VIL Density as a Indicator of Hail across Eastern New York and Western New England

The deployment and continued utilization of the WSR-88D network across the United States has resulted in an increase in warning lead times and verification. As experience grows with the radar, limitations of the system along with new techniques for its better usage are understood. Determining the potential for severe hail (ò 19 mm, 3/4 in.) is one problem facing forecasters who operate the radar during severe weather events.

Several meteorologists have proposed that vertically integrated liquid water content (VIL) could be a useful tool for determining the severe weather potential of thunderstorms, particularly the occurrence of hail. VIL values represent reflectivity data converted into liquid water values. Values associated with severe hail change daily as the environment evolves, forcing the forecaster to be aware of environmental conditions when using VIL as a warning tool. Studies have indicated that dividing the VIL by the echo top would normalize the VIL and produce a range of VIL values, independent of the airmass type, that can be associated with severe hail occurrence and hail size.

Previous VIL density studies have used the grid-based VIL and the echo top products to compute VIL density. This procedure calls for the forecaster to interpolate and extrapolate numerical values from roughly contoured data maps given on the WSR-88D PUP. With the deployment of the WSR 88-D build 9, the cell based VIL and the echo top is computed and displayed on the attribute table on the PUP. This allows the radar operator to easily compute the VIL density.

Data for the past four convective seasons (1993-1997) in which data from the WSR-88D at Albany (KENX) has been available was sampled in this study. Preliminary findings suggest that severe hail events (ò 19 mm, 3/4 in.) increased as the VIL density increased above 3.5 g m-3. When VIL density values were less than 3.5 g m-3, there were very few events of severe hail. Above 4.0 g m-3, nearly all storms produced severe hail.