Preparing for a “Change” in Severe Hail Warning Criteria in 2010

1. Preparing for a “Change” in Severe Hail Warning Criteria in 2010 Brian J. Frugis NWS WFO Albany, NY NROW XI November 4-5, 2009

2. Motivation for Study • NWS ER will be changing severe hail criteria from 0.75” to 1.00” on January 1, 2010 • All current hail studies for the region are based on 0.75” • VIL of the Day • VIL Density • Common warning techniques using dBZ heights relative to the -20° C level • New thresholds and techniques are needed to accurately warn for the new hail criteria

3. What was done • Examined radar returns for each report of severe hail (0.75” or larger) from the major severe cases of hail from the summer of 2009 • July 16th and June 15th hail events • Compiled data and compared averages of each parameter that was examined

4. Data for Study 3 additional hail reports were rejected due to probable errors in locations and/or times

5. Storm Reports for Study

6. Parameters Studied • 50 dBZ Echo Top • 55 dBZ Echo Top • 60 dBZ Echo Top • 65 dBZ Echo Top • Vertically Integrated Liquid (VIL) • Storm Echo Tops (ET) • VIL Density • Height of 50 dBZ Echo Top above -20° Celsius isotherm

7. Why dBZ heights? • In a warning situation, the height of the 50+ dBZ level in correlation with the -20° Celsius level is often examined. The increasing amount of height above this level increases the level of confidence of severe hail in a particular thunderstorm.

8. Why VIL Density? • VIL Density = (VIL/ET) * 1000 • Units are in g m-3 • This helps “normalize” the VIL to produce a common values for thunderstorms producing hail, independent of the particular air mass.

9. Previous Studies on VIL Density • Previous studies have been done examining VIL Density for the ALY CWA • VIL density values of 3.50 g m-3 identified 82% of severe hail based on a 0.75” criteria in a paper by Blaes et. al in 1998 • They also determined that a VIL density of 3.70 g m-3 would depict 91% of severe hail cases based on a proposed 1.00”criteria (based on 154 total reports) • However, this study also mentioned that this isn’t reliable since there would also be a False Alarm Rate (FAR) of 48% based on the data set used • There were limited 1.00” reports in their database • It was recommended that additional studies be done

10. Methodology • Used FSI 4D Storm Investigator to get heights of different dBZ thresholds • Also used 4 panels displays of the lowest tilts from adjacent radar sites (BGM, OKX, TYX and CXX) • Used D2D gridded VIL and ET displays to get specific values at times of each report

11. Limitation of Radar Location • Radar Location • The KENX radar’s cone of silence made judging echo tops difficult for storm’s close to the RDA • A solution to this was to view other radars, such as KTYX, KBGM, KCXX, and KOKX.

12. Limitation of Radar Location • Radar Beam Blockage • Portions of the Catskills (whose terrain reaches heights of 4180 feet) can block the lowest tilts of the radar beam • This could give inaccurate VIL values for storms in these areas

13. Problems with Hail Reports • Size is not always accurately measured • Spotters don’t always use rulers and some reports are from the untrained public • Location and time • Some locations and times listed on hail report may be estimated and some spotters may not have given their exact latitude and longitude • Have tried to include only the reports that made sense based on radar for this study • Some hail reports may have occurred in unpopulated areas or in places where spotters were not available

14. Truncation and Rounding Errors • Estimating ETs of various levels is difficult higher up and further away from the storm

15. 0.75” vs. 1.00” Hail

16. Severe vs. Non-Severe(based on new warning criteria)

17. Severe vs. Non-Severe(based on new warning criteria)

18. Storm Echo Top vs. VIL Based on a severe hail criteria of 1.00”

19. VIL Density Threshold

20. VIL Density as an Indicator of Severe Hail • Not a good correlation between increasing VIL Density and severe hail in these cases as there was still a high FAR • May be based on small data set of only 70 severe reports • Previous data set by Blaes et al. did not have a large number of hail cases over 1.00” • Their study had large FAR of 48% for a value of 3.70” (despite POD of 91%) showing low correlation for VIL Density and severe hail based on a 1.00” criteria • This study also had a FAR of 48% and POD of only 76%

21. Summary • The 50 dBZ ETs are in general about 10 kft above the -20° Celsius level • Average height is around 32.6 kft • The dBZ echo tops (for 50, 55, 60 and 65 dBZ) are about 6-7 kft higher in a severe over a non-severe storm based on the new criteria • No particular values of VIL Density seem to correlate well in our area for severe hail • At least based off this limited sample

22. Future Work • Compile statistics from other events from 2009 as well as previous years • Will differences between severe and non-severe storms be similar? • Create a new “VIL of the Day” nomogram for operational use • Attempt to see if VIL Density has a better correlation with a larger database

24. Acknowledgements • Many thanks to Joe Villani and Tom Wasula for their help and guidance on this project!

25. Questions? Any questions or comments? Brian.Frugis@noaa.gov