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Severe Thunderstorm and Tornado Warnings at Raleigh, North Carolina

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Severe Thunderstorm and Tornado Warnings at Raleigh, North Carolina Severe Thunderstorm and Tornado Warnings at Raleigh, North Carolina Debra K. Hoium,* Allen J. Riordan,* John Monahan,+ and Kermit K. Keeter* ABSTRACT The National Weather Service issues public warnings for severe thunderstorms and tornadoes when these storms appear imminent. A study of the warning process was conducted at the National Weather Service Forecast Office at Raleigh, North Carolina, from 1994 through 1996. The purpose of the study was to examine the decision process by documenting the types of information leading to decisions to warn or not to warn and by describing the sequence and timing of events in the development of warnings. It was found that the evolution of warnings followed a logical sequence beginning with storm monitoring and proceeding with increasingly focused activity. For simplicity, information input to the process was categorized as one of three types: ground truth, radar reflectivity, or radar velocity. Reflectivity, velocity, and ground truth were all equally likely to initiate the investigation process. This investiga- tion took an average of 7 min, after which either a decision was made not to warn or new information triggered the warn- ing. Decisions not to issue warnings were based more on ground truth and reflectivity than radar velocity products. Warnings with investigations of more than 2 min were more likely to be triggered by radar reflectivity, than by velocity or ground truth. Warnings with a shorter investigation time, defined here as "immediate trigger warnings," were less frequently based on velocity products and more on ground truth information. Once the decision was made to warn, it took an average of 2.1 min to prepare the warning text. In 85% of cases when warnings were issued, at least one contact was made to emergency management officials or storm spotters in the warned county. Reports of severe weather were usually received soon after the warning was transmitted—almost half of these within 30 min after issue. A total of 68% were received during the severe weather episode, but some of these storm reports later proved false according to Storm Data. Even though the WSR-88D is a sophisticated tool, ground truth information was found to be a vital part of the warn- ing process. However, the data did not indicate that population density was statistically correlated either with the num- ber of warnings issued or the verification rate. 1 • Introduction lent thunderstorms, 500 floods, 1000 tornadoes, and several hurricanes" (Friday 1994). Clearly, in this en- "In a typical year, the United States can expect a vironment, timely and accurate severe weather fore- staggering assault by the elements: some 10 000 vio- casts are essential to protect the public. Environmental conditions that lead to the develop- ment of severe thunderstorms and tornadoes are rea- * Department of Marine, Earth and Atmospheric Sciences, North sonably well recognized and can be forecast with Carolina State University, Raleigh, North Carolina. +Department of Statistics, North Carolina State University, demonstrable skill (Johns and Doswell 1992). Raleigh, North Carolina. However, there is as yet no reliable method to predict #National Weather Service Forecast Office, Raleigh, North the development of a specific severe storm. Thus, in Carolina. the United States there is a two-tier process of issuing Corresponding author address: Dr. Allen J. Riordan, College watches for large threat areas and warnings for spe- of Physical and Mathematical Sciences, North Carolina State cific storms. University, Box 8208, Raleigh, NC 27695-8208. E-mail: [email protected] Severe thunderstorm and tornado watches are In final form 1 August 1997. based on our ability to predict the storm environment. ©1997 American Meteorological Society Since 1954, the Severe Local Storms Unit of the Na- Bulletin of the American Meteorological Society 2539 Unauthenticated | Downloaded 10/10/21 05:36 PM UTC tional Severe Storms Forecast Center, now called the breaks appeared imminent but did not develop to cases Storm Prediction Center, in Norman, Oklahoma, has when supercells produced multiple tornadoes. Since provided national guidance with its wide array of fore- the collaboration began, 72 severe thunderstorm and cast products including the issuing of watches. These 20 tornado warnings have been fully documented. products are issued when there is a strong potential for This paper presents the results of the documenta- severe weather over an area of roughly 64 000 km2 for tion of the severe weather warning process at the Ra- a period of up to about 7 h (Ostby 1992). leigh NWSFO. The objective is to describe how the Once a specific damaging storm is detected or ap- storm coordinator reached a decision to issue or not pears imminent, each local National Weather Service to issue a warning. Of special interest is the role of (NWS) office is responsible for warning the public various WSR-88D products and the influence of within its area of responsibility. Warnings for severe ground truth reports in the decision process. convective storms are issued for counties or parts of counties for periods of generally up to 1 h. Each year the local NWS offices issue thousands of warnings 2. Background across the United States. The decisions for each of these warnings are At the Raleigh NWSFO during this study, the shaped by a wide array of factors including informa- Doppler radar PUP with its twin 19-in. color monitors tion from radar, the meteorological environment, (Crum and Alberty 1993), normally the center of ac- ground truth reports, and the judgment of forecasters. tivity, was located facing the south wall of the opera- The installation of the Weather Surveillance Radar- tions room. In front of the PUP there was space enough 1988 Doppler (WSR-88D) radar at NWS offices has for two staff persons, one to select products to be dis- been aimed at improving the accuracy and timeliness played, hereafter designated as the radar operator, and of warnings (Polger et al. 1994; Klazura and Imy 1993; sometimes a second person, normally to the left, who Bieringer and Ray 1996). Lemon et al. (1992) discuss provided support to the operator since at this position the use of this valuable, but complex, new tool. there was full view of the PUP screens and desktop To gain insights into its warning process, the NWS space for reference to a variety of detailed county Forecast Office (NWSFO) at Raleigh, North Carolina, maps. On the wall facing the radar and visible to the invited selected faculty and students at North Carolina radar operator was a large color map of cities, high- State University (NCSU) to participate in its warning ways, and county boundaries of the warning area. activities. This collaborative effort began in 1992 and Immediately adjacent to the PUP was a computer continues today. It includes the period since the WSR- that, during periods of severe weather, was dedicated 88D became operational. One purpose of the univer- exclusively to typing and transmitting warnings. Also sity involvement was to document and facilitate the adjacent to the PUP was a small table with space for warning process. Under the direction of the NWS one of three autodial phones for ready access to county storm coordinator, university staff were assigned du- sheriffs or emergency personnel in any county of the ties such as analyzing surface mesoscale features, re- warning area. Other phones of this type were located questing ground truth reports, and maintaining a elsewhere in the office so that multiple calls could be detailed time log of the activity at the WSR-88D Prin- processed simultaneously. Completing the line of po- cipal User Processor (PUP). As a result of the collabo- sitions to the left of the PUP was a desk with a ham ration, both researchers and forecasters have learned radio console for one or two SKYWARN volunteers. about how the warning process evolved in real time. Staffing varied greatly with the level of severity of While the benefits of this program cannot be measured the event. In a situation when a severe weather out- objectively, it has provided support for forecasters and break was anticipated, there were typically up to four given researchers insight into the operational arena. NWS personnel, including a radar operator, a warn- Also, the experience of the group has channeled re- ing text editor, at least one person to receive and ini- search toward practical forecast problems and initiated tiate phone calls, and a severe storms coordinator. data collection for future research. Ideally, several of these individuals had no additional To date, the collaborative group has developed a operational duties, such as aviation forecasting, and shared experience of over 60 situations during which could be dedicated totally to the severe weather activ- severe weather was anticipated. These have ranged ity. In these cases, there were typically one or two over a wide spectrum from cases when major out- NCSU volunteers and two SKYWARN ham radio 2540 Vol. 78, No. 7 7, November 1997 Unauthenticated | Downloaded 10/10/21 05:36 PM UTC operators as well. By marked contrast, in other situa- it is helpful to describe the types of storms that oc- tions where one or two thunderstorms became margin- curred in central North Carolina in 1995, the year of ally severe, but widespread or intense activity was not most concentrated study. In this year, 56% of severe anticipated, there were only one or two NWS staff, weather for the Raleigh warning area (not counting borrowed from other operational shift responsibilities flash floods) in Storm Data was reported as thunder- to execute the warning process including monitoring storm wind damage, nearly all of which was described the radar. Of course, interesting times ensued when as downed trees. Hail reports included nearly all the what appeared initially to be a benign situation rap- rest with 27% of the total for hail diameters of 1 in.
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