Lower-tropospheric influences on the timing and intensity of afternoon severe convection over modest terrain in a convection-allowing ensemble

Environmental composites constructed from these subensembles reveal a stronger surface horizontal convergence zone and moisture gradient in STRONG, resulting from 2–3.5 m s−1 stronger southerly winds on the south flank of the convergence zone. The stronger southerlies result from accelerated PBL growth and momentum mixing in the presence of strong low-to-mid-tropospheric vertical shear, which is facilitated by reduced above-PBL static stability in the composite STRONG initial condition.

Stronger time-averaged low-to-mid-tropospheric upward motion coincides with the surface convergence zone in STRONG, and individual CI locations occur at the northeastern edge of the composite vertical motion maximum. Trajectory analysis with STRONG members confirms that the CI locations are consistent with large vertical displacements, and corresponding relative humidity increases leading to decreases in convective inhibition, as the southerly airstream ascends across the convergence zone.