1. Introduction
    This paper addresses the current effectiveness of Tornado Warnings issued by the National Weather Service (NWS) in Melbourne, FL.  Tornado Warnings (TORs) are issued as a direct result of agency mission to protect the life and property of East Central Florida (ECFL) residents, as well as to minimize the impacts of adverse weather on area commerce whenever it is determined that a tornado is occurring or is imminent.  Over the past decade, the heightened skill of warning meteorologists and the advent of the WSR-88D Doppler radar network have certainly improved the overall probability of tornado detection.  Through applied research, training, and advanced technology, ECFL warning meteorologists are now equipped to perform a more thorough assessment of local tornado threat situations than ever before.  Yet, except for simple format changes, the TOR product (the information vehicle itself) has evolved very little and has not facilitated the inclusion of enhanced  information.  The result is a diluted warning text whose effectiveness is hindered when attempting to invoke a desired public response.

    It is believed that the effectiveness of the TOR product can be increased by consistently conveying more about what is known (or not known) with respect to any specific tornado threat situation.  This is especially helpful within peninsular Florida due to the diversity of  tornado environments (cool season/high-shear, warm season/low-shear, and tropical cyclone) when coupled with the knowledge of the general behavior of tornadoes within each  environment.  The aim is directed at increasing the effectiveness of the initial warning through a situational approach  based on forecaster confidence but classified by tornado environment.  Care is taken to never minimize a tornado threat, only to properly define it.  Through automated (software) means of TOR preparation, product-to-product consistency is achieved.  More so, automation has enabled the ability to provide situational Calls-to-Action (CTAs) which speak to the context of the event according to the specific location, timing, or societal demographics.  Justification of this endeavor  stems  from  the post  disaster survey report (NOAA/NWS, 1998) of the ECFL Tornado Outbreak where warning performance statistics were declared as excellent. Nonetheless, forty two people died.  Although many factors contributed to the high casualty count, NWS Melbourne has identified the effectiveness of the TOR warning text as an area where local service improvement can be made.  Valuable post-event insight has been incorporated into both situational definitions and associated CTAs to create improved warnings.

2. Background
    Application of the situational approach was first adopted at NWS Tulsa, OK, yielding positive results (Smith and Piltz, 1999) and serves as the project cornerstone.  Their approach defines tornado warning situations based on forecaster confidence.  Warning confidence is typically obtained via Doppler radar and/or storm spotters.  Based on the informational mix, the situation is then identified with specific terminology used within the body of the warning to include "Doppler radar indicated a developing tornado..." or "Doppler radar indicated a severe thunderstorm capable of producing a tornado..." in situations using radar information alone (absent spotter confirmation).  In situations when spotter confirmation was available, the phrase "Storm spotters reported a tornado..." was used.  This terminology is agreeable with NWS Southern Region Operations Manual Letter  S-8-98 policy (ROML, 1998).

    Although threat assessment based completely on forecaster confidence represents notable improvement, one variation is to first consider the environment.  That is, to factor in the character of tornado (potential worst case) that the current environment can support.  In doing so, it is then possible to situationally differentiate a TOR issued for a landfalling summertime waterspout (based on a confirmed report) from a TOR issued for a strong/violent tornado in the spring (based on radar data alone).  Obviously, the threat to life and property is vastly greater in the latter situation despite the apparent lower confidence of "tornado" existence.  It is the ability to properly convey this difference, without compromising safety, where public service can be greatly improved.  Until now this differentiation was rarely reflected within the actual warning text and caused most TOR products, regardless of the situation, to be generically the same; sometimes too alarming and other times not urgent enough.  Improvement is realized by consistently employing a library of terms and phrases within the warning product according to pre-defined ECFL Tornado Warning Situations.

3. ECFL Tornado Situations
    Smith and Piltz (1999) defined four tornado warning situations based on the mixture of radar and storm spotter information to obtain the degree of confidence of tornado existence and then associated the situation to prescribed verbiage as dictated by NWS Southern Region policy.  NWS Melbourne, then, meshed these with peninsular Florida tornado environments to yield the following ECFL Tornado Situations:

Cool Season/High Shear Environment
  1. Radar Detected Weak to Moderate Mesocyclone with
      Tornadic Potential...T/HS1-Lower Confidence
  2. Radar Detected Strong Mesocyclone with Manually-
      Identified Tornado Vortex Signature(TVS)...T/HS2-
      Medium Confidence
  3. Radar Detected Mesocyclone with Confirming Storm
      Spotter Report...T/HS3-Higher Confidence
  4. Reliable and Timely Storm Spotter Report without
      Supporting Radar Signature...T/HS4-Reactive

Warm Season/Low Shear Environment
  5. Radar Detected Low-level Weak Mesocyclone with
      Tornadic Potential Moving along a Boundary...T/LS1-
      Lower Confidence
  6. Reliable and Timely Storm Spotter Report of a
      Potential Landfalling Waterspout...T/LS2-Reactive
  7. Reliable and Timely Storm Spotter Report  near a
      Radar Detected Outflow Boundary (gustnado)...OR...
      near Rapid Echo Growth Area along an Existing
      Boundary (landspout)...T/LS3-Higher Confidence
  8. Reliable and Timely Storm Spotter Report without
      Supporting Radar Signature...T/LS4-Reactive

Tropical Cyclone Environment
  9. Radar Detected Cell with Persistent Gate-to-Gate
      Low-Level Shear of at Least 0.010 /s having Tornadic
      Potential...T/TC1-Lower Confidence
10. Radar Detected Cell with Persistent Gate-to-Gate
      Low-Level Shear of at Least 0.025 /s or Strong
      Mesocyclone with TVS Signature having Tornadic
      Potential...T/TC2- Medium Confidence
11. Reliable and Timely Storm Spotter Report with
      Supporting Radar Signature...T/TC3-Higher
      Confidence
12. Reliable and Timely Storm Spotter Report without
      Supporting Radar Signature...same as T/HS4-Reactive
 

    Cool season situations represent those tornado scenarios possessing  the greatest potential threat to life and property.  Since the environment is marked by high shear (and sufficient buoyancy), tornadoes that form may become strong/violent.  They are typically  associated with a parent mesocyclone and can remain in contact with the ground for many miles and be fast moving.  The event can affect large areas with the ability for multiple tornadoes to occur at once and/or individual storms to produce several tornadoes.  There is also the danger of nighttime occurrence.

    During cool season situations, wording within the warning text becomes more emphatic with increasing confidence as the potential negative impact for playing down any threat is too great.  If the storm is well-sampled, radar data alone can offer reasonable means of assessing a storm's ability to produce a tornado.  Such was the case during the outbreak of 22-23 February 1998 (Sharp et al., 1998).  Figure 1 represents the information that is consistently included during cases when a strong mesocyclone is detected by radar with a manually identified TVS and is similar to the wording depicted by Smith and Piltz (1999) for parallel Oklahoma situations.

    Contrastingly, warm season situations represent those tornado scenarios possessing localized and brief potential threat.  The environment is marked by high instability but low shear meaning that most tornadoes are not associated with a storm having a rotating updraft.  However, occasionally a low-level weak mesocyclone may organize within storms that form and propagate along a reinforced low-level boundary (such as the sea breeze).  If mid-level mesocyclones are forming, then high shear situations should be used.  For pulse-type storms, movement is much slower and there is an obvious diurnal dependency, maximizing during the time of peak heating.  In the absence of environmental shear, boundaries play an integral role, serving as sources of concentrated vorticity for updrafts to tilt and stretch, as focusing mechanisms for enhanced lift, and as markers of turbulent outflows.  Tornadoes are typically very short-lived and short-tracked and usually of F0 intensity.  However, the rare F1 can occur.  Landfalling waterspouts fall into this category.  Not all landfalling spouts are oceanic "fair weather" vortices.  Some form over the intracoastal waterways, swamps, and large lakes and are associated with precipitating cloud.  Although localized and brief, the threat is legitimate.  In an extreme case in 1996, a large spout formed over north Lake Okeechobee which moved onshore to destroy an occupied travel trailer, resulting in one fatality (Storm Data, 1996).

    Unique wording within the warning text focuses on the threat being brief and localized (Fig. 2).  The tornado threat is very real for those in close proximity of the original warned location but drops off significantly with increasing distance.  Emphatic language is reduced as compared to cool season situations.  The term "landfalling waterspout" (not "tornado") is used whenever that variety of vortex is associated with a non-mesocyclonic storm (Fig. 2).  Due to radar sampling limitations, most warm season tornadoes go undetected and the warning meteorologist must rely on reports from storm spotters, often placing them in a reactive role.  Reliable reports offer high confidence of existence, but the issue of timeliness is critical due to the shortened life span of the hazard.  Therefore, the ability to highlight the potential for reoccurrence is available (i.e., for landspout situations).

    The tropical cyclone tornado environment (Florida) is, in part, dependent on the track and size of the tropical cyclone with tornado occurrence historically clustered within the right front quadrant  and most often within the outer spiral rainbands  (Spratt et al., 1997). Whether hybrid or purely tropical in character, north to northeast moving tropical cyclones over the east Gulf of Mexico are more apt to produce multiple tornadoes over Florida (Hagemeyer, 1995).  The threat is often manifested in families of short-lived F0 or F1 tornadoes that are either spatially or temporally removed from the traditional threats of hurricane wind and surge.  The tornadoes may occur ahead of the approaching cyclone (Opal 1995), as the cyclone exits (Erin 1995), or removed from the cyclone center (Gordon 1994).  Occasionally F2 tornadoes occur which cause the majority of tornado-related casualties and property damage.  On average, parent cells do possess mesocyclones but of a compact physical dimension making them more challenging to detect with radar.

    Wording within the warning text emphasizes a tornado threat that stretches beyond the vicinity of the original warning location or the pathcast of a particular suspect cell due to faster storm motions and the potential for multiple events.  Tornadoes are usually short-lived and occur in families.  In fact, successive tornadoes are possible if organized within a dominant band.  Interestingly, on many occasions the tornadic cells are void of electrical activity.  If confidence is lower, situational descriptive phrasing refers to the cell as a "storm capable of producing a tornado" rather than a "severe thunderstorm...".  The reason is that the storm will likely be absent of thunder and large hail and any damaging wind (in addition to tornado winds) will likely be swirling in nature.  If confidence is increased through improved radar sampling or a report is received, more emphatic wording is used and directed at a particular cell for persons in its path.  Stronger, more persistent cells have a greater potential to produce an F2+ tornado which may have a longer track.  In this situation, the term  "short-lived" is dropped.

4. Automation
         Generation of situational tornado warning text has been automated at WFO Melbourne by modifying the default templates included in the Warning Generation software (Warngen).  This interactive software was developed by the Forecast Systems Laboratory and is included within the Advanced Weather    Interactive   Processing  System  (AWIPS) installed at all NWS offices across the nation.

    Eleven situational warning templates were created from the twelve identified ECFL Tornado Warning Situations (one tropical cyclone situation was similar to a cool season/high shear situation so a common template was shared to conserve space).  Labels were used to delineate the environmental classification; cool season/high shear (T/HS), warm season/low shear (T/LS) and tropical cyclone (T/TC) with confidence descriptors listed along side (Figure 4).

    The template structure was designed so that the forecaster, after determining the situation, is guided through a series of options used to compose the warning text.  These choices include, PATHCAST TYPE, REPORT, SITUATIONAL CALL-TO-ACTION and/or ADD-ON.  Upon selecting the appropriate tornado situation, the BASIS of the warning (radar detection and/or spotter report) is already accommodated.  Warngen has the ability to linearly extrapolate storm motion and state where it will be located at certain times during the warning valid period.  Two pathcast types can create either a list of specific cities and times that a storm will arrive or a simple list of cities that a storm will likely impact.  Next, the storm report option can be selected if a damage report has been received.  It prompts the operator to enter damage information at the final edit screen.  The last option involves CTAs which are designed to further address the storm-specific situation and encourage protective action.  As many CTAs can be selected as needed.  When finished, the Warngen template creates a complete situational warning product based upon forecaster input.  Each template can create warning text that is highly specific to the situation.

5. Situational Calls-to-Action
               Numerous CTAs have been locally developed or modified and then implemented into Warngen.  Many are situational and based on recent post disaster surveys and interviews from the Florida and Oklahoma outbreaks of 22-23 February 1998 and 3 May 1999, respectively.  These CTAs were developed to optimize the complete warning text, moving away from cliched phrases previously used for any/all tornado situations in hopes of reducing casualties.  Among the many unique CTAs are those for RVs and RV Parks, Night-time Tornadoes, Casualties Have Already Occurred, Truth About Underpasses, Tornado Emergencies, Heavy Rain With Tropical Cyclone Tornadoes, and Survivor's Advice (Figure 5).  Ready-made situational CTAs enable the warning meteorologist to speak to the context of the particular tornado.

6. References
Smith, R. D., and S. F. Piltz, 1999: Situation SpecificTornado Warnings at the National Weather
       Service Forecast Office, Tulsa, Oklahoma. National Weather Digest, 23, pp 41-44.
 

Remaining references available upon request.

e:mail david.sharp@noaa.gov