Service Assessment

Spencer, South Dakota, Tornado 

May 30, 1998
Photos: The Spencer tornado viewed from about 4 miles east of town on Highway 38 (courtesy 

of Keith Brown), and the Spencer water tower, an example of the catastrophic damage inflicted 

on the town (courtesy of Dr. Josh Wurman). 

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

National Oceanic and Atmospheric Administration, D. James Baker, 

Administrator, 

National Weather Service, John J. Kelly, Jr., Assistant Administrator

 
Contents 

 

Preface 

Service Assessment Team 

Acronyms 

Event Summary 

Facts, Findings and Recommendations 

Appendix A: Fujita Tornado Intensity Scale 

Appendix B: Spencer, South Dakota, Tornado, May 30, 1998, Chronological Log 

 

Preface 

The violent tornado which struck Spencer, South Dakota, on the evening of May 30, 1998, killed 

six residents and destroyed almost the entire town. The National Weather Service (NWS) 

conducted a Service Assessment to examine the effectiveness of the warning process that 

evening, focusing on the performance of the Next Generation Weather Radar (NEXRAD) 

Weather Service Forecast Office (NWSFO) at Sioux Falls, the usefulness of NWS warnings and 

forecasts from the perspective of our partners in the media and emergency agencies, and the 

public response. 

Service Assessments are critical to the ongoing efforts of the NWS to improve the quality and 

timeliness of our warning and forecast services, especially as offices like NWSFO Sioux Falls 

complete the transition to a fully modernized Weather Forecast Office (WFO). By reviewing the 

performance of one operational team at one forecast office during one such devastating event, we 

look for ways to ensure that the application of new technology and new concepts will yield the 

most timely and accurate warnings and forecasts. 

John J. Kelly, Jr., Assistant Administrator for Weather Services, August 1998 

 
Service Assessment Team 

The NWS assembled a Service Assessment Team to review the effectiveness of the warning 

process during the evening of May 30, 1998, when a violent tornado struck the town of Spencer, 

South Dakota. The team met in Sioux Falls on June 2 and spent the following 3 days in South 

Dakota collecting information and conducting interviews with the NWS staff at the Sioux Falls 

NWSFO, emergency managers, law enforcement officials, media representatives, and a number 

of residents of Spencer. Additional information was obtained from the NWS Storm Prediction 

Center (SPC) in Norman, Oklahoma, from other NWS meteorologists who participated in poststorm damage surveys, and from engineers at the National Institute of Science and Technology. 

The team was comprised of the following individuals: 

Dennis H. McCarthy Team Leader, Meteorologist in Charge (MIC), NWSFO Norman, 

Oklahoma 

Ricky L. Shanklin Warning Coordination Meteorologist (WCM), NEXRAD Weather Service 

Office (NWSO) Paducah, Kentucky 

Patrick J. Slattery Public Affairs Specialist, NWS Central Region Headquarters, Kansas City, 

Missouri 

James G. LaDue Meteorologist (Instructor), WSR-88D Operational Support Facility, 

Operations Training Branch, Norman, Oklahoma 

Randall C. Duncan Emergency Management Coordinator, Cowley County, Kansas, President, 

International Association of Emergency Managers 

Other valuable contributors include: 

D. Gregory Harmon MIC, NWSFO Sioux Falls, South Dakota 

Todd A. Heitkamp WCM, NWSFO Sioux Falls, South Dakota 

David L. Andra Science and Operations Officer (SOO), NWSFO Norman, Oklahoma 

Brian E. Smith WCM, NWSFO Omaha, Nebraska 

William F. Bunting WCM, NWSO Pleasant Hill (Kansas City), Missouri 

Michael Foster SOO, NWSFO Fort Worth, Texas 

David A. Imy Chief, Operations Branch, NWS SPC, Norman, Oklahoma 

David Dowell Graduate Research Assistant, University of Oklahoma 

Long Phan National Institute of Science and Technology 

Dat Duthinh 

William H. Lerner Weather Service Headquarters (WSH), Office of Meteorology, Silver 

Spring, Maryland 

Linda S. Kremkau Technical Editor, WSH, Office of Meteorology, Silver Spring, Maryland 

Argus Leader Daily Sioux Falls Newspaper 

 
Acronyms 

AFOS: Automation of Field Operations and Services 

AWIPS: Advanced Weather Interactive Processing System 

CAPE: Convective Available Potential Energy 

CDT: Central Daylight Time 

CRS: Console Replacement System 

CWA: County Warning Area 

DOW: Doppler on Wheels 

EAS: Emergency Alert System 

EMWIN: Emergency Management Weather Information Network 

GOES: Geostationary Operational Environmental Satellite 

HMT: Hydrometeorological Technician 

MCD: Mesoscale Convective Discussion 

MIC: Meteorologist in Charge 

m/s: meters per second 

NAWAS: National Warning System 

NEXRAD: Next Generation Weather Radar 

NOAA: National Oceanic and Atmospheric Administration 

NOW: Short Term Forecast (product identifier) 

NWR: NOAA Weather Radio 

NWS: National Weather Service 

NWSO: NEXRAD Weather Service Office 

NWSFO: NEXRAD Weather Service Forecast Office 

NWWS: NOAA Weather Wire Service 

PDS: Particularly Dangerous Situation 

PSD: Peripheral Sharing Device 

PUP: Principal User Processor 

RFD: Rear Flank Downdraft 

ROTATE: Radar Observation of Tornadoes and Thunderstorms Experiment 

SEL: Severe Thunderstorm or Tornado Watch (product identifier) 

SFP: State Forecast Product (product identifier) 

SOO: Science and Operations Officer 

SPC: Storm Prediction Center 

SPS: Special Weather Statement (product identifier) 

SVR: Severe Thunderstorm Warning (product identifier) 

SVS: Severe Weather Statement (product identifier) 

SWO: Severe Weather Outlook (product identifier) 

TOR: Tornado Warning (product identifier) 

TVS: Tornado Vortex Signature 

VCP: Volume Coverage Pattern 

WCM: Warning Coordination Meteorologist 

WFO: Weather Forecast Office 

WISE: Warning and Interactive Statement Editor 

WSH: Weather Service Headquarters 

WSR-88D: Weather Surveillance Radar-1988 Doppler 

ZFP: Zone Forecast Product (product identifier) 
Event Summary

Storm Summary 

Summary of Warning and Forecast Services 

Overview 

At approximately 8:40 p.m. (all times Central Daylight Time [CDT]) on Saturday, May 30, 

1998, a violent tornado struck the small town of Spencer, South Dakota. Spencer is in extreme 

western McCook County, about 45 miles west-northwest of Sioux Falls. The tornado killed six 

people, injured more than one-third of the town's 320 residents, and destroyed most of the town's 

190 buildings. Damage is estimated at $18 million. The Spencer tornado (rated F4 on the Fujita 

Tornado Intensity Scale, as described in Appendix A) was one of five tornadoes, along a nearly 

continuous damage track approximately 30 miles long. All five tornadoes were produced by one 

supercell thunderstorm during a 1 hour and 5-minute period. The most devastating damage was 

limited to less than 1 mile along the tornado track, but significant damage (F1 to F3 intensity) 

was inflicted on several farmsteads in McCook County and neighboring Hanson County, 

including damaged or destroyed buildings, loss of crops, and loss of approximately 75 head of 

livestock. 

The six tornado fatalities in Spencer were the first in the NWSFO Sioux Falls County Warning 

Area (CWA) since the Chandler, Minnesota, tornado in June 1992 and the first deaths in South 

Dakota in nearly 28 years. On July 14, 1970, one person was killed by a tornado in Lincoln 

County, just south of Sioux Falls. The last violent (F4) tornado in the Sioux Falls CWA occurred 

on June 7, 1993, just north of Sioux Falls. 

The NWSFO in Sioux Falls issued a Tornado Warning for northern McCook County, including 

the town of Spencer, at 8:32 p.m. based on data from its Weather Surveillance Radar-1988 

Doppler (WSR-88D) and reports from law enforcement officials. The warning was promptly 

disseminated over the National Oceanic and Atmospheric Administration (NOAA) Weather 

Wire Service (NWWS), commercial radio and television, NOAA Weather Radio (NWR), the 

Emergency Alert System (EAS), and the National Warning System (NAWAS). The outdoor 

warning siren at the Spencer Volunteer Fire Department could not be activated apparently due to 

loss of electricity from the storm. 

 

Storm Summary 

By late afternoon on May 30, the atmospheric environment over the north-central United States 

had become conducive to a significant severe weather outbreak. The air mass over southeastern 

South Dakota was extremely unstable (note the shaded area in Figure 1). A dry line/cold front 

combination was pushing slowly to the east and southeast into this unstable air, while at jet 

stream level in the atmosphere, a well-defined wind maximum (a jet streak) was approaching the 

dry line/cold front intersection. One favorable ingredient that was lacking ahead of the dry 

line/cold front was a low-level wind maximum (low-level jet). It was displaced farther east, over 
eastern Iowa and Minnesota (arrow in Figure 1). However, even with the lack of strong low-level 

winds, instability along the advancing dry line/cold front boundaries was sufficient for 

thunderstorm development, and the approaching jet streak provided more than adequate vertical 

wind shear for supercell evolution once storms developed. 

Figure 1. A composite analysis, indicating the synoptic conditions relevant to the Spencer 

tornado as the parent thunderstorm, developed at 6 p.m. CDT. The (*) indicates Spencer, and 

FSD indicates Sioux Falls. Thick lines with arrows indicate the upper and lower level jet 

streams, with relevant isotachs shown in thin, labeled contours. The shaded regions indicate 

areas of high Convective Available Potential Energy (CAPE), implying strongest instability. The 

label of strong tropospheric shear indicates regions with more that 50 knots shear in the 700 to 

300 mb layer. Surface low pressure centers are depicted by an "L." 
Figure 2. A Geostationary Operational Environmental Satellite (GOES)-8 1 km visible image at 

4:32 p.m. CDT (2132 Coordinated Universal Time [UTC]). The letter "C" corresponds to 

towering cumulus; "B" indicates a surface boundary; "*" indicates the location of Spencer; and 

"L" indicates a surface low pressure center.

Towering cumulus formed on the dry line by 4:30 p.m. near the Missouri River, about 90 miles 

west-northwest of Spencer (indicated by a "C" in Figure 2). Convective development in that area 

over the next hour could have been related to the intersection of a weak east-west boundary with 

the dry line. The enhanced band of cumulus in the area marked "B" in Figure 2 provided a hint of 

this boundary. Initial convection produced a brief thunderstorm by 5 p.m., however, stronger 

storms developed by 6:35 p.m. southwest of Wessington Springs, about 60 miles west-northwest 

of Spencer. The thunderstorm that would eventually produce the Spencer tornado evolved from 

this new development. 

The Spencer storm almost immediately split into left and right moving cells with the right-mover 

becoming a supercell as a mid-level mesocyclone developed by 7:26 p.m. The mesocyclone 

grew downward toward cloud base as observed by the Sioux Falls WSR-88D between 7:45 and 

7:50 p.m. During the same time, a rear flank downdraft (RFD) was developing south of the 

mesocyclone. RFD features are common prior to tornadogenesis in many supercells. 

The Spencer supercell began to show stronger tornadic potential at 7:55 p.m., as a spotter 

observed a funnel cloud in northeast Davison County and the Sioux Falls WSR-88D indicated a 

significant mesocyclone in the storm. The first spotter report of a tornado associated with this 

storm came at 8:08 p.m., about 8 miles northeast of Mitchell. Tornado #1 (shown in Figure 3) 
actually began a few minutes earlier in that area, based on observations by members of a tornado 

field research project who were located on Highway 38, south of the storm. The ROTATE-98 

project (Radar Observation of Tornadoes and Thunderstorms Experiment), coordinated at the 

University of Oklahoma, employs two truck-mounted Doppler radars, referred to as Doppler on 

Wheels (DOW), which are designed to gather high-resolution reflectivity and velocity data from 

locations within a few miles of tornadic thunderstorms. 

Even though the first tornado ended northwest of Fulton at 8:15 p.m., the mesocyclone was still 

situated at ground-level. By 8:17 p.m., the low-level circulation produced Tornado #2 (Figure 3). 

This tornado passed about 2 miles north of Fulton, producing F1 to F2 damage before weakening 

about 4 miles west-northwest of Farmer, or 7 miles west-northwest of Spencer, at approximately 

8:22 p.m. The parent mesocyclone, still appearing very strong on the NWSFO Sioux Falls WSR88D, continued moving east-southeast. 

A new damage track (Tornado #3 in Figure 3) began almost immediately, approximately 1 mile 

northeast of the end of Tornado #2. Reports of Tornado #3 were passed from law enforcement 

officers through NAWAS warning points to NWSFO Sioux Falls between 8:23 and 8:29 p.m. At 

8:28 p.m., the Sioux Falls WSR-88D showed a well-defined hook echo in reflectivity data and a 

well-defined circulation (strong mesocyclone) in velocity data (Figure 4). 

From 8:23 to 8:37 p.m., the Spencer tornado tracked through farmland, within 1 mile of the town 

of Farmer, prior to crossing the Hanson/McCook County line and striking the town of Spencer. 

Photographs from storm chasers show that it had grown to a large tornado west of Spencer. 

Apparently, it became so large that some chasers and storm spotters within 2 miles of the tornado 

became confused because of the large dust cloud surrounding the tornado, especially along the 

west and south sides where dust was raised by strong RFD winds. The DOW used by the 

ROTATE-98 project measured velocities of at least 90 meters per second (m/s) (200 mph) during 

this period. 
Figure 3. A composite of tornado damage tracks based on aerial surveys. Eight tracks are 

indicated, however, only the first five are addressed in the text. Relative path length, with and Fscale damage are shown. (Brian E. Smith)
Figure 4. A four-panel radar image from the Sioux Falls WSR-88D showing the Spencer 

supercell at 8:28 p.m. CDT (0128 UTC). The gray shading for the reflectivity panels progress 

from light to dark from 0 to 40 dBZ and again from 45 to 70 dBZ. For the velocity panels, the 

very light shaded region just south (north) of the tornado indicates inbound (outbound) winds 

more than 64 kt (more than 15 kt) respectively.

The impact time of the tornado on Spencer varies somewhat according to the source of 

information. Data from the ROTATE-98 project place the tornado near the western edge of town 

as early as 8:37 p.m., while the local electric company logged power failure in town at 8:42 p.m. 

and survivor accounts run as late as 8:45 p.m. Based on DOW and WSR-88D data, however, the 

town of Spencer experienced violent tornadic conditions from 8:38 to 8:39 p.m. Wind speeds 

observed by the DOW, as the tornado passed through Spencer, reached 98 m/s (nearly 220 mph) 

just south of the tornado center. While tracking through town, dirt and rain wrapping around the 
tornado might have obscured it somewhat as the parent thunderstorm took on high-precipitation 

supercell characteristics. 

After leaving a path of destruction nearly 1-mile wide in Spencer, the tornado curved southeast 

through more farmland, most likely hidden in wrapping rain curtains. This damage track finally 

ended just north of I-90. Tornado #4 (see Figure 3) developed just south of I-90 and tracked 

southeast, followed by the weaker Tornado #5, which dissipated just northeast of Stanley Corner, 

about 30 miles west of Sioux Falls, at approximately 9:10 p.m. 

 

Summary of Warning and Forecast Services 

NWSFO Sioux Falls is a full-service forecast office in the midst of a transition to a modernized 

WFO. The staff provides warning and forecast services for 45 counties in parts of four states 

(South Dakota, Minnesota, Nebraska, Iowa), while maintaining many of the traditional forecast 

office responsibilities for the entire state of South Dakota (e.g., public/zone forecasts). The office 

uses a combination of modern technology, such as the WSR-88D radar, and older systems, such 

as the Automation of Field Operations and Services (AFOS) workstations and NWR consoles 

employing play/record decks for magnetic tapes. (The office operates three NWR stations.) 

The potential for severe weather on May 30 in parts of South Dakota, Minnesota, Nebraska, and 

Iowa was recognized by forecasters the day before. The NWS SPC included the Sioux Falls 

CWA in its Day 2 Severe Weather Outlook (SWO) first issued at 2:55 a.m. Friday, May 29. In 

an update issued at 12:02 p.m. Friday, the SPC forecaster specifically mentioned "EXTREME 

INSTABILITY," "SUPERCELL THUNDERSTORMS," and the "THREAT OF ISOLATED 

TORNADOES." The NWSFO Sioux Falls forecasters were also monitoring the severe weather 

threat for Saturday very carefully. A forecast discussion issued at 9:10 p.m. Friday mentioned 

increasingly favorable conditions for severe weather "...IN THE FORECAST AREA ANYTIME 

FROM LATE TONIGHT THROUGH SATURDAY EVENING." The scenario anticipated by 

forecasters on the midnight shift early Saturday morning called for two rounds of severe 

weather--one early in the morning and the other late in the day. 

The Day 1 SWO issued by SPC at 12:54 a.m. Saturday placed a moderate risk of severe storms 

from eastern portions of the Sioux Falls CWA into Iowa, Minnesota, and Wisconsin. However, a 

slight risk area covered most of the rest of the Sioux Falls CWA, including the area around 

Spencer. The 5:20 a.m. update from SPC included no significant changes in the north-central 

United States. However, based on their local analysis, the NWSFO Sioux Falls forecasters opted 

to upgrade the outlook for their CWA, and in a Special Weather Statement (SPS) issued at 4:35 

a.m. headlined a "...MODERATE RISK OF SEVERE WEATHER TODAY...SOME THIS 

MORNING AND SOME THIS EVENING...." 

Strong thunderstorms developed in south-central and southeastern portions of South Dakota 

early Saturday morning, prompting a severe thunderstorm watch from SPC at 3:22 a.m. and a 

number of severe thunderstorm warnings and statements from NWSFO Sioux Falls. These 

storms subsided around midday. One forecaster who remained on duty from the midnight shift 

(midnight to 8 a.m.) in an augmented staffing configuration for the morning severe weather told 
the day-shift staff as he departed that he would be available to come in early if necessary that 

night to help again with severe weather operations. 

Midday and early afternoon provided a short break. However, a Significant Weather Outlook 

issued by NWSFO Sioux Falls at 11:47 a.m. was already focused on the likely redevelopment of 

thunderstorms "...LATE THIS AFTERNOON OR EARLY THIS EVENING...." While 

formulating the public forecasts to be issued by 4:30 p.m., the NWSFO Sioux Falls forecasters 

planned staffing for the anticipated redevelopment. An updated Day 1 Outlook from SPC issued 

at 2:41 p.m. maintained roughly the same configuration of slight and moderate risk areas as the 

earlier issuance, except for an expansion of the moderate risk area to the west to include extreme 

northeastern South Dakota. A Mesoscale Convective Discussion (MCD) issued by SPC at 12:54 

p.m. highlighted the increasing threat of severe weather in western South Dakota for mid and late 

afternoon. Another MCD issued at 2:19 p.m. focused on the increasing threat from east-central 

South Dakota into parts of Minnesota and Wisconsin. Both mentioned portions of South Dakota 

as candidates for severe weather watches later in the afternoon. 

The SPC issued a Severe Thunderstorm Watch at 3:33 p.m. for much of South Dakota, north of 

I-90, and a Tornado Watch at 3:50 p.m. for an adjacent area from eastern South Dakota through 

southern Minnesota. McCook County was included in the tornado watch. Both watches were in 

effect until 10 p.m. The watches were disseminated before 4 p.m. and the public forecasts were 

issued by 4:30 p.m. As often occurs at NWS forecast offices during late afternoon severe 

weather events, the two forecasters who had worked the day shift remained on duty to augment 

staffing for the evening shift. One forecaster stationed himself at the WSR-88D Principal User 

Processor (PUP) workstation while the other went to the personal computer (PC) workstation 

used to generate warnings and statements. 

The staffing configuration at the start of the evening severe weather event consisted of four 

forecasters (two in the standard public and aviation forecaster roles and two handling warnings 

and statements) and one Hydrometeorological Technician (HMT). One of the staff interns was 

called at home and asked to report for duty by 7 p.m. to take over NWR broadcasts. This brought 

total staffing to six just as severe storms entered the Sioux Falls CWA. The staffing level 

remained at six until 9 p.m. At that time, the forecaster who had already worked an extra 

4 hours from 8 a.m. until noon reported 3 hours early for his upcoming midnight shift. This 

brought the total staff to seven for about 1 hour as the severe storms moved through the Sioux 

Falls area. 

During the severe weather episode on the evening of May 30, WSR-88D data analyzed at the 

PUP workstation and spotter reports became the primary input for warning decisions. Most of 

the key spotter reports were relayed from law enforcement officials through NAWAS warning 

points to the NWSFO. A few came from storm chasers who were in the area. 

The first warning issued by the Sioux Falls NWSFO for this event was a severe thunderstorm 

warning at 6:45 p.m. for Beadle County, the farthest northwest county in the CWA. Over the 

next 4 hours, the Sioux Falls staff issued a total of 11 Severe Thunderstorm Warnings, covering 

15 counties, and eight Tornado Warnings, covering nine counties. In addition, forecasters issued 

five Short Term Forecasts and 26 Severe Weather Statements during the evening. 
The first severe weather report of the evening (three-quarter-inch diameter hail) came from 3 

miles west of Huron in Beadle County at 7:19 p.m., more than 30 minutes after the first warning 

was issued. The first significant report on the storm that would eventually produce the Spencer 

tornado was the 7:55 p.m. funnel cloud report in northeast Davison County from the County 

Sheriff. A Severe Thunderstorm Warning had been in effect since 7:44 p.m. for northeast 

Davison and northern Hanson Counties, but the funnel cloud report combined with radar data led 

to the first tornado warning, issued at 8:01 p.m. for northeast Davison and northern Hanson 

Counties. The first tornado report on the Spencer storm was the 8:08 p.m. report from 

approximately 8 miles northeast of Mitchell. 

The 8:01 p.m. tornado warning received limited dissemination, mainly through NAWAS and 

secondary law enforcement channels. It did not go from the PC used to prepare warning products 

to AFOS and, consequently, did not reach NWWS and other systems feeding the media and 

EAS. The apparent reason for this dissemination problem was a communications traffic 

"bottleneck" at a peripheral sharing device (PSD) serving as the interface between a cluster of 

four PCs and AFOS. One of the NAWAS warning points, having been notified of the tornado 

warning at 8:02 p.m. by NAWAS phone, but not seeing a hard copy of the message on the state 

law enforcement telecommunications system, notified the forecast office at approximately 8:10 

p.m. that the message had not been received. By this time, the tornadic thunderstorm had crossed 

the Hanson County line. 

Just as a new Tornado Warning was being issued for northern Hanson County (at 8:12 p.m.), a 

telephone call came from the Kansas City NWSO relaying a report of a tornado in northern 

Hanson County, west of the town of Farmer. The report came from the WCM from the Kansas 

City NWSO, one of the many storm chasers in southeast South Dakota that evening. Hearing no 

active amateur radio networks or other accessible spotter networks at the time, and not having 

the telephone number of the Sioux Falls office handy, this NWS employee used his personal cell 

phone to call back to his own office. Just 1 minute later at 8:13 p.m., a report of a large tornado 

northeast of Mitchell was relayed by the Mitchell NAWAS warning point. 

At 8:16 p.m., the SPC issued a new Tornado Watch for extreme southeast South Dakota, 

northeast Nebraska, and northwest Iowa. This watch emphasized the "PARTICULARLY 

DANGEROUS SITUATION" (a PDS watch) evolving in southeast South Dakota. The NWSFO 

included McCook County in this new Tornado Watch. 

The next tornado reports came from several law enforcement officials between 8:23 and 8:29 

p.m. The first, relayed through the Mitchell NAWAS point at 8:23 p.m., indicated a tornado 5 

miles northwest of Fulton. Between 8:25 and 8:29 p.m., NAWAS points at Mitchell and Sioux 

Falls (Metro Communications Center) relayed reports from a South Dakota Highway Patrol 

officer, the McCook County Sheriff, and a deputy sheriff all indicating a tornado west of 

Spencer. A Tornado Warning was issued for northern McCook County at 8:32 p.m., and it was 

promptly disseminated through usual means to the media and law enforcement officials (i.e., 

NWWS, NAWAS, EAS, etc.). The NWR log indicates that the warning was broadcast live 

within 1 minute of issuance. 

At 8:50 p.m., the tornado was reported as it reorganized on the south side of I-90, approximately 

5 miles southeast of Spencer. Reports of damage in Spencer started reaching the forecast office 
just after 9 p.m. However, details regarding fatalities and the scope of the destruction did not 

reach the forecast office until after 10 p.m. 

With the exception of the PSD problem, resulting in the missed transmission of the 8:01 p.m 

tornado warning to AFOS, all of the equipment and computer systems used by the staff at the 

Sioux Falls NWSFO functioned well throughout the severe weather episode on the evening of 

May 30. Forecasters changed power at the WSR-88D radar from commercial to generator just 

after issuing the 8:12 p.m. Tornado Warning for northern Hanson County rather than risk loss of 

radar data as severe storms continued to approach Sioux Falls. The power change resulted in 

some loss of archive data but no loss of real-time data critical to the warning process. 

Assessment team interviews with key representatives of Sioux Falls television stations and area 

radio stations and service feedback questionnaires distributed to the media by the Sioux Falls 

WCM indicate a universal satisfaction with the service provided by the NWSFO during the May 

30 severe weather events. Broadcasters who were on the air that evening all indicated they had 

timely and accurate information. One TV meteorologist described the service provided as 

"excellent" and "perfect." 

Interviews with Spencer residents and second-hand information gathered through media accounts 

indicate a wide range in the level of awareness prior to the tornado and a wide variety of safety 

precautions taken during the event. Many survivors had some indication, either through media 

reports or by first-hand observation, of the potential severity of the event, although many 

residents were quite unaware. Many had time to take shelter in a basement or some other part of 

their homes, while others were caught with just enough time to get behind a piece of furniture or 

to lie on the floor. All six fatalities were adults, ranging in age from 62 to 93, and occurred in the 

southern half of Spencer where destruction was worst. 

Apparently, most of those who were watching television were tuned to one of the local network 

affiliates. The three local stations did an excellent job of broadcasting warnings as they were 

issued and providing "cut-ins" with displays of radar images. There were also many comments 

that the commercial radio stations did an excellent job of relaying critical warning information 

and reports. Two stations located in Mitchell played an especially vital role because of their 

proximity to Spencer and their first-hand accounts of upstream severe weather. The cable TV 

provider for Spencer has approximately 95 subscribers, but there was no cable-override system 

for the town. NWR reception from the Sioux Falls and DeSmet transmitters is marginal at best in 

Spencer. No one from the Service Assessment Team or the NWSFO is aware of any regular 

NWR listeners in Spencer. A few residents knew the radio frequency used by local spotters and 

received very timely and valuable information while monitoring on a scanner. 

 
NWSFO Sioux Falls Operational Procedures 

Dissemination of Warning and Forecast Products 

Response 

Service Coordination and Preparedness 

Data and Guidance Available to Forecasters 

FACT: Computer model data and surface and upper air observations, which are key elements of 

the forecasts and severe weather outlooks issued by an NWS forecast office, were available to 

the Sioux Falls staff in a timely manner prior to the severe weather events of May 30, 1998. 

FACT: Severe Weather Outlooks, Mesoscale Convective Discussions, and Severe Thunderstorm 

and Tornado Watches issued by the NWS SPC in Norman, Oklahoma, provided valuable and 

timely guidance to NWSFO Sioux Falls, the media, and emergency officials. The SPC called the 

forecast office just before issuing each of the three watches affecting its CWA during the 

afternoon and evening of May 30. The SPC made an additional coordination call at 7:15 p.m. 

FACT: During the severe weather events surrounding the Spencer tornado, the Sioux Falls WSR88D was the primary source of data on which warnings and other products, such as Severe 

Weather Statements, were based. The radar was operated in Volume Coverage Pattern (VCP) 11 

to provide the highest vertical resolution. The system operated without interruption, except 

briefly for a change to generator power at approximately 8:15 p.m. Radar imagery was available 

at the PUP workstation and on one "slave" monitor at the public forecaster's AFOS workstation. 

FACT: WSR-88D reflectivity and velocity data exhibited supercell characteristics in the 

thunderstorm that would eventually produce the Spencer tornado at about the same time (7:55 

p.m.) the first report of a funnel cloud was received at the Sioux Falls NWSFO. The funnel cloud 

was reported in northeast Davison County, about 22 miles west-northwest of Spencer. WSR-88D 

computer algorithms indicated a mesocyclone at 8 p.m. over extreme northwest Hanson County, 

about 19 miles from Spencer, and a Tornado Vortex Signature (TVS) at 8:38 p.m. near Spencer. 

FACT: Valuable severe weather observations from law enforcement officials were relayed in a 

timely manner through NAWAS warning points at Huron, Mitchell, and Sioux Falls (Metro 

Communications Center). These and a few reports from meteorologists who were storm chasing 

were the most reliable spotter reports available. They contributed significantly to many of the 

Severe Thunderstorm and Tornado Warnings issued during the event. 

Finding 1: There were at least 10 meteorologists from other parts of the country, including NWS 

forecasters, "chasing" the supercell thunderstorm which eventually produced the Spencer 

tornado. Two of these storm chasers provided reports to NWSFO Sioux Falls, but most were 

unaware of how to contact the office. One who did, the WCM from NWSO Kansas City, called 

his own office and asked the staff to relay his report to Sioux Falls. 
Recommendation 1: The NWS Office of Meteorology should form a team of NWS forecasters 

and experienced storm chasers to recommend methods of improving coordination between NWS 

forecast offices and "visiting" meteorologists engaged in storm chasing. 

Finding 2: Most of the key severe weather reports prior to the Spencer tornado originated from 

law enforcement officials. A small contingent of trained spotters from McCook County was 

activated prior to the Spencer tornado, but there is no record of their reports reaching the 

NWSFO at Sioux Falls. The NWSFO Sioux Falls WCM has tried on numerous occasions to 

enlist the aid of area amateur radio (HAM) operators to improve severe weather communications 

in the CWA, but progress has been slow. 

Recommendation 2: Central Region should provide the Sioux Falls NWSFO with examples of 

proven methods of enhancing communications with amateur radio networks for severe storm 

reporting. 

NWSFO Sioux Falls Operational Procedures 

FACT: NWSFO Sioux Falls has a well organized severe weather operation which receives 

strong emphasis in terms of guidance and training from station managers. 

FACT: Warning and forecast operations at NWSFO Sioux Falls are conducted using a 

combination of new and old technology, ranging from the state-of-the-art WSR-88D radar to 

systems, such as AFOS and tape-based NWR consoles which have been in use for two decades. 

The Automated Weather Information Processing System (AWIPS) is scheduled for delivery at 

Sioux Falls in March 1999, and the NOAA Weather Radio 2000 Console Replacement System 

(CRS), delivered in June 1998, will be implemented during the fall of 1998. 

FACT: At NWSFO Sioux Falls, warnings and related products are generated primarily by a twoperson team, consisting of a PUP operator who analyzes WSR-88D data and a warning 

forecaster who prepares warnings and statements at a PC. The PC is equipped with NWS 

developed formatting software for warnings (WISE) and locally developed formatting software 

for statements and Short Term Forecasts. 

FACT: The NWSFO Sioux Falls forecasters conducted a thorough analysis of all meteorological 

data available to them prior to the development of severe storms on the afternoon of May 30 and 

made an accurate and timely assessment of the severe weather threat in their CWA. 

FACT: Through detailed local analysis, forecasters at the NWSFO Sioux Falls decided to 

upgrade the early morning SPC Severe Weather Outlook on May 30 to a moderate risk for their 

CWA. The local outlook accurately specified two episodes of severe weather--one in the 

morning and the other in the evening. 

FACT: Staffing at the peak of severe weather events on May 30 consisted of four forecasters, 

one hydrometeorological technician, and one meteorologist intern. Two forecasters focused on 

warning operations (one at the PUP and one at the PC for warning and statement generation), 

and the intern worked exclusively on NWR programming. 
FACT: Senior forecasters at NWSFO Sioux Falls, well aware of the severe weather potential on 

May 30, made careful plans and adjustments to ensure adequate staffing for the severe weather 

events of the day. Five members of the staff worked a total of 18 hours in addition to their 

assigned shifts. This does not include the undocumented extra hours spent by the MIC the night 

of May 30 after the storm. One forecaster working the midnight shift remained on duty until 

noon Saturday morning, volunteered to return early that night, and was called back to duty at 9 

p.m. 

FACT: Between 5:30 p.m. and midnight on May 30, the NWSFO Sioux Falls staff issued 11 

Severe Thunderstorm Warnings covering 15 counties, eight Tornado Warnings covering nine 

counties, 26 Severe Weather Statements, and five Short Term Forecasts. 

FACT: The lead time on the Tornado Warning issued at 8:32 p.m. for northern McCook County 

was 6 minutes. 

Finding 3: The NWSFO Sioux Falls staff "rose to the occasion," working the extra hours 

necessary to ensure adequate staffing on May 30. The only staffing shortage noted was on the 

day following the Spencer tornado when the NWSFO was besieged by media inquiries from 

around the country, and most of the office managers were engaged in initial damage surveys and 

on-site coordination with the media and local officials. 

Recommendation 3: WSH should alert Regional Directors to advise managers of forecast offices 

to include in significant event staffing plans additional staff for the day or two following an 

event. 

Finding 4: The number and types of warning and forecast products issued by NWSFO Sioux 

Falls during the Spencer event are consistent with that office's stage of evolution to a fully 

modernized NWS WFO. However, the PUP-based severe weather operation limited the 

availability of radar data to forecasters at other workstations. This hampered the issuance of 

more frequent, detailed Short Term Forecasts which could have provided an hour or two "heads 

up" for areas downstream of existing severe weather. 

Recommendation 4: With the implementation of AWIPS, and the increased availability of radar 

data at workstations in forecast offices, NWS should emphasize the utility of frequent, detailed 

Short Term Forecasts to provide a "heads-up" for areas where severe weather is expected in a 1- 

to 2-hour time frame and to provide a "bridge" between watches and warnings. 

Finding 5: Of the 45 warnings and statements issued by NWSFO Sioux Falls during the evening 

of May 30, only 13 contained spotter reports. Finding 2 addressed limited spotter reports initiated 

from the field; however, the PUP-based severe weather operation also limited the ability of the 

NWSFO staff to make probing calls into severe weather areas to obtain reports. 

Recommendation 5: With the implementation of AWIPS, and the availability of radar data at all 

workstations, the NWS regional offices should provide direction to forecast offices on adopting a 

more proactive approach to obtaining spotter reports and incorporating them in their warning 

products. 
Dissemination of Warning and Forecast Products 

FACT: The Sioux Falls NWSFO operates three NWR stations which broadcast warnings and 

forecasts for 25 of the 45 counties in the CWA. The warning alarm on the Sioux Falls NWR 

(WXM-28) transmitter is activated when warnings are issued for McCook County; however, the 

broadcast signal is only strong enough for reliable reception in the eastern part of the county. 

FACT: The NWR equipment used at the Sioux Falls NWSFO on May 30 is older technology and 

uses record/play decks and magnetic tapes. Hardware for the NOAA Weather Radio 2000 

Console Replacement System arrived at the NWSFO in early June. The system will be 

implemented through the fall of 1998. 

FACT: Between 7 p.m. and midnight on May 30, one of the NWSFO Sioux Falls meteorologist 

interns was assigned the specific task of NWR broadcasts. During the peak of the severe 

weather, all of his warning broadcasts were made within 2 minutes of the time the warnings were 

issued. 

FACT: From logs which were available and from interviews with media representatives, the 

EAS functioned properly and was useful in the relay of critical warning information to the 

media. 

FACT: From all accounts obtained by the Service Assessment Team, the dissemination of 

critical severe weather information through the commercial media was excellent. Most of those 

in Spencer who were aware of warnings and ongoing severe weather received information 

through the commercial media. Sioux Falls television stations and two commercial radio stations 

in Mitchell were mentioned most often by those interviewed, however, other radio stations in the 

area, including those in Sioux Falls and Yankton, also played valuable roles in disseminating 

warnings. 

FACT: The cable television provider for Spencer has 95 subscribers in town, however, there was 

no cable override system for local emergency management access. 

FACT: Several residents of Spencer who knew the radio frequency used by local storm spotters 

kept themselves informed on the approaching tornado by listening to the spotter traffic on a 

scanner. 

Finding 6: The Tornado Warning issued at 8:01 p.m. for northeast Davison and northern Hanson 

Counties did not reach AFOS from the PC used to compose warning products. Consequently, the 

warning was not disseminated over NWWS and other distribution systems which feed the media 

and other users. While it is impossible to be certain, the most likely point of failure is the 

peripheral sharing device used to link four PCs with one AFOS communications port. 

Recommendation 6: The Sioux Falls NWSFO should move the PC used to generate warnings to 

a dedicated AFOS port. If a dedicated port cannot be made available, alternatives should be 

explored to reduce the number of PCs sharing the PSD with the warning PC. 
Finding 7: The NWR coverage on May 30 was not adequate to ensure reliable reception of 

severe weather information on standard, home-use receivers in Spencer. 

Recommendation 7: The Sioux Falls NWSFO should review its NWR expansion plan, update it 

if necessary, and redistribute. By spring 1999, the office should identify potential partners for 

obtaining NWR transmitters for each site in their plan. 

Finding 8: In spite of attempts by NWSFO Sioux Falls to promote use of the Emergency 

Management Weather Information Network (EMWIN), there are no known users of the system 

in the Sioux Falls CWA. This system could have been useful on May 30 for the dissemination of 

warning and forecast products to emergency managers, law enforcement officials, etc. 

Recommendation 8: Central Region should contact the appropriate state officials responsible for 

the emergency management programs to encourage emergency managers in the Sioux Falls 

CWA to install EMWIN before the next severe weather season. 

Response 

FACT: In spite of the devastation inflicted on Spencer, many residents obtained sufficient 

warning information through media sources or by first-hand observation and avoided death or 

serious injury by taking appropriate precautions. There was, however, a significant segment of 

the small population who remained uninformed until the destruction began. With the volunteer 

fire department siren not functioning and the nearest NWR station essentially out of range, the 

only practical, readily available sources of warning information for most Spencer residents were 

television and commercial radio. 

FACT: The Spencer tornado occurred when many people were watching television. Viewers in 

Spencer who were tuned to a Sioux Falls station were provided information on the 8:12 p.m. 

Tornado Warning for northern Hanson County and the 8:32 p.m. Tornado Warning for northern 

McCook County. 

FACT: Residents who heard some type of warning in time to take precautions indicated Sioux 

Falls television stations or commercial radio stations as their source of warning information. 

Several residents of the area cited two radio stations in Mitchell, which included listener reports 

with warnings from NWSFO Sioux Falls. 

FACT: Residents who took precautions indicated they took steps to protect themselves from 

flying debris, taking refuge in basements, under stairways, in hallways, and in closets. 

FACT: Basements in many of the homes which were completely destroyed were filled with 

debris. Some residents survived by taking the extra precaution of finding a secondary sturdy 

shelter within the basement. 

FACT: The six persons killed by the Spencer tornado ranged in age from 62 to 93. 
Service Coordination and Preparedness 

FACT: The NWSFO Sioux Falls staff, especially the MIC and WCM, maintain very active and 

effective preparedness and outreach programs. Documentation for 1997 and 1998 shows a very 

long list of spotter training classes, presentations for service organizations, school visits, media 

visits and workshops, and other public activities. Spotter training was conducted in April for 

several of the counties affected by the May 30 severe weather outbreak. Severe weather 

preparedness week was conducted in South Dakota during the week of April 20. All of these 

efforts have no doubt raised the level of severe weather awareness and education throughout the 

NWSFO Sioux Falls CWA. 

FACT: A long-term relationship between the staff at NWSFO Sioux Falls and local media has 

produced an excellent working relationship which played a major role in effective severe 

weather warning operations on May 30. Representatives of the media who were interviewed by 

members of the Service Assessment Team were very familiar with the NWS products and 

services and with the NWSFO Sioux Falls warning operation. 

FACT: The NWSFO Sioux Falls MIC and WCM also have an excellent working relationship 

with the Federal, state, and local officials who have jurisdiction in their CWA. 

FACT: The NWSFO Sioux Falls staff was well prepared for the events of May 30. It is obvious 

to members of the Service Assessment Team that severe weather operations have received a 

great deal of emphasis and that guidance and training for warning operations have been a high 

priority among the NWSFO managers. 

 
Appendix A 

Fujita Tornado Intensity Scale

The Fujita Tornado Intensity Scale is a scale of wind damage intensity which wind speeds are 

inferred from an analysis of wind damage. 

Category Definition and Effect

FO Gale tornado (40-72 mph): Light damage. Some damage to chimneys; break branches 

off trees; push over shallow-rooted trees; damage sign boards. 

F1 Moderate tornado (73-112 mph): Moderate damage. The lower limit is the beginning of 

hurricane wind speed; peel surface off roofs; mobile homes pushed off foundations or 

overturned; moving autos pushed off the roads. 

F2 Significant tornado (113-157 mph): Considerable damage. Roofs torn off frame houses; 

mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; lightobject missiles generated. 

F3 Severe tornado (158-206 mph): Severe damage. Roofs and some walls torn off wellconstructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off 

ground and thrown. 

F4 Devastating tornado (207-260 mph): Devastating damage. Well-constructed houses 

leveled; structure with weak foundation blown off some distance; cars thrown and large 

missiles generated. 

F5 Incredible tornado (261-318 mph): Incredible damage. Strong frame houses lifted off 

foundations and carried considerable distance to disintegrate; automobile-sized missiles fly 

through the air in excess of 100 yards; trees debarked; steel-reinforced structures badly 

damaged; incredible phenomena will occur. 

Appendix B

Spencer, South Dakota, Tornado

May 30, 1998

Chronological Log 

Valid 

Time 

CDT

Product 

Type/Report

Counties 

Affected

NWR Log

Time

EAS 

Log

Time

Remarks

0405 SFP Southeast 

SD 

NA NA "some thunder/ storms could be severe 

south/ east" - tonight

0405 ZFP Hanson, 

McCook and 

Turner 

NA NA "some thunder/ storms could be severe" -

tonight

0435 SPS Southeast 

SD 

NA NA "Moderate Risk of severe weather today"

1100 ZFP Hanson, NA NA "some thunder/ storms could be severe" - 

tonight
McCook, 

Turner 

1147 SPS Southeast 

SD 

NA NA "severe winds, 

large hail and tornadoes possible" /

"spotter activation likely after 4 PM"

1550-2200 SEL8 McCook 1548 NA Tornado Watch #468

1625 ZFP McCook, 

Hanson, 

Turner 

NA NA Tornado Watch for McCook, "severe 

thunderstorms" Hanson and Turner

1730 NOW McCook, 

Hanson, 

Turner 

NA NA Thunderstorms this evening "could 

become severe"

1845-1955 SVR Beadle NA NA 

1850 NOW Huron (city) NA NA storms impact on Huron

1903 SVS Beadle NA NA Pathcast update

1919 .75 inch hail Beadle 3 W of Huron

1923 SVR Beadle NA NA 

1940-2010 SVR Sanborn 1937 NA 10 to 40 miles northwest of Spencer

1944-2015 SVR NE Davison

N Hanson 

NA NA 

1945 TORNADO Kingsbury Between Desmet & Lake Preston

1946 SVS Sanborn NA NA Updated location and

movement

1955 FUNNEL 

CLOUD 

NE Davison Spotter

1955 SVS Beadle 1956 NA Warning Expiration

2001-2035 TOR NE Davison

N Hanson 

NA NA Replaced SVR

(NAWAS only)

2002 TORNADO Kingsbury 5 W Lake Preston

2008 TORNADO Hanson 8 NE Mitchell

2012 TORNADO Hanson W of Farmer. Report from EAX WCM

2012-2045 TOR N Hanson NA NA 

2013 LARGE 

TORNADO

Hanson NE of Mitchell

2013 SVS NE Davison NA NA Cancelled TOR for NE Davison
2015 1.75 inch hail Hanson 7 E Loomis

2016-0300 SEL9 Hanson, 

McCook, 

Turner 

2039 NA Tornado Watch #469 

PDS Watch

2023 TORNADO Hanson 5 NW Fulton.

Report from Mitchell PD

2024-2055 SVR Miner 2026 NA 

2025 TORNADO Hanson 1 W Spencer. 

Report from McCook Co. Sheriff

2029 LARGE 

TORNADO

Hanson W of Spencer. Report from Mitchell PD

2032-2105 TOR McCook 2032 2034 Live and recorded simultaneously on 

NWR

2034 SVS Miner NA NA 

2044 TORNADO McCook 1 Dead, 47 Injured. Extensive damage in 

Spencer

2045 TORNADO Hanson 5 W Emery

2045-2120 TOR Hanson NA NA 

2050 TORNADO McCook 9 N Bridgewater

2052 SVS Miner 2052 NA Cancelled Warning

2054 SVS N McCook 2058 NA Tornado report near I-90 & Bridgewater 

exit

2055 TORNADO Hanson At Alexandria

2055 ZFP Hanson, 

McCook, 

Turner 

NA NA Updated for Tornado Watch #469

2059 SVS Hanson NA NA Tornado report & radar indication near 

Alexandria

2100 Wind G60 W Lyon From Lyon EM

2104-2140 TOR McCook 2106 2107 Live and recorded simultaneously on 

NWR

2109-2140 SVR E Lake, 

Moody 

2110 2112 Live and recorded simultaneously on 

NWR

2110 TORNADO McCook At Bridgewater

2111 SVS McCook 2117 NA Tornado reported near Bridgewater.

Radar indicated near Bridgewater & SW 

of Montrose
2115 Wind 

Damage 

Lyon MN N Russell

2116-2150 SVR Lincoln MN,

Lyon MN 

NA NA 

2117 Funnel Cloud Minnehaha 2 W Wall Lake. Source Spotter

2120 TORNADO Turner 2 E Monroe

2120-2210 TOR Minnehaha 2122 2124 Live and recorded simultaneously on 

NWR

2121 SVS Hanson NA NA Warning Expiration

2125 NOW Most of 

CWA 

NA NA Severe Weather Potential

2128-2200 TOR N Turner 2129 2131 Live and recorded simultaneously on 

NWR

2131 SVS E Lake, 

Moody 

2134 NA Updated location

2134 SVS McCook 2136 NA Updated tornado location (radar)

2136 Wind 

Damage 

Lyon MN 5 N Russell

2138 SVS Lincoln 

Lyon MN 

NA NA Updated storm location

2141 SVS Moody 2143 NA Warning expiration

2144 SVS E McCook 2146 NA Warning expiration

2146 SVS Lyon MN NA NA Warning expiration

2148-2220 SVR Lincoln 

(N&C) 

2149 2150 

2148 SVS N Turner NA NA Updated storm location

2150 SVS Minnehaha 2154 NA Public report of tornado west of Sioux 

Falls

2156-2220 TOR Lincoln 

(N&C) 

2157 2159 

2201-2255 SVR Rock MN 

Lyon IA 

2205 2205 Live and recorded simultaneously on 

NWR

2203 SVS N Turner 2206 NA Warning Expiration

2208-2240 SVR S Lincoln 

Sioux IA 

2210 2211 Live an recorded simultaneously on NWR

2210 SVS Minnehaha 2213 NA Warning Expiration

2218 SVS Lincoln 

(N&C) 

2219 NA Warning Expiration
2228-2300 SVR Nobles 2229 2230 Live and recorded simultaneously on 

NWR

2229 SVS Rock MN 

Lincoln 

2231 NA Warning Expiration

2232 SVS Sioux IA 

Lincoln 

2241 NA Warning Cancellation

2240 ZFP All covered 

by Watch 

#469 

NA NA Remove Tornado Watch

2240 SVS Sioux IA NA NA Warning

Expiration

2242 SVS Nobles MN 2245 NA Updated location & pathcast

2300 SVS Nobles MN 2302 NA Warning Expiration

Notes: For Column 3, Counties Affected, only Hanson, McCook and Turner Counties are listed, 

as applicable, for ZFPs, SPSs, and Watches. 

NA = Not Available or Not Applicable.