Ingredients Cross-Section Case: January 23, 1996
Conditional Symmetric Instability

Cross-sections can also be used to distinguish between regions of CI and CSI as will be illustrated for a snowstorm which covered the southern half of Wisconsin with 3-6 inches of snow in the 12 hours between 18Z on January 23, 1996 and 6Z on January 24, 1996. Up to eight inches were reported in a few east-central Wisconsin counties. Convective bands of moderate to heavy snow were reported during the afternoon of January 23 in southern Wisconsin, spreading to central Wisconsin during the evening. This was documented in the Dousman, Wisconsin NWSFO storm report as a "CSI event" (Haase, 1996).

An inspection of the 800-850 hPa ingredients map from the 18-hour forecast of the NCEP-ETA model run initialized at 0Z on January 23, 1996 provides clear signs of the potential for CSI during this period. A region of negative PVes was present in the southern third of Wisconsin at this time. Toward western Wisconsin, there was weak to moderate forcing for ascent coincident with the region of instability and thus a nonzero value of QPV. In a northwest to southeast-oriented ingredients cross-section (drawn perpendicular to the 500-1000 hPa thickness contours from Richland to Walworth counties in southern Wisconsin, two layers of instability can be seen.

The lower layer of instability from 800-850 hPa corresponded to a region of gravitational stability since -d(theta-es)/dz >0. Furthermore, the contours of geostrophic momentum Mg were less steeply sloped than the contours of theta-es, indicating that CSI was possible because the flow was nearly two-dimensional. With weak to moderate forcing and relative humidity of 90-100% shown in the ingredients maps, this instability could be realized. Due to the limited moisture above 750 hPa, however, very heavy precipitation or thundersnow is not likely. Instead, banded shallow moist convection was expected. This is precisely what was observed, as shown by the radar from 18Z on January 23.

The upper layer of negative PVes between 550 and 650 hPa corresponds to CI because theta-es decreases with height in this layer as shown in the ingredients cross-section. However, the relative humidity at 600 hPa is only 50-60% so it is unlikely that this instability was realized.