Scott F. BlairScott F. BlairNOAA/National Weather Service, Topeka, Kansas

Derek R. DerocheDerek R. DerocheNOAA/National Weather Service, Kansas City, Missouri

Joshua M. BousteadJoshua M. BousteadNOAA/National Weather Service, Omaha, Nebraska

Jared W. LeightonJared W. LeightonNOAA/National Weather Service, Topeka, Kansas

Brian L. BarjenbruchBrian L. BarjenbruchNOAA/National Weather Service, Topeka, Kansas

William P. GarganWilliam P. GarganNOAA/National Weather Service, Topeka, Kansas

An Operational Assessment of the Predictability of Giant Hail Events

“Severe” Hail1 inch diameter (25 mm)

“Significant” Hail2 inch diameter (51 mm)

“Giant” Hail4 inch diameter (102 mm)

Vivian, SD (8.0”) Wichita, KS (7.8”)

2003

20102007

Searcy, AR (5.5”)

Dante, SD (6.9”)

Meadville, MO (6.0”)Aurora, NE (7.0”)

2004 2006

2010

A sample of giants this decade…

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 These high-end events have potential to cause extreme damageextreme damage to to property and a property and a substantial threat to exposed lifesubstantial 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?Where are we at operationally?

Overview

Domain: Domain: 17 states 17 states within central contiguous United States within central contiguous United States accounting foraccounting for 89% 89% CONUS reports CONUS reports (638 /715) (638 /715)

Giant Hail Reports: 1 January 1995 – 31 December 2009Giant Hail Reports: 1 January 1995 – 31 December 2009

Point Density MapOutput: 0.5o x 0.5o

Radius: 1o per reportMax value = 11 reports Reports from Storm Data

Events by Month

82% April–July55% May and June

Reports by Hour

84% 2 PM–9:59 PM CST58% 4 PM–7:59 PM CST

4”+ Hail ClimatologyWithin Domain

* 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 9578TIME...MOT...LOC 2315Z 272DEG 14KT 3973 9591WIND...HAIL 60MPH 4.00IN

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.

Watch Product

Warning Product

Forecasted Maximum Hail Size (2005-09)Forecasted Maximum Hail Size (2005-09)

SPC WatchesSPC Watches NWS Warnings/SVSNWS Warnings/SVS

201 reports reviewed16 forecasts 4”+ (8%)

Avg. underestimated size: 1.64 in1.64 in54% TOR-Watches (108/201)

128 reports reviewed9 forecasts 4”+ (7%)

Avg. underestimated size: 2.13 in2.13 in24% 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’

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:

Environmental Data

SBCAPE MLCAPE MUCAPE ELLR 7-5

LR 85-5

SR 0-2KM

SR 4-6KM

SR 9-11 KM

SRH 0-3 KM

Bulk 0-6 km

EL SR

MID RH

Temp profile (sig levels 0, -10, -20, -30 C)

EntireWind Profiles

MUCAPE 1500-3000 J/kg

Moderate Deep Layer ShearModerate Deep Layer Shear0-6 km Bulk Shear: 0-6 km Bulk Shear: ~~35-50 kts35-50 kts

• Supportive of supercell structuresSupportive of supercell structures

• Vertical pressure gradient forces Vertical pressure gradient forces within storm can lead to significant within storm can lead to significant

updraft accelerationsupdraft accelerations

• Speeds greater than assumed Speeds greater than assumed maximum updraft strength due to maximum updraft strength due to

buoyancy alone (CAPE) buoyancy alone (CAPE)

• May play a vital role in potential May play a vital role in potential for giant hail production (when for giant hail production (when other parameters supportive)other parameters supportive)

WSR-88D Radar Data

573573 Reports with available radar data from 1 Jan 1995 to 31 Dec 2009 Reports with available radar data from 1 Jan 1995 to 31 Dec 2009

1-2 radar sites nearest to lat/lon report selected1-2 radar sites nearest to lat/lon report selected

Radar data downloaded from NCDC Hierarchical Data Storage SystemRadar data downloaded from NCDC Hierarchical Data Storage System

Examined data Examined data 15 minutes prior 15 minutes prior to and to and 5 minutes post 5 minutes post report timereport time

QC Storm Data report times and locationQC Storm Data report times and location• time frequently incorrect (time frequently incorrect (132 corrected – 23%132 corrected – 23%))

Radar data interpolated, then paired with environmental data Radar data interpolated, then paired with environmental data

Distance Distance from RDAfrom RDA

TBSSTBSS Full VCP Full VCP slice of dBZslice of dBZ

Peak Rotational Peak Rotational Velocity (Vr)Velocity (Vr)

Rotation Rotation TypeType

dBZ at sig dBZ at sig temp levelstemp levels

Max dBZ Max dBZ in columnin column

Avg Storm Avg Storm MotionMotion

Storm TypeStorm Type VILVIL VIL DensityVIL Density Height of VrHeight of Vr Max Storm Max Storm Top Top

DivergenceDivergence

Selected max Selected max dBZ heightsdBZ heights

WERWERBWERBWER

50 dBZ 50 dBZ width at 0 Cwidth at 0 C

Parameters investigated:

496 540 556 443 208 26# of cases:

530 560 567 565# of cases:

Hail Growth Zone > 60 dBZHail Growth Zone > 60 dBZ(Median: 66 dBZ)(Median: 66 dBZ)

• Tight clustering of reports, general increase in 50 dBZ echo height as MLT increasesTight clustering of reports, general increase in 50 dBZ echo height as MLT increases• 50 dBZ echo height 50 dBZ echo height >> 30,000 ft 30,000 ft

• 4”+ hail uncommon with MLT 4”+ hail uncommon with MLT << 10,000 ft (function of updraft strength) 10,000 ft (function of updraft strength)

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Higher 50 dBZ echo height produced larger hailHigher 50 dBZ echo height produced larger hailStrong confidence for ‘severe’ hail, but…Strong confidence for ‘severe’ hail, but…

50 dBZ echo height vs. MLT questionable indicator of 4”+ hail size 50 dBZ echo height vs. MLT questionable indicator of 4”+ hail size alonealone

• Melting effects are less for giant hail due to surface areaMelting effects are less for giant hail due to surface area• Supercell structures displace giant hail away from main precipitation coreSupercell structures displace giant hail away from main precipitation core

0.75”-1” diameter hail0.75”-1” diameter hail

Donavon and Jungbluth (2007) Fig. 2

4”+ diameter hail4”+ diameter hail

Maximum Reflectivity

• Max column reflectivity Max column reflectivity >> ~ ~6565 dBZ dBZ

• ‘‘ExtremeExtreme’ dBZ values unnecessary’ dBZ values unnecessary

• Independent of time of year/melting levelIndependent of time of year/melting level

Max Max dBZdBZ

From 573 cases

Digital VIL and VIL Density• Documented issues correlating legacy VIL/VILD to hail sizeDocumented issues correlating legacy VIL/VILD to hail size• Confidence may increase when Confidence may increase when DVILDVIL >> 100 100 kg/mkg/m-2-2 and and VILDVILD >> 77 kg/m kg/m-3-3

GR2AE caps VILD at 13GR2AE caps DVIL at 127

99% reports associated with isolated or embedded supercells

• Persistent mid-level mesocyclone• Presence of WER or BWER

Three-Body Scatter Spike

• Radar microwave scattering artifact associated with large hydrometeors frequently considered good

signal for “very large hail” (NOAA WDTB 2002)TBSSTBSS

37%37%(212)(212)

TBSSTBSS

37%37%(212)(212)

No TBSSNo TBSS

63%63%(362)(362)

No TBSSNo TBSS

63%63%(362)(362)

From 573 cases

Occasionally…

• Downrange echoes masked TBSS signature

• TBSS existed outside the 15/5 study period

Structure Characteristics From 518 cases

CyclonicCyclonic

89%89%(461)(461)

CyclonicCyclonic

89%89%(461)(461)

Anti-Anti-cycloniccyclonic

10%10%(52)(52)

Anti-Anti-cycloniccyclonic

10%10%(52)(52)

Diameter < 5 nmDiameter < 5 nm

Vr = (|Vmin| + |Vmax|) / 2Vr = (|Vmin| + |Vmax|) / 2

Rotational Velocity

Vr

• 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

--------------------------

--------------------------

30,000 ft

10,000 ft

Moderate/Strong Mesocyclone Vr > ~40 kts between 10,000-30,000 ft

Storm-Top Divergence

• STD > 100 kts associated with giant hail events

• Strong divergent flow at summit of convection, previous attempts to correlate hail size

• Doesn’t appear to be good indicator of giant hail alone

RDARDAFrom 428 cases

STDSTD

Giant Hail vs. Large Hail

• Strong updraft and semi- organized structure necessary

• 67 Reports from 2010

• 72% classified as supercell

Giant Hail vs. Large Hail

Conclusion

AcknowledgementsAcknowledgementsGeorge Phillips George Phillips (NWS Topeka)(NWS Topeka)

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

50 dBZ height 50 dBZ height ~35,000 to 45,000 ft~35,000 to 45,000 ft 60 dBZ height 60 dBZ height ~30,000 to 40,000 ft~30,000 to 40,000 ft Hail growth zone Hail growth zone >> 60 dBZ (median 66)60 dBZ (median 66)

Max column reflectivity Max column reflectivity ~69 dBZ~69 dBZ Storm-top divergence Storm-top divergence ~125-175 kts~125-175 kts Rotational velocity Rotational velocity ~40-60 kts~40-60 kts

Supercell storm Supercell storm (WER/BWER common)(WER/BWER common)

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 Provide improved recognition of ‘high-end’ hail events, resulting in more accurate hail size forecasts in convective warnings more accurate hail size forecasts in convective warnings