Meteorological synopsis 1974 Super Outbreak

surface analysis @ 6 pm cdt on april 3, drawn national meteorological center

a powerful spring-time low pressure system developed across north american interior plains on april 1. while moving mississippi , ohio valley areas, surge of moist air intensified storm further while there sharp temperature contrasts between both sides of system. officials @ noaa , in national weather service forecast offices expecting severe weather outbreak on april 3, not extent occurred. several f2 , f3 tornadoes had struck portions of ohio valley , south in separate, earlier outbreak on april 1 , 2, included 3 killer tornadoes in kentucky, alabama, , tennessee. town of campbellsburg, northeast of louisville, hard-hit in earlier outbreak, large portion of town destroyed f3. between 2 outbreaks, additional tornado reported in indiana in morning hours of april 3, several hours before official start of outbreak. on wednesday, april 3, severe weather watches issued morning south of great lakes, while in portions of upper midwest, snow reported, heavy rain falling across central michigan , of ontario.

upper-level winds during super outbreak

by 12 utc on april 3, large-scale trough extended on of contiguous united states, several modest shortwaves rotating around broad base of trough. mid-latitude low-pressure center on kansas continued deepen 980 mb (28.94 inhg), , wind speeds @ 850-mb level increased 50 kn (58 mph) (25.7 m/s (93 km/h)) on portions of louisiana, mississippi, , alabama. due significant moisture advection, destabilization rapidly proceeded apace; warm front near gulf coast dissipated , redeveloped northward on ohio river valley. consequently, cape levels in region rose 1,000 j/kg. however, warm temperature plume in elevated mixed layer kept thunderstorms initiating @ surface. meanwhile, large mesoscale convective system (mcs) had developed overnight in arkansas continued strengthen due strong environmental lapse rates. later in day, strong daytime heating caused instability further rise: 18 utc, cape values in excess of 2,500 j/kg present on lower ohio , mississippi valley. wind speeds in troposphere increased, large-scale lifting overspread warm sector. @ same time, forward-propagating mcs spread tennessee , ohio valleys, evolved first of 3 main convection bands produced tornadoes. first convective band moved rapidly northeast, @ times reaching speeds of 60 kn (69 mph) (30.9 m/s (111 km/h)). however, thunderstorm activity, moment, remained elevated in nature.

by 1630 utc, large mcs began splinter 2 sections: southern part slowed, lagging southeast tennessee, while northern part accelerated, reaching pennsylvania 1930 utc. split related several factors, including band of subsidence on eastern kentucky , western west virginia; local downslope winds on appalachians; , inversion on same area. these factors allowed northern part of mcs accelerate due efficient ducting, while southern part slowed boundary layer warmed , moistened. numerous surface-based supercells began develop in southern area, beginning 1 produced f3 tornado @ 1630 utc near cleveland, tennessee. meanwhile, new band of scattered thunderstorms developed @ 1500 utc on eastern arkansas , missouri; on next 4 hours, band became focus several intense supercells, starting in eastern illinois , southern indiana. in wake of mcs, backing low-level winds, rapid diurnal destabilization, , perhaps cool, mid-level advection had occurred on warm sector, weakening convective inhibition (cinh) layer, , favorable wind profiles bolstered helicity on 230 m²/s²—a combination of factors conducive tornadogenesis. consequently, storms increased in intensity , coverage moved illinois, indiana, , northern kentucky, producing several tornadoes, including first f5 tornado of day, @ 1920 utc, near depauw, indiana. several of storms form between 1920 , 2020 utc became significant, long-lived supercells, producing many strong or violent tornadoes, including 3 f5s @ depauw; xenia, ohio; , brandenburg, kentucky. these storms formed second of 3 convective bands generate tornadoes.

while violent tornado activity increased on warm sector, third band of convection developed @ 16 utc , extended near st. louis west-central illinois. based upon real-time satellite imagery , model data, differential positive vorticity advection coincided left exit region of upper-level jet streak reached wind speeds of 130 kn (150 mph) (66.9 m/s (241 km/h)), thereby enhancing thunderstorm growth. storms grew rapidly in height , extent, producing baseball-sized hail 1720 utc in illinois and, shortly thereafter, in st. louis, missouri, reported severe thunderstorm in afternoon that, while not producing tornado, costliest storm hit city time. 1950 utc, supercells producing f3 tornadoes hit decatur , normal areas in illinois. thunderstorms moved warmer, moister air mass on eastern illinois , indiana, produced longer-lived tornadoes—one of began near otterbein , ended near angola in indiana, distance of 109 mi (180 km). meanwhile, 00 utc southern half of first convective band became indistinguishable new convection had formed farther south on alabama , tennessee in connection convective band two. in area, increasing west-southwesterly wind shear @ levels of troposphere, juxtaposed on near-parallel outflow boundaries, allowed successive supercells, producing strong, long-tracked tornadoes, develop unconstrained outflow in broad region south tennessee eastern mississippi. these storms, forming after 23 utc, produced of powerful tornadoes of outbreak, including 2 f5s near tanner , extremely potent f5 devastated guin in alabama.

michigan not hit hard neighboring states or windsor, 1 deadly tornado hit near coldwater , hillsdale, killing people in mobile homes; however, thunderstorm downpours caused flash floods, , north of warm front in upper peninsula, heavy snowfall reported. activity in south moved towards appalachians during overnight hours , produced final tornadoes across southeast during morning of april 4. series of studies dr. tetsuya t. fujita in 1974–75—which later cited in 2004 survey risk management solutions—found three-quarters of tornadoes in 1974 super outbreak produced 30 families of tornadoes—multiple tornadoes spawned in succession single thunderstorm cell. majority of these long-lived , long-tracked individual supercells.