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
Brian L. Barjenbruch
William P. Gargan

2. “Severe” Hail “Significant” Hail “Giant” 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. Where are we at operationally?
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. 82% April–July 55% May and June 84% 2 PM–9:59 PM CST
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
TIME...MOT...LOC 2315Z 272DEG 14KT
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:
Temp profile (sig levels 0, -10, -20, -30 C)
Entire
Wind Profiles
SBCAPE
MLCAPE
MUCAPE EL LR
7-5
85-5 SR
0-2KM
4-6KM
9-11 KM
SRH
0-3 KM
Bulk
0-6 km
MID RH

10. MUCAPE 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:
Distance from RDA
TBSS
Full VCP slice of dBZ
Peak Rotational Velocity (Vr)
Rotation
Type
dBZ at sig temp levels
Max dBZ in column
Avg Storm Motion
Storm Type
VIL
VIL Density
Height of Vr
Max Storm Top Divergence
Selected max dBZ heights
WER
BWER
50 dBZ width at 0 C

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

14. Hail Growth Zone > 60 dBZ
# of cases:
530
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-2 and VILD > 7 kg/m-3
GR2AE caps DVIL at 127
GR2AE caps VILD at 13

19. 37% 63% Three-Body Scatter Spike TBSS No TBSS
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. Supercell storm (WER/BWER common)
Conclusion
Environmental conditions serving as a sole predictor of giant hail size appears limited
Need sufficient instability (MUCAPE ~ 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 ~ 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)