The radar from 14Z on February 12 shows isolated bands of heavy snow (near white-out conditions were observed in Madison at this time) and the satellite image from 19Z indicates an unstable convective boundary layer with cellular convection and roll clouds.
The instability associated with this event in southern Wisconsin was confined to a very low layer and did not appear on the standard level ingredients maps. The 800-850 hPa 18-hour ETA model ingredients map valid at 18Z on February 12 did not indicate any areas of negative PVes over Wisconsin and only weak scattered QG forcing.
However, a cross-section drawn across southern Wisconsin from LaCrosse to Milwaukee for 18Z on February 12 reveals a layer of negative PVes between 870-950 hPa, with -d(theta-es)/dz < -5 K/m just above the surface and low level relative humidity greater than 70%. Given even a slight forcing, such instability could have been realized. Although little QG forcing is predicted in this cross-section, scattered weak QG forcing can be seen on the 950 hPa surface (not shown) throughout southern Wisconsin around this time. Furthermore, the air temperature just above 900 hPa (which corresponded to the level of instability) was -15 C, the ideal temperature for maximum depositional growth of ice crystals. If the instability was realized, strong vertical motions occurred in a region susceptible to rapid ice crystal growth. Thus, when analyzed from an ingredients perspective using cross-sections, this 18-hour forecast provides significant clues to the potential for the scattered heavy snow showers that passed through Wisconsin in the wake of the strong cold front.