Conditional Symmetric Instability and Frontogenetic Forcing in the 29-30 December 1997 Snowstorm

Stephanie Walker

Senior, The Pennsylvania State University, B.S. Candidate in Meteorology

(Manuscript received in final form 1 May 1998 for Meteo 418W)

ABSTRACT

ETA Model gridded data and snowfall observations were used to determine the role of frontogenetic forcing and conditional symmetric instability (CSI) in the 29-30 December 1997 snowstrom in central Pennsylvania. Various WSR-88D products depicted several mesoscale precipitation bands embedded within a region of broader preceipitation. These bands formed in a region of thermally direct circulation near a jet entrance. The slantwise convective available potential energy (SCAPE) was calculated to access the contribution of CSI to enhanced vertical ascent. The vertical velocities as a result of CSI were found to be 66 cm/s. Low-level frontogenesis and CSI played a major role in contributing to the sharp gradient of snowfall as well as the enhanced snow accumulation in central Pennsylvania on 29-30 December 1997.

1. Introduction

Banded clouds and precipitaion are often found embedded in the widespread region of clouds and precipitation associated with mid-latitude cyclones (Sanders and Bosart 1985). Past case studies have revealed two mesoscale mechanisms responsible for this banded structure, conditional symmetric instability (CSI) and frontogenesis. Michaud et al. (1995) found CSI to be a major contributor to the record-breaking snowfall in State College, PA on 2-3 March 1994. CSI caused flood-producing precipitation bands in Montana between 24-26 September 1986 (Lussky 1989). Others have found the banding to be a combination of the two mechanisms. Sanders (1986) found both CSI and frontogenesis present in the major New England snowstorm of 5-6 December 1981. The banded nature of precipitation in the "megalopolitan snowstorm" of 11-12 February 1983 was mainly due to frontogenetic forcing, but a small area of CSI was present (Sanders and Bosart 1985). CSI and frontogenesis also contributed to several mesoscale precipitation bands that formed over Pennsylvania on 30-31 October 1993 and 4-5 December 1993 (Grumm and Nicosia 1997).

This paper discusses frontogenesis and CSI, as well as the atmospheric conditions necessary for each to be present. WSR-88D radar imagery and GOES 8 satellite imagery show several mesoscale precipitation bands over central Pennsylvania on 29-30 December 1997. Model gridded data and conventional surface and upper-air data are used to diagnose the atmospheric conditiona. The interpretation of this information clearly suggests that both CSI and frontogenesis were contributors to the areas of heavy snowfall across central Pennsylvania.

Selected Figures

State College Doppler radar base reflectivity image from 0300 UTC 30 December 1997. At this time, three narrow, north-south bands of higher reflectivity were embedded in the broader region of precipitation over Centre County, PA.

State College Doppler radar base reflectivity image from 0503 30 December 1997. Another wider band of strong reflectivity, which was oriented WNW to ESE near the Pennsylvania-Maryland border at 0300 UTC, rotated counterclockwise and merged with the thinner bands over Centre County by 0503 UTC.

Vertical cross section of equivalent potential temperature (degrees C, solid) and geostrophic momentum (m/s, dashed) based on the MesoETA 00-hr forecast valid for 0900 UTC 30 December 1998. CSI is suggested over central Pennsylvania in the region between 850 and 450 mb where the equivalent potential temperature surfaces are sloping more steeply than the momentum surfaces.