The formation of a forward-tilting cold front with multiple cloud bands during Superstorm 1993

A mesoscale-model simulation is used to examine the evolution of the cold f ront and accompanying cloud bands in eastern Mexico associated with Superst orm 1993 (12-14 March). The simulated cold front differed in structure and evolution from a classical cold front, in agreement with evidence from obse rvations and European Centre for Medium-Range Weather Forecasts analyses. T he surface cold front, as defined by the leading edge of strong northerlies and cold advection, initially possessed a rearward till with height over s outhern Texas. Within 6 h, the leading edge of the front moved equatorward and developed a large-scale forward tilt of greater than 200 km in the hori zontal from the surface to 700 hPa. This forward tilt occurred as a mid- to upper-tropospheric baroclinic zone arrived from over the Sierra Madre, des cended into eastern Mexico, and interacted with the surface cold front. Emb edded within this large-scale forward tilt was a locally enhanced horizonta l potential temperature gradient that also tilted forward similar to 100 km from the surface to 850 hPa. Tilting frontogenesis associated with ascent at the leading edge of the surface front was responsible for the smaller-sc ale forward-tilting structure. This surface-based ascent is believed to hav e caused the primary cloud band observed from satellite imagery that is coi ncident with the leading edge of the front, whereas a second region of asce nt, elevated at the leading edge of the mid- to upper-tropospheric baroclin ic zone, is believed to have caused the prefrontal cloud band revealed by s atellite imagery. Subsidence behind the forward-tilting cold-frontal struct ure at and above 850 hPa (and concomitant divergence underneath) resulted i n frontolysis of the surface front, and eventually the dissipation of the p rimary cloud band, leading to the dominance of the prefrontal cloud band. F inally, the Superstorm 1993 cold front is compared and contrasted to noncla ssical cold-frontal structures found in the literature and a general contex t for frontal interaction is discussed.