Isentropic Analysis of January 1617 Snowstorm Across Eastern Virginia and Lower Maryland Tim Gingric - PowerPoint PPT Presentation

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Isentropic Analysis of January 1617 Snowstorm Across Eastern Virginia and Lower Maryland Tim Gingric

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Title: Isentropic Analysis of January 1617 Snowstorm Across Eastern Virginia and Lower Maryland Tim Gingric


1
Isentropic Analysis of January 16-17 Snowstorm
Across Eastern Virginia and Lower MarylandTim
Gingrich and Brian HurleyNOAA/NWS Wakefield
VAIsentropic Analysis and ForecastingMillersvil
le UniversityApril 4-5, 2003
2
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.

3
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.

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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.

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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.

27
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.

28
Acknowledgements
  • John Billet Science Operations Officer (SOO)
    NWS Wakefield VA
  • Scott Schumann Information Technology Officer
    (ITO) NWS Wakefield VA
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