The Role of Frontogenetical Forcing and Conditional Symmetric Instability in the Midwest Snowstorm of 30–31 January 1982

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Abstract
On 30-31 January 1982 a modest low pressure system moved through the lower Mississippi Valley into western Tennessee. During this 24-hour period rain changed to snow over central Missouri and Illinois, increasing in intensity over the last 12 hours, In addition, embedded convection took place over east-central Missouri- Illinois including the St. Louis metropolitan area. Overall snow totals were over 25 cm within a narrow band along the edge of the precipitation shield. Frontogenetical forcing together with conditional symmetric instability are discussed as possible physical explanations for the intense precipitation which was not well forecast. It is shown that moderate-strong ascent was part of a thermally direct ageostrophic circulation created by frontogenetical forcing. Frontogenesis is shown both at the surface and aloft to increase in the 12 hour period prior to the heavy precipitation. It is strongest in the low levels and slopes to the west-northwest with height along an advancing cold frontal zone. Both deformation and divergence components of the frontogenetical function equation play key roles in the total frontogenesis. Quasi-geostrophic frontogenesis is also shown to be quite strong, especially at low levels. Q-vector forcing of vertical motion increased with the quasi-geostrophic frontogenesis and helped create a direct thermal circulation of warm air rising in south-central Missouri-Illinois and cool air sinking in northern portions of those states normal to the axis of maximum frontogenetical forcing. Conditional symmetric instability was also diagnosed in the region of the updraft of the direct thermal circulation. This “slantwise convection”, diagnosed where surfaces of constant geostrophic angular momentum slope less than surfaces of equivalent potential temperature, is believed to have both increased the intensity of the updraft and decreased the scale length of the phenomena. The conditional symmetric instability may have helped to create pulselike eruptions of elevated cloud tops seen in satellite imagery and often noted with East Coast cyclogenesis by several researchers.

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