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1984 Soviet Union tornado outbreak
The 1984 Soviet Union tornado outbreak, also known as the 1984 Ivanovo tornado outbreak, was one of only three disastrous tornado outbreaks in modern Russian history (one of the others being the 1904 Moscow tornado) and the third-deadliest tornado outbreak in European history. Occurring on June 9, 1984, the outbreak struck the Ivanovo and Yaroslavl regions north of Moscow, an area over 400,000 km2. At least two of the eleven known tornadoes were violent events, equal to F4 or F5 in intensity on the Fujita scale, based upon observed damages. The deadliest single tornado was posthumously rated at F5 intensity and killed at least 92 people along its long path near Ivanovo and other towns. The tornado, up to wide, caused extreme damage, reportedly annihilating steel-reinforced concrete structures and throwing heavy objects of for distances up to . Another tornado, assessed to have been at least F4 and possibly F5 in intensity, occurred at Kostroma. Severe thunderstorms also produced hail up to in weight, among the heaviest hailstones confirmed worldwide. In all, the entire tornado outbreak killed at least 400 people and injured 213.
==Meteorological synopsis==
On June 8, 1984, a negatively tilted trough caused an extratropical low pressure area to form over the coast of the Romanian People's Republic (now non-Communist Romania). Surface moisture moved north from the Black Sea and caused nearby dew points to rise to ; though at that time these were restricted to Romania and the Ukrainian SSR, dew points were higher than average elsewhere. By 1800 UTC, developing thunderstorms over the Ukrainian SSR spread overnight into the Russian SFSR. Between 00 and 12 UTC on June 9, the strengthening low pressure area moved north-northeast over the northwestern Russian SFSR before undergoing occlusion. In the meantime, a strong cold front rapidly advanced along a line extending south from the surface low, then south of Minsk in the Byelorussian SSR (now Belarus), to near Bucharest. This front separated the drier air mass to the north from the warm, moist air mass near the Black Sea, and strong wind speeds near ground level caused vertical mixing. Therefore, dew points actually dropped before the first tornadoes formed, but nevertheless several factors overcame the lower dew points to produce tornadoes. Among these were a strong upper-level jet stream, clear skies causing daytime heating and instability, strong synoptic-scale lifting leading to ascension of updrafts, and high adiabatic lapse rates promoting thunderstorm development. All these factors combined to produce severe weather near Moscow. The tornadoes occurred in this region because an unstable and moist air mass, supported by warm sea surface temperatures over the Black Sea, had been in place four days before the outbreak began. The unusually strong intensity of the trough in the region on June 8–9, with a 500-millibar geopotential height measured at about 2.7 standard deviations below normal, also favored an intense tornado outbreak.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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