The Response of a Long-lived Mesoscale Convective System to Changes in Boundary Layer Stability

Mesoscale convective systems (MCSs) are well known to make a significant contribution to the  nocturnal maximum in warm season precipitation observed over the Great Plains. Several climatological studies of these systems have been conducted revealing that the majority of these MCSs frequently weaken or dissipate in the morning with the cessation of the nocturnal low-level jet. However, previous research has shown that ~ 28% of these nocturnal MCSs remain steady or strengthen during the late morning. Accurate short-term forecasts of these long-lived convective systems remain a challenge, and guidance from numerical models is complicated by the relative difficulty these models have in accurately predicting nocturnal MCSs. Despite the challenges faced by forecasters, less research has been dedicated to understanding the mechanisms responsible for the persistence of these nocturnal systems into the following day. 

The system presented herein is a long-lived MCS that persisted through much of the diurnal cycle and was poorly forecast operationally. This MCS initiated around 02 UTC in an environment thought to have minimal convective available potential energy (CAPE) and went on to produce numerous wind and hail reports and one EF-1 tornado near 06 UTC. Thus, this system was not completely elevated throughout the nighttime despite the most-unstable CAPE existing aloft. High-resolution simulations using the Weather Research and Forecasting (WRF) model in tandem with Oklahoma Mesonet observations were utilized to study the evolution of the system as it encountered environments with different thermodynamic and kinematic characteristics and to better understand why the system persisted during the daytime as the boundary layer was destabilizing. 

The results of the WRF simulations match the observations well, providing insight into the maintenance mechanisms responsible for the system’s longevity. Despite initiating in a stable, nocturnal environment, the MCS was initially maintained by a surface cold pool. However, further environmental stabilization resulted in the system transitioning from being primarily cold-pool-driven to bore-driven, which corresponded to a lack of observed severe wind reports. However, during the nighttime, the observed system reorganized and accelerated southward, which the simulations attribute to a strengthening rear-inflow jet and accelerating cold pool that overtakes the bore and again dominates the system propagation. As the boundary layer began destabilizing with the onset of solar heating, the organized MCS was able to maintain itself as a cold-pool-driven system before moving off the Gulf coast.