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  • View in gallery

    A hypothetical NWS tornado warning (white polygon) with a tornado path (black heavy line) is shown for a supercell storm (cartoon radar image). Times for warning and tornado duration are listed. The shading (gray) within the warning polygon represents the period prior to the tornado confirmed through NWS products and the period of real-time confirmation (gray hatched).

  • View in gallery

    Study domain with the beginning tornado locations for real-time confirmation (543; red triangles) and without real-time confirmation (819; black squares) from 2007 to 2011.

  • View in gallery

    (left) Day (1013 events) and (right) night (349 events) distributions of beginning tornado locations for real-time confirmation cases (red triangles) and events without real-time confirmation (black squares) from 2007 to 2011.

  • View in gallery

    Box-and-whiskers plot of tornado duration (min) from 543 real-time confirmation cases (gray) compared to 819 cases of no real-time confirmation (black) from 2007 to 2011. The shaded box covers the 25th–75th percentiles, the whiskers extend to the 10th and 90th percentiles, and the median values are marked by the heavy horizontal line within each shaded box.

  • View in gallery

    As in Fig. 4, but for tornado pathlength (km).

  • View in gallery

    Tornado pathlength distribution (%) of real-time confirmation for short- (1204), moderate- (127), and long-track (31) track from 2007 to 2011.

  • View in gallery

    As in Fig. 4, but for the tornado duration (min) following real-time confirmation through NWS products, distributed by total tornado pathlength (km).

  • View in gallery

    Tornado distribution (%) by EF-scale rating (EF1, 314; EF2, 130; EF3, 71; EF4, 24; and EF5, 4) for real-time confirmation events from 2007 to 2011.

  • View in gallery

    Distribution (%) of SPC forecast risk of general thunder (86 cases), slight (688 cases), moderate (460 cases), and high (126 cases) from day-1 convective outlooks for tornado events with (gray) and without (black) real-time confirmation.

  • View in gallery

    Distribution (%) of SPC forecast tornado probabilities of 2% (215 cases), 5% (374 cases), 10% (297 cases), 15% (266 cases), and 30% (112 cases) from day-1 convective outlooks for tornado events with (gray) and without (black) real-time confirmation.

  • View in gallery

    Distribution (%) of SPC convective watches of severe (169 cases), tornado (752 cases), and PDS tornado (324 cases) for tornado events with (gray) and without (black) real-time confirmation.

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Assessing Real-Time Tornado Information Disseminated through NWS Products

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Abstract

Real-time confirmation of a tornado specified in National Weather Service (NWS) warnings and statements is believed to increase the credibility and urgency of these critical warning messages for the end user, because it represents the greatest degree of certainty that the hazard exists. This timely tornado information disseminated in official NWS products and relayed through multiple sources by private and public partners may help the public believe, personalize, confirm, and respond to the warning message. This is the first study to explicitly assess the frequency of real-time confirmation of ongoing tornadoes within NWS products and explore what unique conditions may facilitate or hinder this process. Tornado reports and their respective NWS warnings and statements during a 5-yr period from 2007 to 2011 across the central contiguous United States were compiled and examined. Overall, 40% of tornadoes were confirmed in NWS products in real time. Increasing tornado pathlength, duration, and intensity subsequently resulted in an increasing likelihood of real-time confirmation prior to the tornado dissipating. The time of day was a factor; nighttime tornadoes were 20% less likely to receive real-time confirmation than daytime events. Additionally, increasing tornado forecast risk in products issued by the Storm Prediction Center corresponded to an increasing likelihood of real-time confirmation. Analysis of these data reveals specific scenarios when tornadoes are more or less likely to be reported in real time, providing some guidance for when timely ground-truth information may or may not be available.

Corresponding author address: Scott F. Blair, NOAA/NWS/Weather Forecast Office, 1803 N. 7 Hwy., Pleasant Hill, MO 64080. E-mail: scott.blair@noaa.gov

Abstract

Real-time confirmation of a tornado specified in National Weather Service (NWS) warnings and statements is believed to increase the credibility and urgency of these critical warning messages for the end user, because it represents the greatest degree of certainty that the hazard exists. This timely tornado information disseminated in official NWS products and relayed through multiple sources by private and public partners may help the public believe, personalize, confirm, and respond to the warning message. This is the first study to explicitly assess the frequency of real-time confirmation of ongoing tornadoes within NWS products and explore what unique conditions may facilitate or hinder this process. Tornado reports and their respective NWS warnings and statements during a 5-yr period from 2007 to 2011 across the central contiguous United States were compiled and examined. Overall, 40% of tornadoes were confirmed in NWS products in real time. Increasing tornado pathlength, duration, and intensity subsequently resulted in an increasing likelihood of real-time confirmation prior to the tornado dissipating. The time of day was a factor; nighttime tornadoes were 20% less likely to receive real-time confirmation than daytime events. Additionally, increasing tornado forecast risk in products issued by the Storm Prediction Center corresponded to an increasing likelihood of real-time confirmation. Analysis of these data reveals specific scenarios when tornadoes are more or less likely to be reported in real time, providing some guidance for when timely ground-truth information may or may not be available.

Corresponding author address: Scott F. Blair, NOAA/NWS/Weather Forecast Office, 1803 N. 7 Hwy., Pleasant Hill, MO 64080. E-mail: scott.blair@noaa.gov

1. Introduction

National Weather Service (NWS) tornado warnings are one of the most essential services provided to the general public, private sector, and the integrated warning team (IWT), which is composed of broadcast media, emergency managers, and other decision makers in the realm of public safety (Doswell 2005; Dix and Fieux 2011; NWS/Warning Decision Training Branch 2013). These tornado warnings, communicated through multiple sources, in addition to improvements in technology and public education, can be partially credited for the overall decreasing trend in tornado-related deaths over the past century (Brooks and Doswell 2001; Coleman et al. 2011).

The actual human response to tornado warnings often varies due to complex demographical, behavioral, situational, cultural, and economic factors (Mileti and Sorensen 1990; Perry and Lindell 1991; Legates and Biddle 1999; Sorensen 2000; Sherman-Morris 2010; Brotzge and Donner 2013; Sherman-Morris 2013). Mileti and Sorensen (1990) proposed a model in which a user’s response to a warning progressed through several steps: hearing the warning, understanding the contents of the message, believing the warning is credible, personalizing the warning, and confirming the warning is true, before a physical sheltering response occurs. When a tornado warning is issued, the public frequently seeks out additional information beyond the initial warning before taking protective action in order to believe, personalize, and confirm the message (Mileti and Sorensen 1990; Hayden et al. 2007; NWS 2009, 2011a,b; Lindell and Perry 2012; Sherman-Morris and Brown 2012). Several studies have shown that a critical component to help elicit a response is looking for or hearing confirmation of the threat within the warnings themselves, perhaps because it increases the perceived credibility and urgency of the warning message (Hodler 1982; Tiefenbacher et al. 2001; Andra et al. 2002; Hammer and Schmidlin 2002; NWS 2009; Schumacher et al. 2010; NWS 2011a; Nagele and Trainor 2012).

Ground-truth reports of tornadoes may be received from storm spotters, the public, or other sources that in turn can be disseminated in the initial tornado warning, subsequent warning statements, and local storm reports. Real-time information included in these NWS products serves as an integral component of the warning and sheltering process. In turn, these messages can be used by broadcast media, decision makers, and other users to further clarify the track and location of the hazard as well as to convey the seriousness of the situation to a mass audience. The public frequently turns to television broadcasts as their primary source of tornado information (Legates and Biddle 1999; Brown et al. 2002; Hammer and Schmidlin 2002; Mitchem 2003; Paul et al. 2003; Sherman-Morris 2005, 2010; Hayden et al. 2007; Demuth et al. 2009; Coleman et al. 2011), with real-time tornado information aiding in the public’s willingness to believe the warning message, encouraging a stronger public response.

This is the first study to assess the frequency of real-time confirmation of ongoing tornadoes within NWS products and explore what unique conditions may facilitate or hinder this process, by quantifying these real-time occurrences over a 5-yr period across some of the most tornado-prone regions of the United States. An overview of the data and methods used is provided in section 2, followed by the results and their applications in section 3. Discussion and concluding remarks follow in section 4.

2. Data and methodology

The domain for this study was confined to the central contiguous United States, generally the states east of the Continental Divide and west of the Mississippi River. This region was chosen to encompass a geographic area with the most frequent supercell storm mode and tornado activity (Brooks et al. 2003; Smith et al. 2012). The time interval for this study was from 1 January 2007 through 31 December 2011, to closely coincide with the current era of NWS storm-based warnings and the enhanced Fujita (EF) damage scale rating system that began during 2007 (McDonald and Mehta 2006; NWS 2007).

a. Tornadoes

The Storm Prediction Center (SPC) tornado database compiled from the National Climatic Data Center (NCDC) publication Storm Data (NCDC 2007–11) was used to examine tornado events that occurred within the study domain. Doswell and Burgess (1988) detailed the myriad of challenges within the climatological tornado database inherent with F/EF-scale ratings (hereafter EF), including those classified as “weak” tornadoes (EF0 and EF1). Much uncertainty exists with EF0-rated events, as many tornadoes occur in rural areas and produce little or no damage, ultimately defaulting to the lowest damage rating. Tornadoes rated EF1 and greater are traditionally confirmed by more formal damage assessments, lending slightly better confidence in climatological studies and databases (Rose et al. 2004; Verbout et al. 2006). Therefore, all tornadoes rated EF1 or greater were incorporated into the study.

Information about specific location, time, duration, pathlength, and EF-scale rating were collected for every tornado included in the study. Tornadoes were also classified based on pathlength as short (<24 km; <15 mi), moderate (24–48 km; 15–30 mi), or long (>48 km; >30 mi). In addition, tornadoes were further classified as “day” or “night” occurrences to examine whether visibility differences affect real-time confirmation. All sunrise and sunset times were obtained for each event, and nighttime tornadoes were classified as the time period from 1 h after sunset to 1 h prior to sunrise. Civil twilight ranges from approximately 25 to 40 min across the domain annually, yet uncertainty exists to the reduction of available brightness in a storm environment. Therefore, the daytime–nighttime criteria were chosen to ensure that all of the ambient light during civil twilight was eliminated for nighttime tornado events.

b. Real-time confirmation

A “real-time confirmation” of a tornado was defined as an explicit mention of a confirmed tornado within an NWS text product that was issued prior to the dissipation of the tornado. In cases when an NWS product confirmed that a tornado was sighted but was issued after the tornado dissipated, or when no mention of a reported tornado was included in the products, the tornado event was classified as “no real-time confirmation.” Figure 1 graphically illustrates this methodology, using a hypothetical case of real-time confirmation for a tornado-warned (and tornado producing) supercell and its associated tornado path.

Fig. 1.
Fig. 1.

A hypothetical NWS tornado warning (white polygon) with a tornado path (black heavy line) is shown for a supercell storm (cartoon radar image). Times for warning and tornado duration are listed. The shading (gray) within the warning polygon represents the period prior to the tornado confirmed through NWS products and the period of real-time confirmation (gray hatched).

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

To obtain these data, all available NWS tornado warnings, follow-up warning statements called “severe weather statements,” and preliminary local storm reports associated with each tornado were acquired from the Iowa Environmental Mesonet application (http://mesonet.agron.iastate.edu/cow/). The beginning and ending times of a tornado as recorded in Storm Data were compared to the issuance time of the NWS product that first confirmed the presence of an ongoing tornado. It should be noted there are some limitations to the precision of specific genesis and dissipation times of tornadoes as documented in Storm Data, leading to some uncertainty. In rare situations when multiple tornadoes were ongoing or occurred during the duration of a warning, and NWS products confirmed the sighting of at least one tornado, all tornadoes within the warning were considered confirmed in real time after initial confirmation in the product.

c. SPC products

SPC “day-1 convective outlooks” and their respective “tornado probabilities” as well as any “convective watches” in effect for each tornado were recorded and assimilated with the other dataset obtained from Storm Data to examine whether an increasing forecast risk of severe weather may correlate to an increasing number of “eyes” in the field to report severe weather. The outlook and its tornado probability issued at least 8 h prior to a tornado report were used in the study. This 8-h timeframe was subjectively determined to provide sufficient lead time for emergency managers and other decision makers to organize, contact, and eventually deploy storm spotter networks that might have led to a potential increase of observers in the field.

3. Results and applications

A total of 1362 tornadoes rated EF1 or greater were included in the study, and approximately 40% (543) of these tornadoes were confirmed in real time through NWS products prior to dissipation. Geographically, there appears to be a slight tendency for tornadoes to be confirmed in real time more frequently across the Great Plains states, specifically across the high plains (Fig. 2). Possible reasons for this higher frequency may stem from better visibility across the plains than the more varied terrain found across portions of Missouri, Arkansas, and Louisiana, where hills, dense vegetation, and higher low-level relative humidity impede observations (Ashley 2007; Brotzge and Erickson 2010). Additionally, storm mode may also play a role where supercells, with more easily identifiable storm structure and tornado locations, are more frequent in the Great Plains, with more complex storm modes found in the eastern portion of the study domain (Brotzge and Erickson 2009; Smith et al. 2012). These natural influences likely play some role in supporting or hindering the ability of storm spotters and the public to observe a tornado.

Fig. 2.
Fig. 2.

Study domain with the beginning tornado locations for real-time confirmation (543; red triangles) and without real-time confirmation (819; black squares) from 2007 to 2011.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

a. Daytime versus nighttime tornadoes

Tornadoes occurring during the nighttime hours, defined as the period from 1 h after sunset to 1 h prior to sunrise, were confirmed in real time at a much lower rate when compared to daytime events. Approximately 45% of tornadoes during the day were confirmed in real time versus only 25% at night. Using a Student’s t test, the rate of real-time confirmation of daytime tornadoes is statistically greater than that of nighttime tornadoes, to the 99% confidence level. These findings suggest that the visual and timely confirmation of tornadoes at night is potentially hampered by several factors. One of the biggest hindrances to nighttime confirmation is likely the difficulty of observing tornadoes in the dark (Ashley et al. 2008; Brotzge et al. 2011). The study reveals that a large number of these nighttime events occurred in areas of complex terrain and ample tree cover (Fig. 3), resulting in additional detection challenges beyond the lack of light [Doswell et al. 1999; Fig. 8c in Ashley (2007)].

Fig. 3.
Fig. 3.

(left) Day (1013 events) and (right) night (349 events) distributions of beginning tornado locations for real-time confirmation cases (red triangles) and events without real-time confirmation (black squares) from 2007 to 2011.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Nocturnal tornadoes enhance the vulnerability of the public due to a variety of factors, not only limited to a population asleep at night, as demonstrated by previous studies (Simmons and Sutter 2005; Ashley et al. 2008). It is possible that the lower rate of real-time confirmation of nighttime tornadoes may also delay or decrease the warning response and increase the vulnerability for those that receive notification at night. Operationally, ground-truth reports of tornadoes during nocturnal events generally may become less frequent, and in some cases the quality less reliable due to the observing obstacles. This is not surprising because the lowest levels of a storm become increasingly difficult to observe at night. These nocturnal challenges may be indirectly inferred from the 20% reduction of real-time confirmation between daytime and nighttime tornado events. It is important for the operational meteorology community to be aware of the potential diurnal changes of timely severe weather reports, and does not interpret a lack of reports at night to mean that the hazard is no longer occurring. Furthermore, the other components of the IWT should be aware that real-time confirmation of tornadoes through NWS products may become less frequent or absent at night, but should not construe the shortage of reports as a lower risk when a tornado warning is in effect.

b. Tornado characteristics

The total tornado duration was a notable discriminating factor between events that were confirmed in real time and those that did not receive validation in NWS products prior to tornado dissipation. Minor overlap was found between real-time events versus no real-time confirmation, and median values of tornado duration indicate a large discrepancy between the two datasets, with 14 min for the real-time occurrences as opposed to only 5 min for the events with no real-time information (Fig. 4). The duration of tornadoes that are confirmed in real time is statistically longer than the duration of tornadoes that are not, to the 99% confidence level.

Fig. 4.
Fig. 4.

Box-and-whiskers plot of tornado duration (min) from 543 real-time confirmation cases (gray) compared to 819 cases of no real-time confirmation (black) from 2007 to 2011. The shaded box covers the 25th–75th percentiles, the whiskers extend to the 10th and 90th percentiles, and the median values are marked by the heavy horizontal line within each shaded box.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Typically, there is an inherent amount of time that lapses from when a tornado is observed in the field to the issuance of an NWS product confirming the threat. This process normally can be described in five steps: 1) a tornado is observed by the public, law enforcement, or storm spotters; 2) an individual reports their observation to local law enforcement, a spotter network, or directly to an NWS office through some form of communication; 3) the NWS receives the information; 4) the NWS gauges the credibility of information through environmental and radar-based clues; and 5) the NWS disseminates the ground-truth information through a variety of products and services. In an ideal scenario, this process may only take a few minutes from start to finish, especially if a tornado is anticipated by the NWS. However, there are several potential challenges that can delay the entire communication and dissemination process. While not intended to be a comprehensive list of pitfalls, these challenges that result in a delay of information can stem from 1) confusion of the individual’s location and/or the location of the observed tornado, 2) a communication delay or breakdown between the individual reporting and the receiving party, 3) a lack of knowledge of the local NWS office and/or their phone number, and 4) conflicting or unexpected information within the report. Any one or any combination of these challenges can greatly impede the process of disseminating this critical information; therefore, logic would suggest that a tornado persisting for longer periods of time would invite a greater probability for the ground-truth reporting and NWS dissemination process to confirm the threat prior to tornado dissipation.

Increasing tornado pathlength corresponded to a higher frequency of confirmation of events in real time (Fig. 5). Median pathlengths of 10.5 km (7 mi) were associated with real-time confirmation events, compared to 4.8 km (3 mi) for events with no confirmation in NWS products. Pathlength is statistically greater for tornadoes that are confirmed in real time versus those that are not, to the 99% confidence level. Furthermore, long-track tornadoes (>48 km; >30 mi) were confirmed in 84% of the cases, whereas only 36% of short-track events (<24 km; <15 mi) were verified in real time (Fig. 6). It is believed that increasing tornado pathlength, along with a longer duration, results in an increasing probability that the vortex will move across sufficient geographical area for the public or storm spotters to observe the event.

Fig. 5.
Fig. 5.

As in Fig. 4, but for tornado pathlength (km).

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Fig. 6.
Fig. 6.

Tornado pathlength distribution (%) of real-time confirmation for short- (1204), moderate- (127), and long-track (31) track from 2007 to 2011.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

The amount of time a tornado persisted following the initial real-time confirmation disseminated through NWS products showed a dependence on tornado pathlength and duration (Fig. 7). In cases with positive identification issued in real time, NWS products were able to confirm the threat for short-track tornadoes with a median of 8 min remaining of the tornado’s lifespan. This greatly improved for moderate- to long-track tornadoes, with medians of 23 and 49 min, respectively.

Fig. 7.
Fig. 7.

As in Fig. 4, but for the tornado duration (min) following real-time confirmation through NWS products, distributed by total tornado pathlength (km).

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Intense tornadoes resulting in EF3–EF5 damage were confirmed in real time at a much higher frequency than weaker tornadoes rated EF1 and EF2 (Fig. 8). The most violent tornadoes of EF4 and EF5 intensity were confirmed in 92% and 100% of the cases, respectively, which is in stark contrast to EF1 tornadoes, where only 33% of these events were confirmed in real time. While EF0 tornadoes were not investigated in this study, it is presumed that a similar rate of confirmation to EF1 events, if not lower, would exist for the weakest and traditionally shortest-lived events. The rate of real-time confirmation is statistically greater for weak (EF1) tornadoes versus strong (EF2 and EF3), and for strong versus violent (EF4 and EF5) tornadoes, both to the 99% confidence level. This tendency for the rate of real-time confirmation to increase as intensity increases also mirrors the trends seen with pathlength and duration, which reflects the finding of Brooks (2004) that tornadoes with increasing pathlengths generally result in increasing F-scale ratings. Additionally, Brotzge and Erickson (2010) showed that increasing F-scale ratings and pathlengths resulted in a higher probability of a tornado warning being issued, which was speculated to spawn from improved visual or radar-based detection; these findings not only relate tornado intensity and pathlength as in Brooks (2004) and the current study, but also hint that these scenarios may more often result in real-time confirmation. The results in this study show that longer pathlength and duration and also higher EF-scale rating may result in a higher frequency of these tornadoes being confirmed in real time through NWS products.

Fig. 8.
Fig. 8.

Tornado distribution (%) by EF-scale rating (EF1, 314; EF2, 130; EF3, 71; EF4, 24; and EF5, 4) for real-time confirmation events from 2007 to 2011.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

The results are encouraging when considering that the most destructive, deadly, and long-track tornadoes are likely to be confirmed in real time through NWS products. The ability to confirm the presence of these extreme events in a timely manner benefits and enhances the decision makers’ and broadcasters’ roles in conveying the warning message. However, it is important to clarify the limitations of what these data suggest. While it is probable that a tornado rated as violent (EF4/EF5) will be observed in the field and disseminated through NWS products in real time, there is no guarantee that the actual strength, magnitude, or potential impact of the event can be accurately gauged during the tornado’s lifespan on any regular basis (NWS 2011b). Simply receiving the confirmation of an ongoing tornado does not indicate that a tornado is a specific strength. Only limited research has been conducted to correlate rotational signatures from the Weather Surveillance Radar-1988 Doppler (WSR-88D) super-resolution velocity data with tornado intensity at the surface, with varied success (Kingfield et al. 2012; LaDue et al. 2012; Toth et al. 2013). To date, no robust scientific guidance exists to accurately correlate tornado intensity and its potential ground-level impacts with WSR-88D data available in real time to operational forecasters.

c. SPC outlooks and watches

The authors hypothesize an increasing forecast tornadic risk should be proportional to an increase in preparation and involvement by the severe weather community, ultimately resulting in more eyes in the field to observe severe weather. Previous studies have shown that SPC convective outlooks and watches are heavily used in the pre-event planning activities of emergency managers and storm spotter groups, among others (Doswell et al. 1999; Baumgart et al. 2008). Generally, a forecast containing an increased risk of tornadoes traditionally motivates members of the IWT to use a more aggressive approach in preparation. For instance, local media may dispatch a fleet of television storm chasers and journalists into the field. Emergency managers may alert local law enforcement and deploy local storm spotters in strategic viewing locations across the community (Baumgart et al. 2008). Local and national experienced storm chasers may flock to the highest risk area. The public may also experience a heightened awareness for the potential of hazardous weather and keep a keener eye to the sky. Additionally, NWS staffing is likely to increase during severe weather to balance the workload, potentially allowing for more frequent dissemination of products and services immediately following reception of severe weather reports. In an ideal scenario where an elevated threat of tornadoes exists, a weather-savvy society is in place prior to convective development, producing the highest number of amateur and professional weather observers available in the field. This in turn should result in an increased probability of (visible) tornado observation. In contrast, a low risk or a poorly anticipated severe weather event may result in a muted awareness of a tornado threat and, thus, fewer resources in the field.

The results revealed that SPC convective outlooks, issued at least 8 h prior to each tornado, showed an increased number of tornadoes confirmed in real time with an increased forecast risk of severe weather (Fig. 9). In cases where “general thunderstorms” were forecast, tornado events were infrequently confirmed in real time (24%) whereas “high risk” forecasts were associated with the majority of tornadoes being confirmed through NWS products prior to dissipation (54%). Because the categorical convective outlooks include all threat types of severe weather, forecast tornado probabilities contained in the outlook were also investigated. A similar trend is found with SPC-forecasted tornado probabilities within the outlooks, where the majority of tornadoes were confirmed in real time for the highest probabilities (Fig. 10). In addition, tornadoes occurring in a “tornado watch” were much more likely to be confirmed via NWS products in real time compared to “severe thunderstorm watches” (Fig. 11). This was especially true when “particularly dangerous situation” (PDS) tornado watches were issued; in these situations, 53% of tornadoes were confirmed in real time.

Fig. 9.
Fig. 9.

Distribution (%) of SPC forecast risk of general thunder (86 cases), slight (688 cases), moderate (460 cases), and high (126 cases) from day-1 convective outlooks for tornado events with (gray) and without (black) real-time confirmation.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Fig. 10.
Fig. 10.

Distribution (%) of SPC forecast tornado probabilities of 2% (215 cases), 5% (374 cases), 10% (297 cases), 15% (266 cases), and 30% (112 cases) from day-1 convective outlooks for tornado events with (gray) and without (black) real-time confirmation.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

Fig. 11.
Fig. 11.

Distribution (%) of SPC convective watches of severe (169 cases), tornado (752 cases), and PDS tornado (324 cases) for tornado events with (gray) and without (black) real-time confirmation.

Citation: Weather and Forecasting 29, 3; 10.1175/WAF-D-13-00126.1

SPC short-term forecast products reflected the trends revealed in section 3b, with increasing forecast risk also relating to a higher probability of real-time confirmation. This relationship is likely a consequence of the design of SPC products to forecast the areal probability of severe weather and its magnitude based on anticipated environmental conditions and convective evolution. Tornado events that are long tracked and intense empirically have a tendency to be well forecasted through the suite of SPC short-term products. Additionally, the results at least partially support the notion that an increasing forecast risk of tornadoes may lead to more observers in the field, ultimately providing increasing potential of timely storm reports and subsequent confirmation in NWS products. Therefore, the forecasted tornado environment in the IWT planning stage may increase anticipation that real-time tornado information may or may not be available.

4. Discussion and conclusions

Continued advances in technology over the past decade have resulted in several new avenues for spotters and chasers to provide severe weather reports in real time to the NWS (Pietrycha et al. 2009; Brotzge and Donner 2013). Social media platforms have also enabled the public to quickly share photos or text reports of hazardous weather (Hyvärinen and Saltikoff 2010; Blair and Leighton 2012; Mass 2012). Spotter network reports, streaming video from storm chasers and television stations, Facebook, and Twitter reports have all been utilized during warning operations, helping to accelerate the dissemination of ground-truth information to the multiple users of NWS products. However, as technology advances, it remains critical that adequate investments are made to both technology infrastructures and proficiency training for emerging, diverse tools in the operational setting.

New technology should allow for real-time confirmation to apply beyond human observations with the nationwide completion of the dual-polarization upgrade to the WSR-88D network. A “tornado debris signature” is distinguished when debris is lofted within the parent circulation, revealed on radar as a nonmeteorological echo by a sharp reduction in the correlation coefficient product collocated within a strong velocity couplet (Ryzhkov et al. 2005; Kumjian and Ryzhkov 2008). While this signature will not improve lead time, it has great potential as another tool to identify ongoing damaging tornadoes relatively close to a radar site. This is especially true in limited-visibility, high-precipitation situations or at night in the absence of human ground-truth reports, potentially improving some of the diurnal disparity of timely tornado information. Future research will help better determine how this remotely sensed information can be communicated in an effective and understandable channel to the public. Today’s technologies and those of the future will provide more opportunities for the IWT to monitor ongoing tornadoes, contributing to a quicker transmission of more detailed information to the end user.

Real-time confirmation of a tornado through official and broadcast sources is thought to increase the credibility and urgency of the warning message, which may help play a role in encouraging the public to seek shelter. This study provides a climatology of tornadoes confirmed and disseminated through NWS products prior to dissipation from 2007 to 2011 over the central contiguous United States. Data were also investigated to determine what available signals existed that may facilitate or hinder receiving or transmitting timely tornado reports. The results illustrate the following:

  • Approximately 40% (543) of the tornadoes in the study sample were confirmed in real time through NWS products.
  • The high plains region had a higher occurrence of real-time confirmation, possibly attributed to less vegetative and terrain obstructions, more frequent supercell storm mode, and a high concentration of storm chasers during the months of climatologically favored tornado activity.
  • A strong discriminating factor for a tornado to be confirmed through NWS products in real time was tornado duration (median of 14 min compared to 5 min for nonconfirmed events).
  • The large majority of the most intense and long-track tornadoes were confirmed in real time [approximately 94% of EF4 and EF5 events; 84% of tornadoes pathlength >48 km (>30 mi)].
  • Nighttime tornadoes were 20% less likely to receive real-time confirmation in NWS products compared to daytime events (only 25% of nighttime events confirmed).
  • SPC short-term forecast products (convective outlooks, tornado probabilities, and watches) showed an increasing probability for real-time confirmation as the forecasted risk increased.

This study provides a set of general guidelines for anticipating situations when real-time confirmation of a tornado is more (or less) probable, which may be used to help IWT members and other partners adjust their expectations for the availability of ground-truth information within NWS products. It is important to remember that each tornadic scenario is unique and some deviations from the results found herein are possible. Communication and reporting challenges in the field, variations in population density, seasonal anomalies of severe weather events, and NWS workload or technology issues may potentially lead to further abnormalities in the dissemination of timely tornado information. It is important for broadcasters, decision makers, and other NWS partners to value the information contained within the initial science-based tornado warning, regardless of whether or not validation of the ongoing threat is available, especially because, as this study demonstrates, certain conditions may delay or prohibit confirmation.

NWS offices should continue to strive to proactively issue timely and reliable tornado reports through the suite of available tools, products, and services. An aggressive approach to confirm the threat should be utilized, including the monitoring of social media platforms and electronic field reports and videos from spotters. Additionally, other IWT members should immediately share tornado reports with each other through NWSChat or other means to attain a wider dissemination of critical information. Such team efforts improve the speed of essential tornado updates in official NWS products—in some cases by several minutes—ultimately enhancing the warning message to the end users and potentially achieving a stronger public response to take appropriate action in the face of hazardous weather.

Acknowledgments

The authors gratefully acknowledge Julie Demuth, Jennifer Laflin, Amos Magliocco, Albert Pietrycha, and Jeff Manion for their thorough and beneficial reviews. We also thank two anonymous reviewers for helping to improve the manuscript. The views and opinions expressed in this paper are those of the authors and do not necessarily represent an official position, policy, or decision of the National Weather Service.

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