasonal distribution and other factors, primarily the antecedent saturation.

The strongest Czech windstorms from 1961 to 2010 were evaluated using a weather extremity index. The index combines the return periods of station wind gusts and the size of the affected area allowing determination of the duration of individual events. Of 50 extreme wind events (EWEs), most were synoptic‐scale windstorms that occurred between the end of October and the beginning of March. However, six EWEs were categorized as the convective‐scale windstorms. Four of the strongest Czech and European windstorms were related to one other (Kyrill, the November 1984 storm, Vivian and Wiebke, and Jeanette). Approximately 90% of the synoptic‐scale windstorms were characterized by strong westerly or northwesterly flow at the 850 hPa level and a north–south temperature gradient at the 500 hPa level. The remaining (rather weaker) synoptic‐scale events were characterized by southerly flow and a west–east temperature gradient at the 850 and 500 hPa levels, respectively. To determine the abnormality of windstorms by season, a weather abnormality index was used to evaluate 50 abnormal wind events (AWEs). Throughout the summer, as well as in half of May and September, AWEs were categorized as the convective‐scale windstorms accompanied by weak 850 hPa winds. Apart from two events with easterly winds, the remaining AWEs were accompanied by winds from the southwest and a strong west–east temperature gradient at the 850 hPa level. Ten of the latter events occurred in the last two ten‐day periods of July and first two ten‐day periods of August. All of the events followed hot episodes, and seven of them followed one of the 25 extreme high‐temperature events. A less pronounced relationship between wind and temperature anomalies was present in the colder half of the year from October to March: three of the strongest Czech EWEs were associated with abnormal high‐temperature events (AHEs).

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We present an analysis of a period of high air temperature that occurred in the second half of August 2012 in the Czech Republic (CZ). We use and compare the results of two different approaches for the evaluation of high air temperature events. The Weather Extremity Index (WEI) evaluates the extremity and spatial extent of the meteorological event of interest. The second method is based on the duration of daily maximum air temperature above specific thresholds. In 2012, the high air temperature in the CZ lasted from 18 August to 24 August (18/8 to 24/8). It was connected with the inflow of hot air from northern Africa between the low pressure trough over the eastern Atlantic and the region of high pressure in central Europe. The high air temperature culminated on 20/8 when its maximum was greater than 30C across the whole of the CZ. The highest daily maximum air temperature on record in the CZ with a value of 40.4C was observed at the Dobřichovice station. Our results demonstrate that the studied period was quite extraordinary, occurring so late in the summer with a relatively large areal extent and extremity of detected maximum air temperature. Furthermore, the WEI was found useful for identification of very extreme high air temperature events and facilitated intercomparison in terms of extremity and spatial extent. However, WEI cannot be used for detection of periods with a persistent relatively high air temperature that could have severe impacts on both human activities and natural ecosystems but during which the extremity of observed air temperature values is not very high.