Here we present an alternative framework where moisture is prognostic and is coupled to precipitation through a simplified Betts-Miller parameterization. Unstable Rossby-like wave solutions are obtained that arise from interactions between moisture, convection and the anomalous circulation in the presence of a background temperature gradient. Warm air that is advected south by the anomalous flow is lifted along the sloping isentropes of the monsoon region, which moistens and destabilizes the column to the west of the low-pressure center. The moistened lower troposphere enhances convection which, in turn, causes the low pressure system to intensify through vortex stretching. It is shown that during the active months of the South Asian monsoon unstable growth occurs only if the moist wave can propagate westward against the low-level westerly flow. For parameter values that resemble the observed monsoonal background state, growth of these waves is largest at the synoptic scale. Expanding the framework to include meridional moisture gradients leads to a more general framework that suggests this ``moisture-vortex" instability may operate on other synoptic-scale low pressure systems such as easterly waves.
Supplementary URL: https://journals.ametsoc.org/doi/abs/10.1175/JAS-D-17-0310.1