Ingredients Cross-Section Case: February 12, 1999
Low-Level Instability

On February 12, 1999, following the passage of a strong cold front through the Midwest, very cold air (-18 C at 850 hPa) flowed over a residually warm and moist ground from the previous day's storm (Milwaukee, Wisconsin had a high temperature of 68 F and 0.55 inches of rain on February 11, 1999). The warm surface temperature beneath cold air aloft established a shallow low-level instability as will be shown in this section. The dry air and high winds facilitated the transfer of moisture from the wet ground to the atmosphere, providing a layer of ample moisture in the vicinity of the instability. With weak forcing the instability was realized, resulting in a convective snow event. It is interesting to note that this event was forced by mechanisms similar to those associated with lake-effect snow, namely a large air-surface temperature difference and a source of moisture at the ground.

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.