Evaluation and Application of Conditional Symmetric Instability, Equivalent Potential Vorticity, and Frontogenetic Forcing in an Operational Forecast Environment

James L. Wiesmueller National Weather Service—Baltimore–Washington Forecast Office, Sterling, Virginia

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Steven M. Zubrick National Weather Service—Baltimore–Washington Forecast Office, Sterling, Virginia

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Abstract

The related fields of equivalent potential temperature (θe), geostrophic momentum (Mg), equivalent potential vorticity (EPV), and frontogenetic forcing were computed, analyzed, and evaluated for two mid-Atlantic states snowstorms in which conditional symmetric instability (CSI) is believed to have been present. Observed data as well as model gridded data were used with the National Weather Service PC-based Gridded Information Display and Diagnostic System in evaluating the event, thereby providing both initial and model forecast fields. A brief physical and historical review of CSI is provided as a basis for better understanding and increased proper application of this technique into operational forecasting. The implications of CSI for the operational forecaster are then demonstrated through a diagnosis of events occurring in Maryland and Virginia on 26 February 1993 and 30 January 1995, in which precipitation banding related to frontogenetic forcing and CSI are believed to have occurred. The intent here is to demonstrate that it is possible to improve short-range forecasts of CSI-related precipitation in an operational forecast environment through improved techniques for the recognition and evaluation of CSI via model-derived forecast fields, augmented with WSR-88D radar and other new observing systems.

Vertical cross sections of forecast Mg, θe, layer mean geostrophic frontogenesis, and EPV fields were constructed and revealed regions of conditional symmetric instability and frontogenetic forcing that were nearly coincident with the observed enhanced snow bands. The availability of gridded model output and the increased capability for quickly manipulating those datasets in real time have now enhanced the potential for improved forecasts of CSI-related phenomena in operational forecast settings.

Corresponding author address: Mr. James L. Wiesmueller, National Weather Service, 44087 Weather Service Road, Sterling, VA 20166.

Email: James.Wiesmueller@noaa.gov

Abstract

The related fields of equivalent potential temperature (θe), geostrophic momentum (Mg), equivalent potential vorticity (EPV), and frontogenetic forcing were computed, analyzed, and evaluated for two mid-Atlantic states snowstorms in which conditional symmetric instability (CSI) is believed to have been present. Observed data as well as model gridded data were used with the National Weather Service PC-based Gridded Information Display and Diagnostic System in evaluating the event, thereby providing both initial and model forecast fields. A brief physical and historical review of CSI is provided as a basis for better understanding and increased proper application of this technique into operational forecasting. The implications of CSI for the operational forecaster are then demonstrated through a diagnosis of events occurring in Maryland and Virginia on 26 February 1993 and 30 January 1995, in which precipitation banding related to frontogenetic forcing and CSI are believed to have occurred. The intent here is to demonstrate that it is possible to improve short-range forecasts of CSI-related precipitation in an operational forecast environment through improved techniques for the recognition and evaluation of CSI via model-derived forecast fields, augmented with WSR-88D radar and other new observing systems.

Vertical cross sections of forecast Mg, θe, layer mean geostrophic frontogenesis, and EPV fields were constructed and revealed regions of conditional symmetric instability and frontogenetic forcing that were nearly coincident with the observed enhanced snow bands. The availability of gridded model output and the increased capability for quickly manipulating those datasets in real time have now enhanced the potential for improved forecasts of CSI-related phenomena in operational forecast settings.

Corresponding author address: Mr. James L. Wiesmueller, National Weather Service, 44087 Weather Service Road, Sterling, VA 20166.

Email: James.Wiesmueller@noaa.gov

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