4    Discussion

The NE region of Bangladesh is susceptible to recurrent flash floods in response to the extreme rainfall in Meghalaya India. However, the flash flood event in 2022 previous records, inundating more than 70% of the area of Sylhet and Sunamganj engendered a humanitarian disaster. This event was the response to the extreme rainfall in Cherapunjee, more than 2500mm of accumulated rainfall was recorded by IMD in three days which was greater than that of the last 102 years. It is evident that due to the effect of climate change, these extreme events are expected to be more frequent. Hence, in this study, we have investigated whether there was any influence of atmospheric river, a climatic driver that transports copious moisture through a narrow zone and produces severe rainfall when it is encountered by hills or other elevated regions, for the sudden rise in the accumulated precipitation.
The rainfall data illustrated in Figure 2 shows a significant and sharp increase in accumulated precipitation on 15 June and the highest daily precipitation of 972 mm was recorded on 17 June which drastically receded the day after. Consequently, the water level plots in Figure 3 in response to rainfall shows that the water level stations in the upstream of the Meghna Basin crossed the Danger Level by the sudden rise in the water level due to the rainfall in the Meghalaya. Linking this rainfall event with AR generation, results in Figure 4 show the presence of AR on 15 January that lasted for 12 hours. Contrary to the previous findings about the dynamics of AR where the generation of AR is formed usually in the ocean, AR in this region was generated due to the existing moisture due to South West Monsoon in the Indian sub-continent. Similar findings have been observed by Hu [51] from their analysis of tropical moisture contribution in the U.S. Northwest Coast where they found that the ARs associated with 2-day greater precipitation contributed less than 50% from the tropics although the ARs were formed in the ocean. Furthermore, the contribution of jet streams and low-level jets can intensify the movement of moisture along the Himalayan region during the South West Monsoon. Consequently, it favors the generation of AR in NE India that landfalls and generate extreme precipitation in Meghalaya depicted in Figure 4.
The anomaly in the forecast of the water level especially for the upstream stations and their rapid response to the rainfall of the Meghalaya shows the possibility of supplanting the current Flash Flood Forecasting method by using the information of the Atmospheric River formation and its landfall. Analysis results show that the danger level of water was overtopped on 16 June in Lourergarh station in Figure 3 and considering the window, Atmospheric River landfall information will be provided two days before the extreme flash flood. Additionally, the 3 days and 5 days forecasts underestimated the water level, hence the efficiency is limited for early warning purposes. Therefore, atmospheric Rivers can be a new horizon for flash flood forecasting. However, additional events need to be investigated for a firm understanding of the applicability of Atmospheric River as an indicator for flash flood forecasting.

5    Conclusion

The NE region of Bangladesh is highly vulnerable to recurrent flash floods, primarily caused by the extreme precipitation from Meghalaya, India. Nonetheless, the intensity of the floods in June 2022 exceeded the past catastrophic events caused by the unprecedented rainfall of 102 years in Cherapunjee and Mawsynram. In contrast to the usual onset of the monsoon season in India, an atmospheric river developed on June 15, 2022, progressing across the northeastern region of India and Bangladesh, and making landfall in the NE region of India. Analysis of Wind Speed and Geopotential height at 500 hPa pressure level illustrated the movement of high-speed jets across the land area of India flowing towards the Pacific Ocean over Bangladesh through a narrow corridor. Consequently, it along with the local convergence of moisture favored the formation of the Atmospheric River. The ample amount of moisture carried by the atmospheric river resulted in intense rainfall from June 15th to June 17th. The comparison of 3 days and 5 days of deterministic water level forecast with the observed one showed that the forecasts underpredicted the water level when the runoff in response to the extreme rainfall from 15 June in Meghalaya started. Therefore, it is hoped that integrating AR into the forecast system will enhance the current forecasting mechanism of flash floods.