The extent and variability of snow covers are important parameters in hydrological and meteorological systems (Udnaes et al., 2007; Brown and Armstrong, 2010). In mountainous and snow-covered basins, snowmelt and its runoff play an important role in changes in the flow regime and have a major share in the production of flow and water resources. Snow reserves of mountain basins are among the important and reliable sources of the country. Snow accumulation in high-altitude parts of the mountains controls the seasonal pattern of runoff in the lower parts. The gradual melting of snow is considered an important source of river flow for feeding groundwater aquifers, as well as a crucial source of fresh water for domestic, agricultural, and hydroelectric uses. However, rapid and early melting of snow covers can impose significant environmental hazards. Communities that live along rivers are forced to flee their homes, due to the sudden melting of snow covers and the possibility of flooding. In case of rapid melting of snow covers, the melted water reaches the downstream of the basin in a short amount of time, resulting in losing the opportunity for infiltration and feeding of groundwater aquifers. The rapid melting of snow covers leads to severe avalanches in mountainous areas and causes the transfer of huge volumes of snow masses to downstream areas, imposing destructive effects on downstream of the basin. Concerning the geographical position of Iran relative to the general atmospheric circulation, precipitation occurs in the cold half of the year in most parts of the country. The precipitation period starts from the middle of October and continues until the end of May. In the southern half of the country, this precipitation period is even shorter, andit does not coincide with the peak consumption period. Therefore, freezing rains in the cold period of the year areideal phenomenon in arid and semi-arid countries such as Iran, as most of these precipitations are stored naturally behind mountain dams for the construction of dams with the lowest cost, in addition to supplying water to springs during the warm period of the year with gradual melting. Karkheh catchment is located in the west of Iran, in the middle and southern regions of Zagros Mountains. The area of Karkheh is equal to 50764 square kilometers. Karkheh River is formed by joining the main rivers of Gamasiab, Qarah Su, Seimare, and Kashkan, each of which, along with the lower part of Karkheh River, has catchmentswhich form the main sub-catchments of Karkheh. The mountainous areas of the catchment with about 27645 square kilometers are mostly concentrated in the eastern and middle parts while the plains and foothills with about 23119 square kilometers generally cover the northern and southern parts of the catchment. In terms of longitudinal and transverse expansion, massive heights of the Zagros Mountains are located at the upstream of the catchment, where most of the rainfall falls in the form of snow and accumulates during the cold period of the year. This massive snow is the source of the springs that supply the water of the Karkheh River and the water needed by the extensive agricultural fields, towns, and villages downstream of the catchment during the spring and summer seasons. In some years, the arrival of early warm waves in the last months of the cold period of the year (March and early April) causes rapid and intense snowmelt in the upstream basin and therefore, significant amounts of valuable water reserves become unreachable. Consequently,this issue causes flood risks at the upstream of the dam and sometimes necessitates discharging water from damsdue to their limited capacity. The results of previous research indicate that snow cover decreased in most parts of the world in recent years. For example, Seluchi et al. (2006) analyzed the synoptic and thermodynamic nature of the 2003 heat wave in the South American subtropical region and introduced the existence of stable atmosphere and the advection of temperature and humidity as the main cause of the creation of the heat wave and its intensity. Stewart et al. (2004) predicted the flow time of snowmelt runoff in the northwestern United States under climate change conditions. By using climate models and considering temperature and precipitation changes in the 21st century in the affected areas, it was found that the snowmelt runoff flew about 30 to 40 days earlier. By evaluating the climate change in Michigan province of United States, winter rainfall changes from snow to rain due to increased temperatures. Further, it is expected that the number of snowy days by the end of this century decreaseseach year by 30 to 50%in optimistic scenarios and between 45 to 60% in pessimistic scenarios. By studying anomalies and changes in the dynamic structure of summer synoptic patterns in southwestern Iranbased on the warm wave index, Jamalizadeh et al. (2019) concluded that temperature abnormalities in the region had a positive trend at all levels and this upward trend had sometimes reached up to one degree.
Bednorz (2009) prepared combined maps of sea surface pressure and 500 hPa level while examining the synoptic conditions of rapid melting of snow covers in Poland and Germany plains for days when the snow depth decreased by 5 cm. The results highlighted that low- and high-pressure systems caused rapid melting of snow cover in the North Atlantic and the Mediterranean Sea (NAO positive phase), respectively. MacDonald et al., (2012) studied a number of snow models to complement observations, evaluate important snow processes, and develop appropriate models for Chinook wind conditions. The results of modeling indicated that snowmelt occurs during Chinook winds and snow covers are heavily melted or destroyed completely. By using non-parametric tests, Changchun et al.,(2008)investigated the annual temperature and precipitation of 19 meteorological stations over a period between 1958 and 2002, and examined the effects of climate change on the snow cover of the Tarom River catchment. The results showed that the altitudes between 2500 and 5000 meters were a sensitive area and affected by climate change. Hayhoe et al. (2010) predicted that Chicago temperatures would rise between 2 and 3.5 degrees Fahrenheit in the near future (2010-39) and 2.5 to 9 degrees Fahrenheit between 2040 and 2069, although summer temperaturewould rise between 5 and 19 degrees Fahrenheit by the end of this century. Yonggang et al. (2013) examined the effect of climate change on the snowmelt runoff of the Kaido catchment in northwest China, using the HADCM3 model output with different scenarios. The results revealed an increase in runoff in spring and a significant decrease in summer. Sharma et al. (2012) used MODIS sensor data from 2000 to 2011 to investigate snow trends in the sub-basins of the Jhelum River catchments in the northwestern Himalayas. Based on the findings, the decreasing trend of snow cover is observed in all sub-catchments, especially in Banihal sub-catchment.
In another study, Huang et al. (2017) measured the snow cover of the Tibetan Plateau, by using MODIS sensor daily images during the period 2001-14and applying the non-parametric Mann-Kendall test. The resultsrevealed a significant decrease in the level of snow cover at the plateau surface, especially at its high areas. Cohen and Saito (2003), and Dery et al. (2005) found the correlation with delay between meteorological signals with the level of snow covers. Falarz (2007) mentioned the positive correlation between the snow cover and the North Atlantic Oscillation (NAO) in October (winter). The lack of stability and dependence of snow cover on atmospheric circulation in the twentieth century are almost related to the circulations of local changes/European meridian. Mayr and Armi (2008) showed that the blowing of warm winds and increased temperature could be large enough to lead to more evaporation, as well as melting mountain snow. The occurrence of Foehn wind can be a challenge in mountainous areas with recreational centers and ski slopes, in addition to its effects on water resources. By examining the historical trend of early thermal waves, the present study seeks to analyze the conditions of thermal waves in terms of frequency and duration, aiming to inform relevant authorities to respond appropriately in the case of increasing trend of waves or their durations. On the other hand, the patterns and origin of these heat waves are identified by evaluating the synoptic of thermal waves. Ultimately,by monitoringthe atmospheric synoptic conditions, the necessary warnings to the inhabitants of the river and its tributaries are given by the relevant organizations in case of the occurrence of patterns similar to heat waves.