LAUSR

Funding information University of Bergen, Norway, This study focuses on the analysis of the extreme precipitation event in Central Nepal on 19 July 2007 which was part of a sequence of rain events leading to the devastating South Asia flood of 2007. We investigate synoptic-scale conditions using reanalyses and attribute moisture sources with a Lagrangian moisture source diagnostic. Further, we characterize the mesoscale precipitation event with a high-resolution numerical simulation. The simulation reveals an intense wide convective event with a simulated 40 dBZ echo core of considerable horizontal extent (1, 550 km) exceeding a height of 12 km. Initially small convective cells were invigorated by high CAPE and a potentially unstable layer at mid-tropospheric levels. This layer reached conditional instability adding latent energy to the system. Isolated convective cells organized upscale into a wide intense convective system, fuelled with moist low-level inflow. The result was torrential rain with over 250 mm within 24 hr. Several synoptic-scale conditions contributed to the intense development: (a) supply of moist air with the help of a typical monsoon break condition flow pattern, (b) anomalously significant moisture sources along this path due to prior precipitation events, (c) an upper-tropospheric trough orienting the atmospheric flow against the Himalayas with associated quasi-geostrophic forcing creating a favourable environment for convection, and (d) destabilized stratification due to an upslope flow. This analysis encompasses multiple scales and shows how a wide intense convective system, not unusual for this region, can be intensified by distinct synoptic constituents.