Isentropic Analysis of January 16-17 Snowstorm Across

Eastern Virginia and Lower Maryland

Tim Gingrich and Brian HurleyNOAA/NWS Wakefield VA

Isentropic Analysis and Forecasting

Millersville UniversityApril 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, 2003.

• 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 750-700 mb layer (mid levels).

• Snowfall prediction is based on an empirical relationship between the mixing ratio (or specific humidity) within the 750-700 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 750-700 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 750-700 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