The role of jet streak regeneration forced by a deepening continental planetary boundary layer in the explosive surface cyclogenesis of 28 March 1984

Numerical modeling studies of the 28-29 March 1984 explosive cyclogenesis case revealed that key features within the antecedent environment can be traced back into Texas 33 hours before the event. Experiments showed that unseasonable lower-tropospheric warming, facilitated by minimal soil moisture, strong surface sensible heating, and down slope flow off the Mexican Plateau, forced the development of a deep Planetary Boundary Layer (PBL) and a reduction of surface pressures over southern Texas. The hydrostatic response to the heating forced a mid-level thickness perturbation and an intensification of the mid-tropospheric pressure gradient force. Large divergence tendencies within the mid-level flow initiated differential vertical motions, folding of the isentropes, and the formation of a Mid-tropospheric Unbalanced Front (MUF).;Coincident with the development in southern Texas, the PBL over the Edwards Plateau deepened to near the 550 mb level. Vertical fluxes of horizontal momentum led to the formation of a lower-tropospheric mesoridge, which resulted in parcel divergence and an acceleration towards the lower surface pressure over southern Texas. The accompanying cold air advection induced an intensification of the mid-level pressure gradient force and a rapid turning of the winds off the Mexican Plateau. The proximity of hot air over elevated Plateau region facilitated frontogenesis by cross-stream confluence deformation. This incipient jet/front system propagated eastward across Texas where it encountered a strong SE to NW directed pressure gradient force. The advancement of the Polar Front at low-levels enhanced the mid-tropospheric pressure gradient force over the Arklatex and accelerated the jet northeastwards into Mississippi by 1200 UTC 28 March.;Mid-tropospheric jet/front system began to decay after 1200 UTC 28 March. The thermally direct circulation about the weakening frontal system sagged southeastwards and phased with a thermodynamically indirect circulation surrounding the large-scale system. This merger of large-scale and subsynoptic scale systems provides a focusing mechanism for the initiation of a squall line. Vertical wind shear, surface sensible heating and weak static stability provide an environment conducive to explosive cyclogenesis.