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Abstract. Flood is known as one of the most distractive natural disaster worldwide. Therefore, its prediction is of great importance from the socio-economical point of view. Despite the great improvement in computational techniques and numerical weather prediction approaches, so far, in Iran, an acceptable flood prediction method has not yet been introduced. The main aim of this study is to recognize and classify the patterns of synoptic systems leading to torrential rainfalls in a watershed basin located in south-west of Iran. In this research, 20 major floods characterized by high rainfall intensities and severe damage were selected. The pattern, extension, and the direction of movement of the selected synoptic maps from surface to 500 hPa pressure levels were identified. Furthermore, the position of cyclones, anti-cyclones and mid-level trough lines were carefully tracked and classified into different groups. The results show that the major severe floods occurring in Dalaki watershed river basin are mainly influenced by strengthening of the center of Sudan heat low (SHL) and the coincidence moisture feeding by the Indian Ocean and Mediterranean Sea. It was found that simultaneous merging of the SHL system and Mediterranean frontal system would intensify the flood intensities over the basin. The mean positions of high pressures, low pressures, the Red Sea trough lines and 1015 hPa isobars of the major floods are also discussed.

Purpose The purpose of this paper is to assess the spatial and temporal variations of extreme hot days (H*) and heat wave frequencies across Iran. Design/methodology/approach The authors used daily maximum temperature (Tmax) data of 27 synoptic stations in Iran. These data were standardized using the mean and the standard deviation of each day of the year. An extreme hot day was defined when the Z score of daily maximum temperature of that day was equal or more than a given threshold fixed at 1.7, while a heat wave event was considered to occur when the Z score exceeds the threshold for at least three continuous days. According to these criteria, the annual frequency of extreme hot days and the number of heat waves were determined for all stations. Findings The trend analysis of H* shows a positive trend during the past two decades in Iran, with the maximum number of H* (110 cases) observed in 2010. A significant trend of the number of heat waves per year was also detected during 1991-2013 in all the stations. Overall, results indicate that Iran has experienced heat waves in recent years more often than its long-term average. There will be more frequent and intense hot days and heat waves across Iran until 2050, due to estimated increase of mean air temperature between 0.5-1.1 and 0.8-1.6 degree centigrade for Rcp2.6 and Rcp8.8 scenarios, respectively. Originality/value The trend analysis of hot days and heat wave frequencies is a particularly original aspect of this paper. It is very important for policy- and decision-makers especially in agriculture and health sectors of Iran to make some adaptation strategies for future frequent and intense hot days over Iran.

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