**Assessing the Need to Change the Hail Size Warning Criterion to One
Inch:**

**A National Weather Service Field Office Evaluation**

**1. Introduction**

> The mission of the National Weather Service (NWS) requires the
> issuance of warnings alerting for weather conditions that will
> threaten lives or cause damage. Severe thunderstorm warning criteria
> are the same as those that define a thunderstorm as being "severe",
> namely ¾ inch hail or larger, and 50kt (58mph) winds or greater.
>
> The current hail criteria for a Severe Thunderstorm Warning (hail of ¾
> inch or greater) has been in effect since March 1954. This hail size
> apparently was chosen for aviation considerations as the smallest size
> of hailstones that cause significant damage at airplane speeds between
> 200 and 300 mph.
>
> From observations and storm report data, hailstones less than one inch
> in diameter occur much more frequently than larger hailstones, and do
> not cause property damage or serious injuries unless accompanied by
> severe winds. In Kansas during the timeframe January 1, 1950 through
> May 31, 2005 the number of hailstones less than one inch in diameter
> accounted for 40.7% of all hailstone reports. Secondly, only six
> fatalities have been recorded in the United States due to hail. The
> fatalities span three centuries, with one fatality in the 1700's, two
> in the 1800's, and three in the 1900's. These values were acquired
> from NCDC's online Stormdata records.
>
> A field evaluation of the one inch hail criterion for warnings
> occurred from the period March 1 through August 31, 2005. The
> evaluation included the following NWS weather forecast offices
> (WFO's), which handle at least a portion of Kansas: Wichita, Dodge
> City, Goodland, Hastings, Topeka, Pleasant Hill, and Springfield. The
> evaluation included the entire county warning areas for these offices,
> covering all of Kansas, a small portion of eastern Colorado, a portion
> of central and southern Nebraska, and all of western Missouri. In
> terms of external customer input, the focus was on the media outlets
> servicing each WFO and local Emergency Managers.

2\. **Description of Evaluation**

a.  **Purpose**

> The low threshold set for hail has led to excessive warnings in which
> our external users deem as less credible and may lead to complacency
> in regards to the protection of life and property. The complacency
> that occurs with the issuance of a severe thunderstorm warning is the
> driving force behind this evaluation. It is our goal to decrease the
> number of severe thunderstorm warnings based solely on hail, so that
> the media, emergency managers, and the general public react
> accordingly when a severe thunderstorm warning has been issued.
> Basically, at what hailstone size does one react with the appropriate
> call to action to protect them from danger.
>
> When one does an Internet search on hail damage, there are thousands
> of papers and articles on this subject. A common theme when reviewing
> many of these papers is that hail size alone is not the sole cause of
> damage. Hail damage comes from a combination of the **Size**, its
> **Duration**, and the **Velocity**. When one factors out the Duration
> and Velocity and just looks at size alone, only a few papers were
> found.
>
> A very extensive research project was one entitled ["Atlas of the
> Edmonton Tornado and Hailstorm:]{.underline} A Decade of Research by
> Robert B. Charlton, Bradley M. Kachman, Lubomir Wojtiw, Department of
> Earth and Atmospheric Sciences, University of Alberta." In their
> study, they found that one began having hail damage to automobiles
> when the size became "Walnut" or larger. Walnut was defined as about
> 7/8" in diameter. For roof damage, it was larger. This was
> substantiated by Mr. Jim D. Koontz in his paper; ["What are the
> Effects of Hail on Residential Roofing Products?"]{.underline} In his
> paper he tested several different types of roofing material against
> different sizes of hail and differing Terminal Velocities. Once again,
> when one considers free falling hail, it took one inch or larger to
> cause indentations or fractures on any of the roofing material that
> was tested.
>
> Because of these studies, we believe when considering hail and
> factoring out the accompanying wind, one inch is a better criteria for
> the initiation of damage. As a side note, there was a study conducted
> by State Farm Insurance in 2002 where they used steel balls to
> replicate hail falling on vehicles and roofs, and in their test the
> smallest diameter ball they used was 1" inch in diameter. A company
> that pays out millions of dollars each year in weather related claims
> and not even testing what hail could do below 1" is a pretty strong
> testament to the reason for changing the criteria.

b.  **Procedures**

> During the evaluation period, the participating offices increased
> their hail warning criterion to one inch. For hail below one inch
> (that occurs with non-severe winds), other products will be issued, as
> decided by the individual offices. For WFO's Wichita, Springfield,
> Pleasant Hill, a "Significant Weather Advisory" (under SPS header) was
> issued for hail less than one inch, winds less than 50kts (58mph), and
> to give areas of concern a heads up as to what type of hazardous
> weather may be approaching. These products were issued prior to the
> issuance of warnings. However, focus was on the warnings issued while
> in effect. WFO's Dodge City, Goodland, Hastings, and Topeka utilized
> an enhanced Short Term Forecast (STF) to convey storm information
> below the one inch hail criteria and 58 mph winds.

The following **were not** affected by the field evaluation:

1)  All storm reports of large hail that define a severe thunderstorm,
    including ¾ inch hail, were listed in local storm reports (LSR\'s).
    This evaluation did not impact the historical record of hail reports
    of at least ¾ inch in diameter.

2)  Convective watches issued by the Storm Prediction Center continued
    to include the ¾ inch hail criterion that defines a "severe
    thunderstorm".

3)  The wind speed criterion for warnings remained at 50kts (58mph).

The field evaluation focused on the following:

> 1\) Impact on external customers.

2)  Impact on WFO warning decision-making process.

**3. Findings**

a.  **Impacts on External Customers**

> ***[Wichita, KS]{.underline}***
>
> The one inch hail evaluation was driven by the end user of our
> products. The Media and Emergency Management community had stressed
> for years that there is a need to raise the minimum criteria for a
> severe thunderstorm warning based on hail. They have conveyed that it
> will help in many different facets in attempting to illicit a proper
> response from the general public and there hazardous weather
> operations. Over warning will result in complacency, which could
> ultimately result in failure to follow the appropriate safety
> guidelines for a severe thunderstorm warning.
>
> *"Our operations saw a marked decrease in the number of call outs we
> initiate for severe thunderstorm warnings. The new NWS initiative was
> a blessing in that we can dove tail this criterion with our Standard
> Operating Plans and continues to reduce our call outs for incidents
> which present no real threat.*
>
> *The reduction in unnecessary warnings has directly resulted in some
> cost reductions in our operations. We have discussed this with our
> administration and Board of Commissioners and they have indicated
> strong support for continuing this initiative by NWS and local
> Emergency Management"*
>
> ***Jim Schmidt, Director Butler County Emergency Management***
>
> *"I think the one hail initiative is very beneficial. Lower numbers of
> severe thunderstorm warnings, in my opinion, make for more effective
> warnings"*
>
> ***Randall Duncan, Director Sedgwick County Emergency Management***
>
> ![](media/image1.wmf){width="2.751388888888889in"
> height="2.2125in"}Data from an informal survey of Central and South
> Central Kansas show that the general public is in favor of raising the
> hail criteria. Of the respondents, nearly 43% felt that a warning
> should be issued once a hail stone reaches one inch in diameter. This
> is in direct contrast with the Customer Satisfaction Survey of the
> general public, see question below. It has been shown that the wording
> within the Customer Satisfaction survey in regards to one inch was
> very leading. It has been shown that insurance companies do not even
> begin to study hail stone damages until the size reaches one inch in
> diameter. So, to state that penny sized hail without severe wind
> criteria dents vehicles can be misleading. It should also be stated,
> that it is unrealistic to protect vegetation from severe hazardous
> weather. Lastly, the responses offered for the question continues the
> leading trend towards a ¾ inch bias. Selection (a.) below utilizes the
> word "kept" and then mentions no change. Why would anyone that reads
> this question answer anything other than what they are being led
> towards, no change?
>
> (a.) The NWS currently considers a thunderstorm severe when winds are
> at least 58 mph (which can cause large tree limbs to break) and/or
> hail is ¾ inch in diameter (about the size of a penny which can damage
> vegetation, dent vehicles, etc.) or larger.\
> \
> 15.  Do you think the hail criterion should be?
>
> a\. Kept at ¾ inch (no change)\
> b. Smaller than ¾ inch\
> c. Increased to 1 inch (about the size of a quarter)
>
> \*\* It is recommended that this question be disregarded when trying
> to asses the need for changing the hail criteria to one inch in
> diameter.
>
> ![](media/image2.wmf){width="2.838888888888889in"
> height="2.2958333333333334in"}That said, the informal survey also
> shows when the public is more inclined to take precautionary measures
> once the hail stone size reaches one inch in diameter. Nearly 30% of
> those asked stated that they would move away from windows once the
> hail stone size increased to one inch in diameter. Surprisingly,
> nearly 44% stated that they would wait until the hail stone size
> reached 1.75 inches in diameter. If the goal of a severe thunderstorm
> warning is to illicit a response, this data shows that our warnings
> are not successful in getting the public to take action. This question
> should be a true measure of whether or not our severe thunderstorm
> warning criteria based on hail stone size should be raised. Our
> mission is the protection of life and property through the issuance of
> warnings. However, if our warnings for severe thunderstorms are not
> being paid attention to and no one is seeking appropriate shelter,
> then we need to change our criteria to match the changes needed to
> illicit an appropriate safety action.
>
> ***[Topeka, KS]{.underline}***
>
> Discussion with meteorologists from our two TV stations with a weather
> staff indicated that the meteorologists at both stations liked the
> experiment. Overall, they felt that there were fewer interruptions to
> programming for minor events, and that this resulted in more attention
> paid to events with more significant severe weather.
>
> ***[Pleasant Hill, MO]{.underline}***

-   What do our users think?

    -   Questionnaire sent to all media and EM's (223 separate contacts)

    -   67 respondents (30%)

    -   Four questions were asked (results listed):

> 1\. What size hail do you feel would cause damage to property other
> than crops? (67 responses: 24 media, 43 EMD's)
>
> ![](media/image3.wmf)  
>
> ![](media/image4.wmf)
>
> ![](media/image6.wmf)2. For what size hail would you want to receive a
> Severe Thunderstorm warning? (Remember, this is only based on hail and
> not wind or tornadoes) (64 responses: 23 media, 41 EMD's)
>
>  
>
> ![](media/image7.wmf)
>
> 3\. Based on your experience in previous years, compare services
> provided in NWS warnings for hail in 2005: (66 responses: 24 media, 42
> EMD's)
>
> ![](media/image9.wmf)
>
> ![](media/image10.wmf)
>
> *4.* Based on your experience in previous years, compare services
> provided in NWS warnings for thunderstorm wind in 2005: (66 responses:
> 24 media, 42 EMD's)
>
> ![](media/image12.wmf)
>
> ![](media/image13.wmf)

-   Feedback indicates strong signal for improved services for hail

-   91% of respondents indicated improved or no change noted to services
    for hail.

-   50% of respondents indicated perceived improved services using the
    larger hail criterion.

-   9% of respondents indicated perceived decrease in services using the
    larger hail criterion.

-   40% of respondents listed quarter hail or larger as an effective
    threshold for a warning.

-   Compared to just 25% of respondents that listed penny hail or larger
    as an effective threshold for a warning.

> There was nominal negative feedback on using larger criterion this
> season. While several positive comments as seen below.
>
> ***Customer Testimonials***
>
> *"Service from the NWS has been excellent in our jurisdiction and we
> are comfortable with the current experimental hail warning size."
> **Tim Mosier, EMD Pettis County, MO***
>
> *"Warnings are good -- I think before, the warnings were issued for
> the smallest of hail. I've seen a lot less warnings issued this year
> for 'minimally' severe storms with extremely small hail size. This,
> IMO, is a good improvement!"*
>
> ***Eric Aldrich, meteorologist, KOMU-TV Columbia, MO***
>
> *"The warnings are taken more seriously by the general public.
> Although this change has reduced the number of severer thunderstorm
> warnings issued during the severe weather "The forecasting office in
> Pleasant Hill has changed their warning criteria for hail this season
> to Quarter size before issuing a severe thunder season, now when a
> warning is issued, it is unique enough people take it more seriously.
> The forecast office in Pleasant Hill has also spent a lot of time
> explaining the hail warning change to the local emergency Managers/
> local weather meteorologists so we realize when the NWS issues a
> warning for hail that serious damage is likely - not just another
> warning that usually causes minor or no damage and people then ask why
> was a warning issued for this storm. This is the cause for people to
> eventually ignore all warnings."*
>
> ***Mark Widner, Independence, MO EMD***
>
> *"The reduction in number of warnings did not hamper our efforts in
> any way, in our opinion.  I had no complaints about not receiving
> warnings on marginal storms from anyone\...although I must admit that
> we did do a lot of radar reports on \'non-severe\' storms, utilizing
> your \'significant weather alerts\'.  I\'d like to see that continue
> as it did this year\...gave us a \'head start\' on a handful of storms
> that ultimately became severe\...and also kept listeners informed on
> storms that were clearly of concern, but did not prompt a warning." *
>
> ***Jeff Elliott, KFMZ/KZBK-FM, Brookfield, MO ***
>
> ***[Dodge City, KS]{.underline}***
>
> During the regular spring severe weather preparedness presentations
> and during other civic group meetings, the issue of changing severe
> weather criteria was discussed. An extreme high percentage (99%) was
> not in favor of any changes. When the issue of changing criteria for
> hail to one inch was discussed, many times folks had very opinionated
> responses.
>
> In knowing that crops could not be protected, folks still wanted to
> know about the smaller hail (\<1"). Quite a few times people expressed
> interest in warnings based on hail size less than ¾ of an inch.
>
> Were services to our customers improved by going to the one inch
> criterion? Probably not, again based on what the public of our CWA
> wants. A few more Short Term Forecasts were issued, but not a
> significant increase.
>
> ***[Hastings, KS]{.underline}***
>
> What size hail do you feel would cause damage to property other than
> crops? (Results from Media and EMs)
>
> 0 -- Pea
>
> 1 -- Penny
>
> 2 -- Nickel
>
> 6 -- Quarter
>
> 4 -- Golf Ball
>
> 0 -- Tennis Ball
>
> For what size hail would you want to receive a Severe Thunderstorm
> warning? (Remember, this is only based on hail and not wind or
> tornadoes)
>
> 0 -- Pea
>
> 1 -- Penny
>
> 2 -- Nickel
>
> 6 -- Quarter
>
> 4 -- Golf Ball
>
> 0 -- Tennis Ball
>
> Based on your experience in previous years, compare services provided
> in NWS warnings for hail in 2005.
>
> 0 -- Decreased Significantly
>
> 2 -- Decreased Slightly
>
> 5 -- No Change
>
> 5 -- Increased Slightly
>
> 0 -- Increased Significantly
>
> Based on your experience in previous years, compare services provided
> in NWS warnings for thunderstorm wind in 2005.
>
> 0 -- Decreased Significantly
>
> 3 -- Decreased Slightly
>
> 3 -- No Change
>
> 5 -- Increased Slightly
>
> 1 -- Increased Significantly
>
> ***[Goodland, KS]{.underline}***
>
> Emergency managers which responded to four survey questions sent in
> late August commented that warning services in 2005 were either
> slightly or greatly improved over previous years. One respondent
> stated there were no changes to services in 2005. No respondent
> mentioned a decrease in service. Although the survey question was
> written intentionally so as not to lead to a particular response...it
> is difficult to determine if there truly was any bias. The mere fact
> that someone asked for their opinion regarding quality of service
> could elicit a positive response in and of itself, and result in a
> survey response as an improvement in service. When asked about the
> size of hail they feel should warrant a Severe Thunderstorm Warning,
> the responses ranged from "pea" (0.25") to "quarter" (1.00") with most
> responding "nickel" (0.88"). When asked what size hail they felt would
> cause damage to property other than crops, the survey responded with
> values ranging from "penny" (0.75") to golf ball (1.75") with most
> responding nickel (0.88") to quarter (1.00").

b.  **Impact on Internal Operations**

> ***[Wichita, KS]{.underline}***
>
> It was found that the services provided throughout the one inch hail
> evaluations improved immensely. In years past, a generic STF would be
> issued covering a one to two hour time frame alerting users of what
> they may expect in terms of hazardous weather. However, with the
> advent of the evaluation, new products were utilized to convey
> hazardous weather below the one inch and 58 mph warning criteria. One
> of those products was a Significant Weather Advisory. This product was
> issued under the auspice of a Special Weather Statement (SPS).
> Forecasters were able to utilize WARNGEN software to quickly and
> accurately create the product for issuance. The SPS header allowed for
> broader dissemination to the end user via television and pagers.
> Depending on the STAR system on local cable companies determined
> whether or not the SPS would crawl across the screen on The Weather
> Channel. The Significant Weather Advisory brought about new and more
> detailed information to the customers that once were not available.
> Since the Significant Weather Advisory was issued for criteria less
> than warning criteria, the public was more aware of what each storm
> was producing and where it was moving. The Significant Weather
> Advisory was also issued to give counties downstream from severe
> storms a heads up as to what they can expect hazardous weather wise,
> which in turn gives them ample time to take precautionary measures.
>
> The staff at WFO Wichita felt that the issuance of Significant Weather
> Advisories help to take the pressure off of the warning decision
> process (WDP) in dealing with marginal storms. (i.e. hail varying
> between ½ and ¾ inch). The staff at Wichita also felt that they were
> able to allot more time to storms that possessed stronger
> characteristics which may lead to larger hail. Per one forecaster
> regarding the issuance of Significant Weather Advisories;
>
> *"This allows the Warning Decision Maker (WDM) to put most time/effort
> into the most significant storms.  This eliminates the fretting over
> marginal storms that might produce dime hail when there are more
> threatening storms to watch."*
>
> ***Lead Forecaster WFO Wichita***
>
> The staff at WFO Wichita was in full support of increasing the minimum
> criteria for hail to one inch in the issuance of a severe thunderstorm
> warning.
>
> ***[Topeka, KS]{.underline}***
>
> 1\. Was the Warning Decision Process [easier]{.underline} or
> [harder]{.underline} during the one inch hail evaluation?       \
> \
> **Summary**: The answers varied from harder to easier to the same.
> Most felt they had some difficulties with the new criteria since they
> were used to issuing based on criteria for ¾" hail.
>
> 2\. How did the issuance of enhanced STF's affect your Warning
> Decision Process?\
> \
> **Summary**: Once again the answers varied...but for the most part,
> the forecasters liked the Enhanced Short Term Forecast way of handling
> storms expected to produce hail \< one inch.
>
> 3\. Do you think the one inch hail evaluation made your duties in
> severe weather more or less challenging? (this goes beyond the warning
> decision process into overall severe weather operations).      Why?
>
> **Summary**: A lot of the responses didn't really address the overall
> challenge of the weather event, but reiterated their experiences
> making warning decisions. Several mentioned that in busy events, they
> were less worried about the smaller hail and thus able to concentrate
> on more significant storms, and this helped.
>
> As a HMT, do you think the one inch hail evaluation made your duties
> in severe weather more or less challenging?    
>
> **Summary**: As you can see, while fewer warnings were issued, the
> HMTs did not feel that this made severe weather overall less
> challenging. Enough calls were made and LSRs issued that workload did
> not change a lot.
>
> ***[Pleasant Hill, MO]{.underline}***
>
> Was the warning decision process (WDP) easier or harder during the one
> inch hail evaluation? Why did you answer as you did?

-   6 out of 8 respondents felt the warning decision process was easier
    this year.

-   Most common reasons why they felt the process was easier:

    -   Storms that produce 1" or bigger hail tend to have more
        persistent updrafts, many times accompanied by at least weak
        rotation.

    -   Forecasters could 'baseline' the environment with the penny
        report, then be in better shape to anticipate hail 1" or larger.

    -   Storm structure signatures (tight reflectivity gradient, 50 dBz
        to a certain height) much better defined with respect to the
        larger hail criterion.

    -   Remaining two respondents felt their WDP was about the same as
        previous years.

-   Reasons for their responses were:

    -   Issuing a significant weather advisory took about the same about
        of interrogation and 'button pushing" in Warngen as issuing a
        warning

    -   Storms of marginal strength still required constant surveillance
        for intensification.

> How did the issuance of significant weather advisories affect your
> WDP?

-   6 out of 8 respondents felt the SWA helped in their warning decision
    process.

-   Most common reasons why they felt the process was easier:

    -   Helped with a "step-up" process in thinking.

    -   Instead of sitting on a storm, felt they could issue a
        value-added product to give a better "heads-up" to those areas
        downstream of a storm. Previous use of general nowcasts with
        storm-specific nowcasts made it more difficult to convey the
        area of greatest concern.

    -   SWA could be issued for a wide range of convection -- from pea
        hail and gusts to 35 mph to nickel size hail and 55 mph winds.

    -   Several forecasters said that they liked the SWA.

    -   Remaining two respondents felt there was nominal effect on their
        WDP.

-   Reasons for their responses were:

    -   Issuing a significant weather advisory took about the same about
        of work as issuing a warning or nowcast.

    -   Storms of marginal strength still required constant surveillance
        for intensification.

> Do you think the one inch hail evaluation made your duties during
> severe weather more or less challenging? Why did you answer as you
> did?

-   6 out of 8 respondents felt their duties were less challenging.

-   Most common reasons why they felt the process was easier:

    -   They could focus their attention on the higher impact events
        (storms of greater severity and greater threat).

    -   They felt they could publish more information on marginal storms
        than they could in the past.

    ```{=html}
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    ```
    -   Remaining two respondents felt their duties were similar to when
        we used the smaller criterion.

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-   Reasons for their responses were:

    -   Issuing a significant weather advisory took about the same about
        of work as issuing a warning or nowcast.

    -   Storms of marginal strength still required constant surveillance
        for intensification.

> Was there confusion over what product(s) to issue?

-   Not really. SPS (significant weather alert) was used for more than
    just penny/nickel hail, so this vehicle allowed the staff to
    actually publish more information this year than in year's past. We
    did add an expiration time to the SPS to alleviate concern over when
    the product terminated.

> Was there a duplication of products when significant weather
> advisories had to be upgraded to warnings?

-   Yes, but in the same vein as when an SVR is upgraded to a TOR. Staff
    used text within the SPS to advise areas that a warning may be
    issued shortly.

> ***[Dodge City, KS]{.underline}***
>
> In polling the staff, they did not find stress level decreasing at
> all. In fact, sometimes there was more stress in deciding if the hail
> was 1 inch or less than 1 inch. The majority of the warnings for
> marginal storms stated "hail up to the size of quarters is possible".
>
> Did the number of warnings decrease this year compared to the same
> period in 2004? Yes. But, there were perhaps only 20 instances when
> radar meteorologists did not warn for storms that they thought might
> be producing hail less than 1 inch (\<5% decrease). The number of
> warnings issued each year is highly variable in any case. The
> following shows the number of warnings and events over the last 12
> years:
>
> ![](media/image15.png){width="1.73125in" height="1.875in"}
>
> ![](media/image16.wmf){width="5.245833333333334in"
> height="3.045138888888889in"}
>
> Finally, from January 1, 2005 through September 12, 2005 there were
> 163 reports of hail less than 1 inch in diameter reported and 225
> reports of hail one inch or greater.

***[Hastings, NE]{.underline}***

> WFO Hastings used the enhanced STF process. The staff didn't have any
> problems deciding which product to issue or when to issue. In
> Nebraska, the STF is transmitted on the LETS system. I am not sure
> about the Kansas system. All media outlets obtain the STF through
> their wire services. Radio stations in our CWA do use the STF and
> several broadcast them as written. Our office has mixed feelings on
> the use of the significant weather advisory. This would be an
> additional product our users would have to look for and might increase
> the confusion of what gets issued when and under what heading for both
> our user and staff. The STF for showers and thunderstorms up to those
> producing small hail and some wind, SPS for bigger hail and stronger
> winds up to severe criteria, and finally an SVR for severe events. Our
> customers have liked the concept of keep it simple.
>
> This year's severe weather season has been somewhat unusual. Most of
> the hail events we have had were for hail in excess of 1 ½ inches.
> Only a few cases occurred where there might have been a concern
> between ¾ and 1 inch hail and there was only a small percentage of
> times where warnings were not issued because of the higher criteria.
> The staff continued to communicate with the counties during
> thunderstorm events to determine if hail was occurring and its size.
> We didn't get a good test of the new criteria, the processes and the
> affects to our customers.
>
> Summary of staff input.

1.  Was the WDP easier or harder during the one inch hail evaluation and
    why?

> The overall response was that there wasn't much change to the warning
> process. It was noted there was very little to help decipher between a
> storm producing ¾ inch hail and one that was producing 1 inch hail.
> There were some adjustments to thresholds, but this had little impact
> on the WDP. Some of the newer forecasters had some adjustment period
> at the beginning of the season, but were able to quickly change their
> thought process to the 1 inch criteria. I liked how one forecaster put
> it, "The decision process was the same. The storms needed to be
> evaluated for severe potential"

2.  How did the issuance of enhanced STFs affect your WDP?

> The responses showed there was little affect on the warning process. A
> few liked the ability to cover marginal events with the STF. Another
> forecaster liked the ability to "ramp up" to the warning process.
> Having the higher criteria for warnings allowed for more time to
> "talk" about the storms rather than going from nothing to warning.

3.  Do you think the one inch hail evaluation made your duties in severe
    weather more or less challenging?

> This was pretty much a wash. The majority of the staff felt that the
> change in criteria was too small to make any impact on the duties. If
> there was a decrease in warnings issued, it was offset by the issuance
> of STFs. A couple did comment that they had a small amount of extra
> time to monitor the storm. Storms need to be monitored and evaluated
> which is all part of the severe weather duties.
>
> ***[Goodland, KS]{.underline}***
>
> During the one-inch hail initiative, WFO Goodland issued Short-Term
> Forecasts (NOWcasts) for events below severe criteria, and either
> Severe Thunderstorm or Tornado Warnings (and their followup Severe
> Weather Statements) for events that met or exceeded severe criteria.
> Our philosophy was that NWS already had a vehicle for discussing
> events considered non-severe (NOW), so WFO GLD chose not to issue
> Special Weather Statements (SPS) for events considered "almost
> severe". It was felt that an added layer of products for marginal
> situations (SPS) would only serve to confuse the end user, since that
> would lead to situations where a NOW is in effect for general
> convection, an SPS for convection not quite severe, then either an SVR
> or a TOR for severe storms.
>
> For events below severe criteria, the office issued NOWs via the
> WarnGen application, which gave specific storm movement and timing
> based on radar. This provided more detailed information for marginal
> storms versus a more general statement that would be hand-written
> without using WarnGen. Our office estimates that perhaps 10 percent
> fewer warnings were issued in 2005 because of the hail threshold of
> 1.00" versus 0.75". Part of the reason for such a low reduction in the
> number of warnings is the fact that 2005 brought more large hail
> events in the Goodland area of responsibility when compared to the
> last several years. In 2005, approximately 60 percent of hail events
> were over 1-inch, while in 2003, only 40 percent of hail events were
> over 1-inch. Many storms this year were prolific hail producers rather
> than marginal events. This is attributed to the happenstance of
> environmental conditions during severe weather episodes this year.
> Another contributor to the estimated small reduction in warnings
> involves the fact that the High Plains environment is typically
> characterized by a drier lower atmosphere which leads to the potential
> for more damaging wind events as compared with offices further east.
> Therefore, the warning forecaster in many instances issued warnings
> for storms that met the wind criterion for severe, while at the same
> time the hail threat was marginal (0.75" to 0.88" diameter).
>
> The estimate of 10 percent fewer warnings is based on actual 2005
> statistics. Of the 369 Severe Thunderstorm Warnings that were issued,
> only 52 of those warnings (14%) verified exclusively by a report of
> 0.75" or 0.88" hail. All other warnings issued verified by hail
> meeting or exceeding 1.00" or by wind damage or a report of wind speed
> of 58 mph or greater. This means that in a perfect world, if every
> radar operator were able to discriminate every thunderstorm that did
> not reach the 1.00" hail threshold, that 14% fewer warnings would
> occur. Since this is not a perfect world and radar operators are not
> able to discriminate with such accuracy, a number of 10% fewer
> warnings seem more plausible.
>
> The survey of local emergency managers indicated no perception of
> diminishing service this year, and to the contrary, an increase in
> service was mentioned in almost all cases. This would seem to indicate
> that fewer warnings from NWS are not necessarily perceived as a
> decrease in service from a customer viewpoint.
>
> Although not specifically asked in a survey, several informal contacts
> with emergency managers indicated a perception that warnings in 2005
> would be a bit more "meaty". In other words, they perceived with the
> hail threshold increased to 1.00" that if a warning were to be issued
> it would have more "merit". In addition, some emergency managers
> stated they take less action for severe thunderstorm warnings if the
> wording is not for higher-end hail and wind. But this was an extremely
> small sample of customers.

# When the staff was asked if the warning decision process was easier or harder during the 1-inch hail initiative, 4 answered easier, 2 answered harder, and 2 answered about the same. Those replying the process was harder indicated the difficulty in differentiating the difference on radar between 0.75" and 1.00" diameter hail, especially after many years of mentally correlating radar signatures with 0.75" hail. Those responding easier stated it is rather easy to get 0.75" hail on the High Plains, and it allowed more time to watch a storm to determine if it would become more intense without worrying about a few stray penny hail reports.

> When the staff was asked whether duties were more or less challenging
> during the evaluation, 4 answered no change, 2 responded more
> challenging and 2 responded less challenging. The staff members
> responding "no change" mentioned although there may fewer warnings,
> each storm must still be thoroughly interrogated, and there are other
> threats including strong wind and tornadoes which also need to be
> considered for each decision. Those answering "more challenging"
> mentioned the difficulty in distinguishing between 0.75" and 1.00"
> diameter hail given the current technology.

***[Springfield, MO]{.underline}***

A.  Improved services through more effective products ( stf, sps, and
    warnings)

    a.  Issues concerning the issuance of the three products

        i.  Confusion was not a problem, however, there was some
            question whether to use the SPS as a "ramp up" product, or
            skip the SPS and go right to the SVR.

    b.  Was there a duplication of products when significant weather
        advisories had to be upgraded to warnings.

        i.  Yes there were several instances when storms continued to
            intensify after an advisory was issued requiring an upgrade
            to a warning. This is not unlike upgrading a NOW to a
            warning. Many issued long term SPS for a county(ies), then
            interrogated reflectivity further for advisory vs. severe
            pulse storms.

    c.  Was the warning decision process (WDP) easier or harder during
        the one inch hail evaluation? Why?

        i.  Many staff members thought the process was easier. This was
            especially true for the many borderline events that did not
            look as strong yet produced nickel-sized hail, while other
            storms appeared to be marginally severe but only produced
            peas or dime size hail. Storms expected to produce one inch
            hail were consistently stronger in appearance, thus the one
            inch criteria seemed to cut out a lot of the guessing game
            and the borderline severe storm cases.

        ii. Difficulty did increase once the summer pulse season began
            as radar thresholds for advisories vs warnings were less
            clear with rapidly developing storms, especially when
            attempting to determine when maximum intensity was reached.

    d.  How did the issuance of significant weather advisories affect
        your WDP.

        i.  Advisories were treated as seriously as warnings. The same
            attention and storm interrogation were needed to determine
            the extent of the hail size.

        ii. Do you think the one inch hail evaluation made your duties
            in severe weather more or less challenging?

        iii. **Nearly neutral - l**ess challenging for differentiating
             between severe and non-severe storms, but more challenging
             when differentiating between SPS-worthy and non-SPS-worthy
             storms.

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    a.  **Science Behind the Warning Decision Process**

> ***[Topeka, KS]{.underline}***

1.  I think it was a little harder to adjust to the larger hail size and
    verifying it because we have been used to VIL of day or reflectivity
    structure relevant to 3/4 inch hail. Once I adjusted my thinking and
    techniques the 1 inch hail was about the same process as the 3/4
    inch hail.

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2.  Initially it was harder I thought.  Looking for some correlation
    with a \"VIL of the day\".   The idea of using the 40 to 45
    percentile seemed to have worked and gave some confidence later in
    the test. 

3.  Easier to warn for a 1.0 inch or greater hail threat.  I was more
    confident in determining updrafts capable of producing hail 1.0 inch
    or greater based on Z \>= 60dBZ @ the height of the -20 C
    temperature. Of course these values would be lower with any updraft
    containing a mid-level mesocyclone.  Harder to warn for wind events.
    Some times base V at the height of the 0.5 degree elevation angle
    may not show very strong radial velocities (especially if the
    sampled radial velocity bins are perpendicular to the radar beam).
    Therefore when using the old 0.75\" hail criteria you may have had a
    SVR out on a marginally severe storm that produced a
    downdraft/outflow winds in excess of 58 MPH.

4.  It was a little harder. I could never get a good feel from typical
    storm structure/algorithm output (e.g. VIL of the hour/day) whether
    to issue in marginal situations based on the new criteria. It is
    like there is always a gray area when issuing in marginal
    situations. With the new criteria, the gray area is larger\...does
    that make sense? Of course, once we start getting some ground truth,
    that made it much easier.

5.  Harder. Determining what reflectivity/rotation/VIL levels produced
    1\" hail was more of a challenge.

> ***[Pleasant Hill, MO]{.underline}***
>
> What did you use to determine hail size?

-   Staff relied heavily on hail size nomogram, developed by WFO Des
    Moines.

-   Nomogram correlates 50 dBz height and freezing level, for
    anticipated hail size.

-   Process worked out well this year.

-   Journey forecaster also compiling hail climatology and working on
    local hail study.

> ***[Dodge City, KS]{.underline}***
>
> Incorporated a change in the warning process, adding a few thousand
> feet to the height of the 50dbz threshold which seemed to be
> scientifically sound. Did it help? It's hard to tell.
>
> ***[Hastings, NE]{.underline}***
>
> For initial parameters, our office utilized the hail size nomogram and
> made adjustments to dbz heights as needed as the event progressed.
>
> ***[Goodland, KS]{.underline}***
>
> Staff members agreed that given the radar technology and scientific
> theory available, it is extremely difficult to differentiate between
> 0.75" diameter vs. 1.00" diameter hail from a warning standpoint. The
> staff used various methods for warning issuance in regard to hail. One
> method used was whether or not persistent updraft rotation was
> present, and if so, that storm was assumed to be more likely to
> produce larger hail. The presence of a Three-Body Scatter Spike (TBSS)
> would also imply large hail. Most of the staff utilized radar
> reflectivity data to locate the height of a 50 dBZ core. Most staff
> members used the height of the -20 C temperature and made mental
> adjustments as to how high the core needed to be in relation to the
> altitude of the -20 C temperature, but the "mental adjustments"
> varied. No nomograms were used.
>
> ***[Springfield, MO]{.underline}***
>
> Nearly all staff members adjusted to higher 50-60 bdz thresholds, with
> most techniques now factoring in AWOC trainings on -20 ^o^C levels and
> effects of mesocyclones on hail size.

**4. Conclusion**

> A field evaluation, including seven Central Region WFO's, occurred
> during the summer of 2005 to evaluate the impact, positive or
> negative, of changing the hail warning criterion to one inch. The
> evaluation was not focused on changing the criteria that define a
> severe thunderstorm, or to provide clear evidence of the need to
> change the hail warning criterion. Instead, the focus was on gathering
> data, internally and externally, that provide some objective guidance
> on the direction WFO's should follow to meet the needs of the NWS
> mission as well as those of external customers, which include the
> general public. Of note, not one complaint was received throughout the
> evaluation.
>
> The One Inch Hail Evaluation was deemed successful by the NWS offices,
> Media, and Emergency Management. Thus, the following recommendations
> are being offered.
>
> Recommendation:
>
> Continue One Inch Hail Evaluation for all offices serving the state of
> Kansas.
>
> Recommendation
>
> Expand One Inch hail Evaluation to all offices within the Central
> Region of the NWS and offer participation to any other office outside
> of Central Region.

Recommendation

> Conduct the evaluation with better organization, by creating an
> intranet post-event feedback form and develop an in depth survey to
> gauge staff and public perception.
>
> Recommendation
>
> Continue to issue hail events less than one inch in Local Storm
> Reports (LSR) and StormData.

Recommendation

> Create a standardized method for verification so that metrics may be
> developed to determine forecasters skill in identifying storms
> producing hail of one inch or larger.

Recommendation

> For all participating office to issue Significant Weather Advisories
> for criteria less than one inch and to provide a "heads up" to
> counties for approaching storms. The best scenario would be to create
> a new PIL for the Significant Weather advisory for easy retrieval of
> media outlets to place maps in corner of television broadcast.

Recommendation

> Enhance communications with partner agencies like FAA and DOD to
> solicit feedback on criteria change.