Late-May 1998 tornado outbreak and derecho

NWS damage survey map tracking the tornado families in southeast South Dakota

The first severe weather of the outbreak was reported at 12:30 p.m. in southeast Montana. Several hours later a supercell thunderstorm produced 2.75-inch (7.0 cm) hail across southeast Montana, kicking off the outbreak in earnest. Numerous reports of very large hail were received throughout the outbreak with the largest official report of 3.00 inches (7.6 cm) 10 miles (16 km) north of St. Lawrence in east-central South Dakota. The hail itself produced thousands of dollars in damage. Many reports of severe straight-line winds and damage were also reported. Numerous storm chaser reports suggest that significant severe weather events also occurred in the sparsely populated area traversed by the storm.

Later that evening, a supercell in southeastern South Dakota produced a series of tornadoes. The family of tornadoes that crossed the Spencer area was observed by a Doppler On Wheels (DOW) radar (Wurman et al. 1997, Wurman 2001). The DOW observed the tornadoes from before 8:04 through 8:45 pm local time (01:04–01:45 UTC)(Alexander and Wurman 2005) and the passage of a destructive F4 tornado through Spencer itself from 8:37-8:38 (01:37-01:38 UTC). DOW measurements of tornadic winds over the largely destroyed southern portion of Spencer have permitted the first (and only as of December 2006) direct comparison of measured winds with F (or EF) Scale damage ratings as reported in the above referenced articles. Peak observed Doppler winds of near 115 m/s (260 mph) corresponded well with the documented F4 damage.

The DOW observations showed that the list of tornadoes derived from damage surveys alone, and the F-scale rating of that damage, may be incomplete and underestimate actual tornado intensity (Wurman and Alexander 2005). Single tornadoes may be mis-characterized as multiple tornadoes due to breaks in the observed damage. DOW measurements suggest that the F4 tornado may have a multiple-vortex structure as it struck Spencer.

Spencer, South Dakota

F4 tornado
The Spencer, South Dakota tornado
Max. rating1	F4 tornado
Fatalities	6 fatalities, 150 injuries
Damage	$18 million (1998 USD)
Areas affected	Spencer, South Dakota
1Most severe tornado damage; see Fujita scale

The Spencer, South Dakota F4 tornado was the most destructive and second deadliest tornado in the history of the state. It was also the fifth deadliest tornado of the year.[1] It began as a large, dust-cloaked tornado northwest of Farmer, South Dakota in Hanson County, concurrent with the demise of the "Fulton" tornado. Continuing toward the east-southeast, it struck several farmsteads before crossing the Hanson/McCook County line a half mile west-northwest of Spencer. At this time, the tornado was being observed by University of Oklahoma's Doppler on Wheels crew, whose mobile Doppler radar data showed up to 220 mph winds in the tornado only a few decameters above the ground.[2]

This tornado carved a 3⁄8 mi (0.60 km) wide path directly across the town of Spencer between 8:38 and 8:44 pm CDT, destroying or damaging all but a few houses on the Northeast side of town, and blowing over the water tower. Several homes in town were swept from their foundations by the tornado. The tornado killed six people, injured 150 (which is more than one-third of the town residents), and destroyed most of the town's 190 buildings. Many trees and power lines were downed, and vehicles were destroyed as well. 5 of the 6 fatalities occurred in an apartment building that collapsed. Damage was estimated at $18 million.[3] The population of the town diminished soon after to less than half of what it was previous to the tornado, from 315 to 145 in April 1999.

This derecho got its start from a developing low pressure system that moved into the northern Great Plains and Great Lakes. The derecho formed from the same storm system that spawned the Spencer, South Dakota Tornado, which killed six people. The supercell thunderstorm which produced that tornado transitioned into the derecho which killed another six people. It would become the most destructive natural disaster to hit the Upper Midwest in recent memory.

On Sunday, May 31, Friday's cold front, which stalled over southern Pennsylvania, started moving quickly back northward as a warm front as strong low pressure approached the Great Lakes. North of the warm front, most of the Hudson Valley and Western New England were under a cool and stable marine air mass, as a result of southeast winds from the Atlantic Ocean. Around 8 am, the warm front was located roughly over the Mohawk River to central and northern Massachusetts, producing powerful thunderstorms. After the warm front's passage, sunshine broke out and strong heating commenced across the Northeast with temperatures quickly rising from the 50s through the 70s into the 80s, and dewpoints skyrocketing from the lower 40s into the upper 60s. surface winds began blowing from the southeast at around 30 mph, while the winds at mid-levels were from the southwest at 60 to 80 mph, and the winds at jetstream level were still ranging from 120 to 150 mph from the west. High levels of instability were present as well. This created an unusually highly sheared environment in the Northeast.

The Storm Prediction Center issued a High Risk for severe thunderstorms across Northern Pennsylvania, most of New York, Western Massachusetts and Western Vermont. A High Risk had never before been issued for the Northeastern United States, and hasn't been since this event. This indicated an abnormally dangerous weather situation for this region of the country, a situation more common in the Plains States and Midwest.

Around 1 pm, the Michigan derecho was moving into New York from Ontario and rapidly weakening. The decaying squall line broke apart into discrete convection over New York and Pennsylvania, which quickly re-intensified into tornadic supercells.

The most destructive tornado of the day was an F3 that tore through Mechanicville and the adjacent town of Stillwater. It caused major damage to Mechanicville's old industrial section located on Route 4 and 32 along the Hudson River. One of the two historic smokestacks (visible from 2 miles away) was knocked down by the tornado. In 2005, the other smokestack and the conjoined building were bulldozed. The tornado was rated F3 (winds estimated at 200 MPH at the time) on the Fujita scale, and was 970 yards (890 m)iles) wide. In total, 70 homes and businesses were completely demolished, and hundreds more damaged. Extensive deforestation occurred along the path as well. 68 people were injured, but no loss of life occurred. Pink insulation was reported falling from the sky ahead of the tornado, in Valley Falls, New York, 12 miles east of Mechanicville. Film negatives from Mechanicville were reportedly found there as well.

Numerous other strong tornadoes touched down across upstate New York and Pennsylvania that evening, several of which reached F2 to F3 intensity. One F3 caused major damage in the city of Binghamton. The other F3 caused major damage in the village of Windsor. This outbreak was likely the most intense, widespread, and long duration severe weather event in modern New York state history. Other less intense tornadoes touched down in surrounding states.

• List of derecho events
• June 2012 North American derecho
• Derecho and tornado outbreak of April 4–5, 2011

• "The Southern Great Lakes Derecho of 1998" (Storm Prediction Center)

• "Upper Midwest hit by hurricane-force storm" (CNN)
• Detailed SPC overview
• NWS overview
• CIMMS satellite imagery and meteorological analysis
• CSWR Doppler-on-wheels Spencer page