One-Inch Hail Initiative

1. One-Inch Hail Initiative Spring Media Seminar March 7th 2009 Jim Sieveking National Weather Service WFO St. Louis, Missouri

2. 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.

3. 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

4. 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.

5. 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.

6. 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

7. 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.

8. 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.

9. What Can We Expect? 2006-2008 Severe Thunderstorm Warning Verification

10. 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.

11. 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…

12. 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

13. 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?