Multiscale aspects of an extreme precipitation event over Nepal in September 2024

On 26–28 September 2024, torrential rainfall struck Nepal during the late monsoon season, causing flooding, landslides and extensive damage. This study examined the multiscale processes contributing to this extreme precipitation event, focusing on intraseasonal oscillations, synoptic-scale circulations, and mesoscale cloud/precipitation systems. A quasi-biweekly intraseasonal oscillation dominated over South Asia during the event, featuring a monsoon low-pressure system over the Indian Peninsula and an anticyclone to its east, both propagating westward. The pressure gradient between them sustained strong southerly moisture transport toward the Himalayas, establishing a persistently humid environment and orographic lift along the southern slopes. In contrast to reports of previous extreme precipitation events in Nepal, the atmospheric circulation responsible for the 2024 event was primarily of tropical origin, with minimal influence from the midlatitudes. Characteristic mesoscale cloud/precipitation systems also developed around the Himalayas. The highest daily precipitation during the event was recorded on 27 September; stratiform systems with relatively modest storm top heights developed over the southern slopes, generating surface precipitation rates of > 100 mm h− 1 through warm-rain processes. Rain gauges across the glacierized basin (3500–5000 m asl) recorded exceptionally high daily and hourly precipitation rates, highlighting the extension of intense rainfall to unusually high elevations.