One-Inch Hail Initiative
Spring Media Seminar
March 7th 2009
Jim Sieveking
National Weather Service
WFO St. Louis, Missouri
The Kansas Experiment
Beginning with the 2005 convective season, Kansas
weather forecast offices have conducted a 1” hail
criteria experiment. These offices have issued severe
thunderstorm warnings for thunderstorms producing
hail sizes of 1” or greater, therefore raising the severe
hail criteria from ¾ inch to 1 inch diameter.
An optional Significant Weather Alert or Advisory
issued under the Special Weather Statement (SPS)
header was used to highlight thunderstorms that
produced hail less than one inch.
The Kansas Experiment Cont’d
The reasoning for this change in service was
based primarily on two factors:
– Scientific research indicates 1” is a more
meaningful threshold for defining hail
associated with property damage
– Feedback from local partners desiring fewer
warnings and EAS activations for marginal
events
The Kansas Experiment Cont’d
• Media outlets located in the Kansas Experiment
have expressed approval. They have received
fewer viewer complaints about breaking into
programming and they feel the warnings we do
issue are more meaningful.
• Media representatives stated that their user
feedback suggests individuals now know that
when a Severe Thunderstorm Warning is issued,
the threat of damage is genuine. In essence, the
warning is carrying more credibility to the public.
The Kansas Experiment Cont’d
• The Kansas Emergency Management community
is in favor of continuing the 1” Hail Initiative.
• They noted fewer spotter activations during the
demonstration period helped to alleviate the
weariness that can set in from over-activating
volunteers for storms that are not expected to
pose serious threat to life and property.
The Kansas Experiment Cont’d
Scientific studies indicate hail alone, smaller than
1” in diameter, is not associated with significant
property damage.
“Hail Damage Threshold Sizes For Common Roofing Materials” (Marshall, et. al. 2002)
“The April 1999 Sydney Hailstorm” (Stephen Yeo, Roy Leigh and Ivan Kuhne, 1999)
http://www.haagengineering.com/ehail/chas/eHail/hailstorm.html
HAIL SIZE (IN) TYPICAL DAMAGE THRESHOLDS
1”
Threshold size for damage to most lightweight asphalt composition shingles.
Thin and/or deteriorated wood shingles, shakes- occasionally punctured or
cracked. Single-pane windows, thin skylight shells- cracked or broken.
1 ¼”
Threshold size for damage to most heavy weight asphalt composition
shingles, wood shingles, and older medium shakes. Automobile body metal-
dented; galvanized metal vents- dented. Older plastic skylights- cracked or
broken.
1 ½”
Threshold size for damage to clay tile, slate, medium shakes, and modified
bitumen single-ply membranes. Automobile body metal- extensive denting.
1 ¾”
Threshold size for damage to heavy shakes and concrete tiles . Metal vents-
caved in. Bare spots and blisters on deteriorated built-up roofs- bruised or
punctured.
The Kansas Experiment Cont’d
• One of the main goals of the initiative was to reduce
the number of overall warnings, and the number of
spotter activations for marginal, non-damaging
events.
• A process was designed to ascertain a reasonably
accurate estimate of the warning issuances each
office saved by raising the hail criterion to one inch
in diameter.
• Results indicated a 35% reduction in the number of
severe thunderstorm warnings issued in the
experiment.
What Can We Expect?
2006-2008 Severe Thunderstorm Warning Verification
Experiment To Be Expanded
Based on the experience and evaluations of the
Kansas demonstration project, the decision has
been made to expand the initiative to all Central
Region offices, including St. Louis.
The official start date of this change in service is
scheduled for April 1st 2009.
Special Weather Statement
(SPSLSX)
WFO St. Louis will issue Special Weather
Statements with the header outlining the
specific threat for sub-severe t-storms.
So instead of…
…SIGNIFICANT WEATHER ADVISORY…
It will say something like…
…STRONG THUNDERSTORM MOVING TOWARD DOWNTOWN ST. LOUIS…
References
• Gallway, J.G., 1989: The evolution of severe thunderstorm criteria within
the weather service. Weather and Forecasting , 4, December 1989, 585-592
• Hales, J.E., 1987: An examination of the National Weather Service sever
local storm warning program and proposed improvements. NOAA Tech.
Memo. NWS NSSFC-15.
• Hales, J.E., 1988: Improving the watch/warning system through use of
significant event data. Preprints. 15th Conference Severe Local Storms,
AMS.
• Marshall, T.P, R.F. Herzog, S. J. Morrison, and S. R. Smith, 2002: Hail
damage threshold sizes for common roofing materials, Preprints, 21st
Conf. on Severe Local Storms, Amer. Meteor. Soc., 95-98.
• Stephen Yeo, Roy Leigh and Ivan Kuhne,1999: The April 1999 Sydney
Hailstorm, Reprinted from the Natural Hazards Research Centre, Natural
Hazards Quarterly, June 1999, Vol 5 issue 2
Discussion
• What do you believe the public’s perception will be to this
change, if any?
• Do you believe this change in severe hail criteria will cut
back on the number of times you will have to interrupt
regular scheduled broadcasting?
• Do you believe that your station will broadcast the Special
Weather Statement?