Mechanisms governing the persistence and diurnal cycle of a heavy rainfall corridor

Observations and convection-permitting simulations are used to study a 12-day warm-season heavy precipitation corridor over the central U.S. plains and Mississippi River valley regions. Such precipitation corridors, defined by narrow latitudinal widths (~3°-4°) and only modest north-south drifts of their centroids (<2° day^-1), often yield extreme total precipitation (100-250 mm), resulting in both short-term and seasonal impacts on the regional hydrologic cycle. The corridor precipitation is predominately nocturnal and located several hundred kilometers north of a quasi-stationary surface front. There, hot, dry air from the daytime boundary layer located underneath a persistent upper-level anticyclone requires large vertical displacements along the axis of the southerly lowlevel jet (LLJ) above the front to eliminate convection inhibition (CIN). Composites reveal ~500 J kg^-1 of average convective available potential energy (CAPE) when this air reaches the southern edge of the precipitation corridor. Despite the relatively modest CAPE, convection is favored by the large reductions in CIN along the vertical displacements and by high ambient midtropospheric relative humidity located above, which is influenced by persistent nightly convection in the region. Though internal feedbacks resulting from the large nightly spatial coherence of convection (including enhanced midtropospheric relative humidities and frontogenetic daytime sensible heat flux gradients owing to residual cloudiness) are favorable for maintaining the corridor, its persistence is most sensitive to largescale external factors. Here, changes to the intensity and position of the large-synoptic upper-tropospheric anticyclone are associated with changes in the frequency of strong LLJs and the surface frontal position, dramatically affecting the intensity and stationarity of the precipitation corridor.