Wednesday, 1 August 2001
On the formation of a strong snowfall gradient in a Midwestern snowstorm: Wave synergy, deformation, and the TROWAL
The snow event of 13-14 March 1999 across southern Missouri featured a sharp northern boundary in the accumulated snowfall field. A practical outcome of this behavior was forecasts for significant snow accumulations in such populated locations as St. Louis and Columbia, MO, where ultimately no snow fell. As a forecast problem, this storm was clearly in need of further scrutiny. Our analyses demonstrated that the snowfall was due largely to ascent within the trowal airstream of the storm system. The sharp snowfall gradient on the northern periphery of the precipitation field was focused by a very dry airstream flowing parallel to and just north of the trowal airstream. Together, the dry and trowal airstreams constituted the western arm of an axis of dilatation within a synoptic-scale deformation zone. The deformation zone was due in part to a shortwave in the northern stream flow, which outran the larger and more vigorous southern storm. In effect, the northerly flow west of the southeastward-moving northern system impinged upon the southerly flow east of the eastward-moving southern storm system; the western branch of both airstreams forced the sharp snowfall gradient across southern Missouri.