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.

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.

Several hours later a supercell thunderstorm produced 2.75-inch (7.0 cm) hail across southeast Montana, kicking off the outbreak in earnest.

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.

Continuing toward the east-southeast, it struck several farmsteads before crossing the Hanson/McCook County line a half mile west-northwest of Spencer.

The derecho formed from the same storm system that spawned the Spencer, South Dakota Tornado, which killed six people.

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.

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 (164 km/h) sustained for a number of minutes and gusts up to 140 mph (230 km/h) also in Dodge County and/or adjacent sections of Fond du Lac County.

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.

Extensive damage to Grandville (approximately 30 miles (48 km) inland) led to the city being closed off, with no traffic allowed to enter in the day after the derecho's passage.

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).

Buildings in Toronto lost numerous windows and significant damage was observed in areas such as Trenton, Napanee, Picton and Kingston.

Overall, the derecho traveled 975 miles (1,569 km) from southern Minnesota to central New York in 15 hours at an average speed of 65 mph (105 km/h).

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.

The decaying squall line broke apart into discrete convection over New York and Pennsylvania, which quickly re-intensified into tornadic supercells.

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.

Numerous other strong tornadoes touched down across upstate New York and Pennsylvania that evening, several of which reached F2 to F3 intensity.

This outbreak was likely the most intense, widespread, and long duration severe weather event in modern New York state history.