Issues with Transitioning to a New Severe Hail Criteria

1. Issues with Transitioning to a New Severe Hail Criteria Presented by: Matt Steinbugl Contributions from: Rich Grumm andJohn LaCorte NOAA/NWS State College, PA Northeast Regional Operational Workshop NROW XI Albany, NY November 4-5, 2009

2. We will look at… • Background • 1” hail - Why and What ?? • Impacts and Implications • FAR vs. Low frequency • Focus on winds • Implementation/Outreach • Climatology • ER, US, PA • Moving ->> Forward • Training Focus • 1” Hail detection techniques • Dual-Polarization • Favorable environments • Summary

3. Background • Current Criteria = ¾” (penny) ; New Criteria = 1” (quarter) • Implementation - January 2010 (Eastern and Southern Region) • CR started in April and WR followed in June 2009 • Impetus for change: • Better representation of hail size that produces damage • Recent engineering study validates the increase in hail size threshold (Marshall et al.) • Reduce user complacency to numerous severe thunderstorm warnings • This should add more credibility to the end user • Surveys were mixed but research and climatology support the criteria change • From a service improvement/operations perspective, implementation may be easier than previously anticipated • From a verification perspective, impact likely on FAR (at least initially) but perhaps not as much as one might think

4. Implications and Impacts • Regional performance – increased FAR • ER analysis based on last two years of verification suggests potential FAR increase by as much as 30% • This is most likely an artifact of the verification as penny and nickel size hail will no longer verify SVRs (whether issued for wind/hail or both) • Oct 2006 - Sept 2009 ER SVR hail distribution suggests the frequency of 1” or greater hail is relatively low (less than 50% and about 35% on average) • Becoming aware of regional/local hail climatology might make our jobs easier ! • Severe wind detection techniques now at the forefront of severe weather warning decision making • Several calibrated 1” hail techniques to leverage (via CR) • Training efforts will need to focus on wind signatures as Tstorm wind gusts now become the driving factor when issuing SVRs

5. 1” hail in most ER counties occurs less than 1-2 times a year if that !!! From Salem et al.

6. Oct 2006- Sept 2009 ER SVR Hail by WFO The average frequency (%) of hail >= 1” is about 36% of all SVR hail reports (black line) in the last 3 years

7. Hail Climatology ** Hail data might be biased toward relative sizes

8. Moving Forward ->> • Short term (Now until Dual-Polarization) • Realize and understand the low frequency of 1” hail occurrence and use this to refine warning decision making/philosophy • Engage in external outreach to educate and ensure a smooth transition for our customers • Better understanding of wind events (MARCs, etc) • Suspect about 50% of verified storms were by wind alone • Few measured verification winds, mostly estimated based on damage • What really constitutes a damaging wind? • Leverage (1”) hail detection techniques • Need to adjust to fit environments (Eastern U.S. vs. the Plains) • Identify favorable environments • Mid Level Lapse Rate Anomalies, >= 6.0C/km good indicator

9. Dual-Pol • Medium term (Duel-Polarization Era) • Dual-pol to provide significant benefits in regards to 1” hail criteria change • Polarimetric radars transmit and receive both horizontal and vertical polarization radio wave pulses. Therefore, they measure both the horizontal and vertical dimensions of cloud and precipitation particles. This additional information leads to improved radar estimation of precipitation type and rate. • Able to detect hailstones rather than infer their presence based on current WSR-88D limitations

10. Summary and Conclusions • Realize and understand that 1” hail is a low probability event and use this information to your advantage • Despite potential (negative) impact in FAR, the increased criteria should generally be transparent and warning operations will likely remain “business as usual” • Main driver for warning issuance now becomes severe winds - need to focus our efforts here and perhaps commence an extensive damage study similar to the ones conducted for hail to determine new threshold

11. Summary and Conclusions • Leverage current hail detection techniques and adjust or “re-calibrate” to local environment (rather than attempting to develop a new criterion) • There is no silver bullet ! • Develop mid level lapse rate anomalies to identify big hail days (Some offices already doing this ??) • Foresee little to no impact on the overall number of warnings issued (warnings largely driven by wind threat) • Dual-polarization will provide great benefits in hail detection (still a few years out)

12. Future Research • Develop better radar techniques to identify storms capable of producing damaging winds • Refine CR detection techniques to fit Eastern U.S. environments and use big events as case studies • Develop a more extensive 1” hail climatology database • 4. Collaborate on regional studies

13. Acknowledgements/References • Rich Grumm (CTP) • John LaCorte (CTP) • Rosemary Auld (ER) • Dave Manning (ER) • Dave Radell (ER) • Dave Beachler (CTP) • See me for references (there are several)