Scott F. Blair NOAA/National Weather Service, Topeka, Kansas Derek R. Deroche

1. An Operational Assessment of the Predictability of Giant Hail Events Scott F. Blair NOAA/National Weather Service, Topeka, Kansas Derek R. Deroche NOAA/National Weather Service, Kansas City, Missouri Joshua M. Boustead NOAA/National Weather Service, Omaha, Nebraska Jared W. Leighton NOAA/National Weather Service, Topeka, Kansas Brian L. Barjenbruch NOAA/National Weather Service, Topeka, Kansas William P. Gargan NOAA/National Weather Service, Topeka, Kansas

2. “Severe” Hail 1 inch diameter (25 mm) “Significant” Hail 2 inch diameter (51 mm) “Giant” Hail 4 inch diameter (102 mm)

3. A sample of giants this decade… 2003 2004 2006 Aurora, NE (7.0”) Meadville, MO (6.0”) Searcy, AR (5.5”) 2007 2010 2010 Dante, SD (6.9”) Vivian, SD (8.0”) Wichita, KS (7.8”)

4. Overview • Giant hail infrequent phenomenon – less than 1% of US hail reports • Likely underrepresented in Storm Data (rural areas, lack of widespread aggressive spatial verification) • These high-end events have potential to cause extreme damage to property and a substantial threat to exposed life • Media/EM partners take additional actions when giant hail imminent (‘severe’ not created equal) • Operational prediction of giant hail (size in general) has been challenging • Need to determine the predictability of giant hail -and- if possible, improve advanced recognition of these events • Sample benefits from investigating known upper threshold • hail-fall character to establish signals Where are we at operationally?

5. Giant Hail Reports: 1 January 1995 – 31 December 2009 Domain: 17 states within central contiguous United States accounting for 89% CONUS reports (638 /715) Point Density Map Output: 0.5o x 0.5o Radius: 1o per report Max value = 11 reports Reports from Storm Data

6. 4”+ Hail Climatology Within Domain Events by Month 82% April–July 55% May and June Reports by Hour 84% 2 PM–9:59 PM CST 58% 4 PM–7:59 PM CST

7. Watch Product THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS ABC. TORNADOES...HAIL TO 4.0 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. Warning Product • * AT 800 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A SEVERE THUNDERSTORM CAPABLE OF PRODUCING GRAPEFRUIT SIZE HAIL. • LARGE DESTRUCTIVE HAIL WILL OCCUR WITH THIS STORM. FOR YOUR SAFETY MOVE INDOORS NOW...AND STAY AWAY FROM WINDOWS. • LAT...LON 3966 9579 3966 9601 3977 9598 3979 9597 3983 9578 • TIME...MOT...LOC 2315Z 272DEG 14KT 3973 9591 • WIND...HAIL 60MPH 4.00IN

8. Forecasted Maximum Hail Size (2005-09) SPC Watches NWS Warnings/SVS 201 reports reviewed 16 forecasts 4”+ (8%) Avg. underestimated size: 1.64 in 54% TOR-Watches (108/201) 128 reports reviewed 9 forecasts 4”+ (7%) Avg. underestimated size: 2.13 in 24% TOR-Warned (44/186)* • 20% NWS warnings forecasted penny-quarter sized hail • Most NWS tornado warnings contained no hail size information • NWS tendency to use golf ball (1.75”) or baseball (2.75”) to convey ‘large hail’

9. Environmental Data • 479 Unique Giant Hail Events from 1 Jan 1995 to 31 Dec 2009 • Unique Event: A giant hail report > 100 km from surrounding reports • and/or reports separated > 3 hours • North American Regional Reanalysis (NARR) data obtained from NCDC NOAA National Operational Model Archive and Distribution System (NOMADS) website. • NARR dataset 32 km, 3 hourly regional reanalysis for North America • Used the closest available dataset to the hail report time • Parameters investigated:

10. MUCAPE 1500-3000 J/kg

11. Moderate Deep Layer Shear • 0-6 km Bulk Shear: ~35-50 kts • Supportive of supercell structures • Vertical pressure gradient forces within storm can lead to significant updraft accelerations • Speeds greater than assumed maximum updraft strength due to buoyancy alone (CAPE) • May play a vital role in potential for giant hail production (when other parameters supportive)

12. WSR-88D Radar Data • 573 Reports with available radar data from 1 Jan 1995 to 31 Dec 2009 • 1-2 radar sites nearest to lat/lon report selected • Radar data downloaded from NCDC Hierarchical Data Storage System • Examined data 15 minutes prior to and 5 minutes post report time • QC Storm Data report times and location • time frequently incorrect (132 corrected – 23%) • Radar data interpolated, then paired with environmental data Parameters investigated:

13. 496 # of cases: 540 556 443 208 26

14. 530 # of cases: 560 567 565 Hail Growth Zone > 60 dBZ (Median: 66 dBZ)

15. ------------------------------------ ----------------------- • Tight clustering of reports, general increase in 50 dBZ echo height as MLT increases • 50 dBZ echo height > 30,000 ft • 4”+ hail uncommon with MLT < 10,000 ft (function of updraft strength)

16. 0.75”-1” diameter hail 4”+ diameter hail Donavon and Jungbluth (2007) Fig. 2 • Higher 50 dBZ echo height produced larger hail • Strong confidence for ‘severe’ hail, but… • 50 dBZ echo height vs. MLT questionable indicator of 4”+ hail size alone • Melting effects are less for giant hail due to surface area • Supercell structures displace giant hail away from main precipitation core

17. Maximum Reflectivity • Max column reflectivity > ~65 dBZ • ‘Extreme’ dBZ values unnecessary • Independent of time of year/melting level Max dBZ From 573 cases

18. Digital VIL and VIL Density • Documented issues correlating legacy VIL/VILD to hail size • Confidence may increase when DVIL>100 kg/m-2and VILD>7 kg/m-3 GR2AE caps DVIL at 127 GR2AE caps VILD at 13

19. From 573 cases Three-Body Scatter Spike • Radar microwave scattering artifact associated with large hydrometeors frequently considered good signal for “very large hail” (NOAA WDTB 2002) TBSS 37% (212) No TBSS 63% (362) • Occasionally… • Downrange echoes masked TBSS signature • TBSS existed outside the 15/5 study period Structure Characteristics From 518 cases Anti- cyclonic 10% (52) • 99% reports associated with isolated or embedded supercells • Persistent mid-level mesocyclone • Presence of WER or BWER Cyclonic 89% (461)

20. Rotational Velocity -------------------------- 30,000 ft -------------------------- 10,000 ft Moderate/Strong Mesocyclone Vr > ~40 kts between 10,000-30,000 ft • Strong mesocyclone within the preferred hail growth region • With buoyancy (CAPE) and storm structure information, infer updraft strength from Vr to support giant hail potential Vr = (|Vmin| + |Vmax|) / 2 Vr Diameter < 5 nm

21. Storm-Top Divergence • Strong divergent flow at • summit of convection, previous • attempts to correlate hail size • STD > 100 kts associated • with giant hail events • Doesn’t appear to be good indicator of giant hail alone STD From 428 cases RDA

22. Giant Hail vs. Large Hail • Strong updraft and semi- organized structure necessary • 67 Reports from 2010 • 72% classified as supercell

23. Giant Hail vs. Large Hail

24. Conclusion • Environmental conditions serving as a sole predictor of giant hail size appears limited • Need sufficient instability (MUCAPE ~1500-3000+ J/kg), moderate deep layer shear (0-6 km bulk shear ~35-50 kts), and favorable vertical temperature profile • WSR-88D data provides increased confidence in identifying giant hail when the following parameters are present • Supercell storm (WER/BWER common) • 50 dBZ height ~35,000 to 45,000 ft • 60 dBZ height ~30,000 to 40,000 ft • Hail growth zone >60 dBZ (median 66) • Max column reflectivity ~69 dBZ • Storm-top divergence ~125-175 kts • Rotational velocity ~40-60 kts Difficult to cross-check ‘null cases’ due to limited spatial verification (4”+ diameter hail likely underreported in supercells) Provide improved recognition of ‘high-end’ hail events, resulting in more accurate hail size forecasts in convective warnings Acknowledgements George Phillips (NWS Topeka)