Evolution of a Surprise Snowfall in the United States Midwest

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  • 1 Department of Atmospheric Sciences, University of Illinois, Urbana, IL 61801
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Abstract

On Friday, 10 December 1982, a modest, yet unforecasted snowfall occurred in a band extending from the Ohio valley states eastward to western New York State. Aside from this case study representing a crucial forecasting problem, the scientific issues suggested by the results of this examination are especially intriguing. The precipitation was associated with neither a surface cyclone nor an obvious surface front. Although the precipitation began in the vicinity of quasi-geostrophic ascent, the details of the precipitation pattern are better explained by the atmosphere's susceptibility to moist slantwise convection. Additionally, the ascent associated with this precipitation event during its later stages in Illinois was pan of an elevated thermally direct frontal circulation. The relatively strong ascent on the warm side of this frontal circulation was likely assisted by the low moist symmetric instability in the same region.

The synoptic-scale flow pattern played a role in the evolution of this precipitation through quasi-geostrophic ascent, weakened environmental moist symmetric, stability, and geostrophic frontogenetic flow. However, in the western part of the precipitation band, the moisture responsible for the precipitation onset is shown to have been transported from the Texas Gulf Coast into the Midwest by a low-level wind maximum. The depth of this moist layer ranged from 20 to less than 150 mb. and its horizontal extent was about 200 km—dimensions which are substantially smaller than synoptic scale. The limited depth of the moist layer may have contributed to this precipitation event being missed by the operational Limited-Area Fine-Mesh Model (LFM), which has only six tropospheric layers, averaging 150 mb in depth.

Abstract

On Friday, 10 December 1982, a modest, yet unforecasted snowfall occurred in a band extending from the Ohio valley states eastward to western New York State. Aside from this case study representing a crucial forecasting problem, the scientific issues suggested by the results of this examination are especially intriguing. The precipitation was associated with neither a surface cyclone nor an obvious surface front. Although the precipitation began in the vicinity of quasi-geostrophic ascent, the details of the precipitation pattern are better explained by the atmosphere's susceptibility to moist slantwise convection. Additionally, the ascent associated with this precipitation event during its later stages in Illinois was pan of an elevated thermally direct frontal circulation. The relatively strong ascent on the warm side of this frontal circulation was likely assisted by the low moist symmetric instability in the same region.

The synoptic-scale flow pattern played a role in the evolution of this precipitation through quasi-geostrophic ascent, weakened environmental moist symmetric, stability, and geostrophic frontogenetic flow. However, in the western part of the precipitation band, the moisture responsible for the precipitation onset is shown to have been transported from the Texas Gulf Coast into the Midwest by a low-level wind maximum. The depth of this moist layer ranged from 20 to less than 150 mb. and its horizontal extent was about 200 km—dimensions which are substantially smaller than synoptic scale. The limited depth of the moist layer may have contributed to this precipitation event being missed by the operational Limited-Area Fine-Mesh Model (LFM), which has only six tropospheric layers, averaging 150 mb in depth.

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