Monday, 5 November 2012
Symphony III and Foyer (Loews Vanderbilt Hotel)
The National Weather Service has changed the criterion for severe hail from 0.75 in. (1.9 cm) to 1.00 in. (2.5 cm) as of 5 January 2010. Many techniques have been developed for forecasting severe hail, such as the Vertically Integrated Liquid (VIL) of the Day method, VIL density and using reflectivity echo (dBZ) heights relative to the -20°C level. However, these techniques were all originally developed based on the legacy 0.75 in. severe hail criterion. Previous studies have also been based on combined large hail and severe wind reports. In an attempt to better forecast hail with the new criterion in place, the Albany (ALY) hail study project examined over 380 hail reports from the NWS Albany County Warning Area (CWA) from 2005-2010. This study has determined the reflectivity echo height values at various dBZ thresholds (50, 55, 60 and 65 dBZ), as well as gridded VIL, Storm Echo Top (ET), VIL Density and several other parameters at a storm-scale level. The study also calculated mean and median values for severe hail from this dataset and produced a variety of tables and graphs, which would be potentially useful to a warning forecaster in an operational setting.
In order to evaluate the use of the data, the results of the ALY hail study were applied to two cases from May 2012. These dates were chosen because it featured both severe and non-severe hail, resulting from supercells and multicells. The freezing level and height of the -20° C isotherm, based off the KALY upper air soundings, were at typical levels for warm season convection in the Northeast. According to local storm reports entered into StormData, these two severe weather episodes produced a total of 45 hail events. These particular events were analyzed to see how well the mean and median values from the hail study correlated to the storms responsible for producing both severe and non-severe hail.
A storm-scale analysis of several of these hail events will be presented as examples of how the application of the hail study values can be used in an operational setting for increased confidence in the occurrence of severe hail. The values will also be shown in conjunction with other methods of storm interrogation of base and derived legacy and dual polarization radar products.
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