Isentropic Analysis of January Snowstorm Across Eastern Virginia and Lower Maryland Tim Gingrich and Brian Hurley NOAA/NWS Wakefield VA Isentropic Analysis and Forecasting Millersville University April 4-5, 2003

Introduction 4-8” of snow fell in less than 6 hours across eastern Virginia and the Lower Eastern Shore, during the evening and early morning hours on January 16-17, While the accumulations were generally well forecast, the duration and intensity of the snow bands were unexpected. The timing, location, and intensity of convective bands were examined through isentropic post-analysis, using both ETA and GFS model data 0-12 hours preceding the event. Isentropic analysis did support a relatively short yet heavy burst of snow across eastern Virginia and the Lower Eastern Shore.

Data and Methodology The following model data were collected: 12Z 1/16/03 operational GFS/ETA runs, with forecast valid times between 00Z – 12Z on January 17th. Isentropic Analysis was performed using theta surfaces between 285K and 300K, specifically looking at pressure, wind, and moisture parameters. Model cross sections were viewed to analyze the convective potential. The Isentropic Mixing Ratio (IMR) or Garcia technique (Garcia, 1994) was evaluated for snowfall prediction. Radar imagery was used to verify precipitation coverage and intensity.

Snowfall Prediction through Isentropic Analysis The IMR or Garcia technique (Garcia, 1994) for snowfall prediction was evaluated before and following the event. The IMR technique assesses the degree and duration of lift and moisture within mb layer (mid levels). Snowfall prediction is based on an empirical relationship between the mixing ratio (or specific humidity) within the mb layer and the duration of strong vertical ascent. During non-convective snowfall events, 6 (12) hour snow totals using the IMR method are approximated as 1x (2x) the mixing ratio, so long as the strong moist ascent remains present. During convective events, 6 (12) hour snow totals from the IMR method are doubled from the non-convective approximations, or 2x (4x) the mixing ratio.

Conclusion Total snow accumulations compared favorably to the IMR projections, even across areas where the character of the precipitation was convective. Over central Virginia, mixing ratios between mb ranged between 2-3 g/kg, which would approximate to 2-3” of snow in a 6 hour period per the IMR technique.

Conclusion As the system pushed farther east, low level trajectories began to originate from the gulf stream waters south of eastern North Carolina. The infusion of diabatic warming (latent heat) allowed for intensification/amplification of the system by the time the warm conveyor belt had reached eastern Virginia and the Lower Eastern Shore, resulting in a stronger south to north transport of moisture. Mixing ratios between mb averaged closer to 3 g/kg across eastern Virginia and the Lower Eastern Shore, as the precipitation structure began to transition into convective bands. A modified IMR projection would support a 6” snowfall accumulation in 6 hours in this region, which was close to the observed storm total.

Acknowledgements  John Billet – Science Operations Officer (SOO) NWS Wakefield VA  Scott Schumann – Information Technology Officer (ITO) NWS Wakefield VA