Biography of the Storm
The following events lead up to the events of 8:40 PM.
The potential for severe weather on May 30 in parts of South Dakota, Minnesota, Nebraska, and Iowa was recognized by forecasters the day before.
2:55 a.m. Friday, May 29 - The NWS included the Sioux Falls area in its Severe Weather Outlook (SWO). In an update issued at 12:02 p.m. Friday, the forecaster specifically mentioned "EXTREME INSTABILITY," "SUPERCELL THUNDERSTORMS," and the "THREAT OF ISOLATED TORNADOES."
12:54 a.m. Saturday - Another Severe Weather Outlook placed a moderate risk of severe storms for eastern portions of the Sioux Falls County Warning Area.
4:35 a.m. Saturday - Sioux Falls forecasters issue a Special Weather Statement (SPS) headlined a "...MODERATE RISK OF SEVERE WEATHER TODAY...SOME THIS MORNING AND SOME THIS EVENING...."
11:47 a.m. - A Significant Weather Outlook issued by NWS Sioux Falls and was already focused on the likely redevelopment of thunderstorms "...LATE THIS AFTERNOON OR EARLY THIS EVENING...."
2:19 p.m. - A Mesoscale Convective Discussion (MCD) 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.
3:33 p.m. - A Severe Thunderstorm Watch for much of South Dakota, north of I-90 is issued, 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.
4:30 p.m. - Towering cumulus formed on a line near the Missouri River, about 90 miles west-northwest of Spencer producing a brief thunderstorm.
6:35 p.m. Stronger storms have developed 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.
6:45 p.m. - The first warning issued by the Sioux Falls NWS for this event was a severe thunderstorm warning for Beadle County. 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.
7:19 p.m. - The first severe weather report of the evening (three-quarter-inch diameter hail) came from 3 miles west of Huron - more than 30 minutes after the first warning was issued.
7:26 p.m. - The Spencer storm almost immediately split into left and right moving cells with the right-mover becoming a supercell as a mid-level mesocyclone.
7:45 to 7:50 p.m. The mesocyclone grew downward toward cloud base. 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.
7:55 p.m. - The Spencer supercell begins to show stronger tornadic potential. A funnel cloud was reported in northeast Davison county by the County Sheriff. A Severe Thunderstorm Warning had been in effect since 7:44 p.m. for northeast Davison and northern Hanson Counties.
8:01 p.m. - The funnel cloud report combined with radar data led to the first tornado warning, issued at for northeast Davison and northern Hanson Counties.
** The 8:01 p.m. tornado warning received limited dissemination, mainly through NAWAS (National Warning System) and secondary law enforcement channels. 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 (Automation of Field Operations and Services). One of the National Warning System points, having been notified of the tornado warning at 8:02 p.m. by 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.
8:08 p.m. - The first tornado report on the Spencer storm was the report from approximately 8 miles northeast of Mitchell. This is Tornado #1 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.
8:12 p.m. - A new Tornado Warning was being issued for northern Hanson County. At the same time a telephone call came from the Kansas City National Weather Service Office relaying a report of a tornado in northern Hanson County,
west of the town of Farmer. The report came from 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 warning point.
8:15 p.m Tornado #1 ends northwest of Fulton. The mesocyclone was still situated at ground-level.
8:16 p.m. - A new Tornado Watch was issued 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. McCook County was included in this new Tornado Watch.
8:17 p.m. The low-level circulation produced Tornado #2. 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 continued moving east-southeast.
A new damage track (Tornado #3, the Spencer Tornado) began almost immediately, approximately 1 mile northeast of the end of Tornado #2.
From 8:23 to 8:37 p.m., the Spencer tornado tracked through farmland, within 1 mile of the town of Farmer.
8:23 to 8:29 p.m. - Tornado reports come from several law enforcement officials. The first 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.
8:32 p.m. - A Tornado Warning was issued for northern McCook County - it was broadcast live within 1 minute of issuance.
8:40 p.m. (approximately) An F4 tornado virtually destroys the town of Spencer, SD.
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 Doppler on Wheels and NWS data, however, the town of Spencer experienced violent tornadic conditions from 8:38 to 8:39 p.m. Wind speeds as the tornado passed through Spencer, reached 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 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.
The information above was contained in an Event Summary issued by the National Weather Service.