Late-May 1998 tornado outbreak and derecho

The Late-May 1998 tornado outbreak and derecho was a historic tornado outbreak and derecho that began on the afternoon of May 30 and extended throughout May 31, 1998, across a large portion of the northern half of the United States and southern Ontario from southeastern Montana east and southeastward to the Atlantic Ocean. The initial tornado outbreak, including the devastating Spencer tornado, hit southeast South Dakota on the evening of May 30. The Spencer tornado was the most destructive and the second-deadliest tornado in South Dakota history. A total of 13 people were killed; 7 by tornadoes and 6 by the derecho. Over two million people lost electrical power, some for up to 10 days.

The derecho was the most violent line of thunderstorms observed on earth during the 1998 calendar year according to the National Weather Service review shortly after the year was over. It was the climax of an unusually heavy derecho season in the North-Central United States and adjacent parts of Canada and the Northeast United States, and this storm combined the characteristics of the two major forms of derecho, the serial and progressive derecho, known as a hybrid derecho. At various points of its evolution, it displayed textbook or record manifestations of supercell and derecho-related phenomena such as the right-mover supercell, evolution of supercells into a linear meso-scale feature which rapidly became a derecho, cumulonimbus with overshooting top and dome, bow echo, bookend vortices, regular and rotor downbursts, gust front, gustnado, rear inflow notch, classic derecho radar signature, effects of infrasound and atmospheric electricity, haboobs, and wind effects on bodies of water including seiches and exposure of bottoms of water features by the wind. The disturbance which was originally the derecho finally disappeared off the coast of Norway more than a week later.

May 30 South Dakota event
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 in 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 in 10 mi 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 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 had a multiple-vortex structure as it struck Spencer.

Storm chasers William T. Reid, Martin Lisius, Keith Brown, and Cheryl Chang forecasted and intercepted the large and destructive Spencer, South Dakota tornado moments before it entered the town. The chase team was positioned east of town and first observed the tornado at approximately 8:35 pm and captured numerous photographs, video, and 35mm motion picture film of the event as it unfolded.

Spencer, South Dakota
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. It began as a large, dust-cloaked tornado northwest of Farmer, South Dakota in Hanson County, concurrent with the demise of the second "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 the University of Oklahoma's Doppler on Wheels crew, whose mobile Doppler weather radar data showed up to 220 mph winds in the tornado only a few decameters above the ground.

This tornado carved a 3/8 mi 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. 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.

May 30–31 Southern Great Lakes derecho
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.

Minnesota
The most damage in Minnesota occurred at the northern edge in Sibley and McLeod Counties. Winds ranged from 80 - 100 mph in those two counties.

After the derecho raced through Minnesota, tens of thousands of trees were blown down. There were 500,000 customers without power. Over 100 homes were destroyed or damaged beyond repair. Twenty-two people were injured. The derecho caused $50 million in damage in southern Minnesota and northern Wisconsin.

Wisconsin
The derecho raced across Wisconsin in only three hours killing one person in Washington County when a tree fell through the roof and onto her bed where she was sleeping. It injured 37 people in Wisconsin. Many utility companies and emergency customers said that this was the most damaging straight-line wind thunderstorm event in 100 years. Five thousand homes and businesses were damaged and 24 were destroyed.

An area of south-central, south-east, and east-central Wisconsin reported wind gusts of over 100 mph from this thunderstorm complex with an all-time official state record gust of 128 mph 3 mi north-east of Watertown in Dodge County. As with other derechos like the July 4, 1977 blow-down in northern Wisconsin, there were other unofficial reports of higher winds as well as estimates of such, including winds of 102 mph sustained for a number of minutes and gusts up to 140 mph also in Dodge County and/or adjacent sections of Fond du Lac County.

The roar of the wind from the derecho was audible up to 30 mi away as the storm traversed this region. Road signs and other metal structures were found thrown about and even violently twisted in the area of 100 mph-plus wind from north-east Dane County through Dodge and Fond du Lac and Washington counties—evidence of both straight-line winds and rotor downbursts were widespread in this area as well as many other points along the path of the storm, and the sound of wind and perhaps hail and the effects of infrasound from some parts of the storm complex was also reported. In the latter case, some people reported strange feelings of pressure, and dogs, cats, and raccoons went wild.

The derecho also caused boating accidents by generating a seiche on Lake Michigan which was reported to be around 10 ft high as it first struck the Michigan coastline of Lake Michigan further north in Muskegon County.

Central Great Lakes
The storm raced through Michigan in only two hours at an average speed of 70 mph. Four people were killed in Michigan, and 146 were injured. Total damage was estimated at $172 million (1998 dollars). 250 homes and 34 businesses were destroyed. In Grand Haven the Story & Clark smokestack at the Piano Factory Condominiums was destroyed when the force of the high wind caused it to crumble, and trees collapsed all over the city, some falling onto roofs. Damage in Spring Lake was worse, due to a highly localized zone of higher winds. The Mill Pointe Condominiums suffered serious damage, including the collapse of one unit. Others were subsequently removed. A factory lost its roof in the storm, and a number of businesses were damaged severely, one beyond repair. Country Estates Mobile Home park also suffered serious damage. Extensive damage to Grandville (approximately 30 mi inland) led to the city being closed off, with no traffic allowed to enter in the day after the derecho's passage. A woman was killed in Pinconning, about 130 mi north of Detroit, when a tree fell on her house.

This derecho would go on to break the record for biggest power outage ever in the state of Michigan (but later surpassed by the 2003 North America blackout). 860,000 people lost power, slightly more than the number from the Southern Great Lakes Derecho of 1991. It blew down five 345-kilovolt transmission towers owned by Consumers Energy.

A total of 13 counties in Central Lower Michigan were declared federal disaster areas.

One person drowned in Ontario when his boat turned over from the derecho's strong winds. Buildings in Toronto lost numerous windows and significant damage was observed in areas such as Trenton, Napanee, Picton and Kingston. Heavy thunderstorm activity was also reported in Ottawa and Montreal, but without damage.

It caused $300,000 worth of damage in central New York before reforming into a tornado outbreak at around 11 am on May 31.

Summary
Overall, the derecho traveled 975 mi from southern Minnesota to central New York in 15 hours at an average speed of 65 mph. It became one of the most damaging derecho events in North America's history, causing $300 million (1998 USD) in damage.

May 31 New York and Pennsylvania tornadoes
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, Midwest and Deep South.

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 ydiles) 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.