## 1 Introduction  
Out of all the climate and weather extremes, drought is one of the most damaging and little understood phenomena [1], with wide-ranging and far-reaching impacts [2]. It brings severe devastation in numerous aspects, including the natural environment, agricultural output, and living water [3]. The American Meteorological Society (AMS) identified four distinct types of drought: hydrological, meteorological, agricultural, and socioeconomic. These categories are determined by the environmental and socioeconomic elements that are impacted by the drought [4]. Scientific study on flash drought has been ongoing since 2002 [5] and the term \"flash drought\" was developed to call attention to this extraordinary drought type and to better differentiate these occurrences from more typical droughts [6]. It looks as though they occur in a \"flash\", making them and their consequences more challenging to forecast and plan for [7]. A flash drought differs from a traditional drought in its rapid onset, intensification, and short duration [8]. The attention of community and academic researchers was drawn to the flash drought event when a severe drought occurred in the central and western plains of the United States in the spring of 2012 [9]. Flash droughts occur as a result of elevated evapotranspiration (ET) rates produced by abnormally high temperatures, strong winds, and intense solar radiation. These conditions are accompanied by little or no rainfall [10], leading to reduced soil moisture levels [11]. When there is a lack of rainfall coupled with severe meteorological phenomena that enhance the demand for evaporation, such as elevated temperatures, various elements work together to cause a rapid increase in evaporation-transpiration processes and the reduction of soil moisture (SM) [12]. When this condition lasts for a prolonged period ranging from several days to several weeks, the evapotranspiration (ET) attributes undergo a transition from being regulated by energy to being governed by moisture. Consequently, this transition gives rise to a sudden onset of drought known as a flash drought, which is quickly accompanied by an increase in vegetation stress [13], and the stress can cause serious impacts on the vegetation health and productivity. Long-term flash droughts have the potential to increase environmental unsustainability, human mortality, and food and water insecurity because of their unpredictable nature and propensity to cause simultaneous severe events like heat waves and wildfires [14]. While significant progress has been made in understanding flash droughts, the methods for detecting and characterizing them are still evolving and not yet standardized [15]. The evaporative stress index (ESI) [16], standardized evaporative stress ratio (SESR) [17], standardized precipitation evaporation index (SPEI) [18], evaporative demand drought index (EDDI) [19], and soil moisture volatility index (SMVI) [20] are some of the drought indices adopted by researchers for determining flash drought events. While studying major climatic variables like precipitation, root-zone soil moisture, temperature, and the actual and projected evapotranspiration that are used to define flash droughts, it was discovered that during flash droughts, the root-zone soil moisture exhibits the clearest signal [21]. Likewise, various satellite-based indicators have been utilized to observe the vegetation's reaction to flash drought, such as the leaf area index (LAI) [22], solar-induced chlorophyll fluorescence (SIF) [23], normalized difference vegetation index (NDVI) [24], gross primary productivity (GPP) [25], and net primary productivity (NPP) [26]. The normalized difference vegetation index (NDVI) serves as a crucial indicator utilized for determining the vegetation greenness, providing information about the vegetation status [27]. In the context of flash droughts, the NDVI plays an important role in monitoring vegetative stress and dynamics in response to sudden changes in soil moisture and precipitation levels [28].