The wrap-around quadrant of wintertime extratropical cyclones characteristically produces heavy snowfall, often accompanied by high winds. The snowfall is coincident with the so-called “trough of warm air aloft” (trowal), a band of warm moist air aloft wedged between the cyclone’s advancing dry slot to the south and the warm frontal surface to the north. When viewed with radar, the precipitation produced within the trowal region is organized in narrow mesoscale bands that propagate from the southern to the northern side of the trowal, have a wavelength of 15-20 km, and are nearly parallel to the geostrophic shear vector in the layer defining the trowal. In this paper, we present detailed high-resolution measurements of the thermodynamic and precipitation structure of the trowal region of two extratropical cyclones. Cross-band wind and thermodynamic analyses were derived from high resolution (~ 20 km) dropsondes obtained during flights of the NCAR Electra Aircraft, airborne ELDORA radar data, WSR 88-D Level II radar data, and Rapid Update Cycle (RUC) model analyses. The observations reveal in unprecedented detail the fine-scale structure and sharp transition zones near the trowal and dry slot boundaries, the structure of the low-level jet, and the relationship between the heavy snowfall and vertical motions within these features. From the dropsonde data and analyses using the RUC Model, we evaluate the potential roles of conditional symmetric instability and gravity waves in the formation of these bands.