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From May 2–8, 1999, a significant tornado outbreak took place across much of the Central and parts of the Eastern United States. During this week-long event, 152 tornadoes touched down (including one in Canada), more than half of them on May 3 and 4 when activity reached its peak over Oklahoma, Kansas, Nebraska, Texas, and Arkansas. The most significant tornado first touched down southwest of Chickasha, Oklahoma, and became an F5 before dissipating near Midwest City. The tornado tore through southern and eastern parts of Oklahoma City and its suburbs of Bridge Creek, Moore, Del City, Tinker Air Force Base and Midwest City, causing $1 billion in damage. With a total of between 66 and 74 tornadoes, it was the most prolific tornado outbreak in Oklahoma history, although not the deadliest. ==Meteorological synopsis== The outbreak was caused by a vigorous upper-level trough that moved into the Central and Southern Plains states on the morning of May 3. That morning, low stratus clouds overspread much of Oklahoma, with clear skies along and west of a dry line located from Gage to Childress, Texas. Air temperatures at 7:00 a.m. Central Daylight Time ranged in the mid to upper 60s °F (upper 10s to near 20 °C) across the region, while dew point values ranged in the low to mid 60s °F (mid to upper 10s °C).〔(Meteorological Summary of the Great Plains Tornado Outbreak of May 3-4, 1999 )〕 The Storm Prediction Center (SPC) in Norman, Oklahoma, a division of the National Weather Service, initially issued a slight risk of severe thunderstorms early that morning stretching from the Kansas-Nebraska border to parts of southern Texas, with an intended threat of large hail, damaging winds and tornadoes.〔(Severe Weather Outlook at 6:30 a.m. CDT on May 3, 1999 )〕 By late morning, the low cloud cover began to dissipate in advance of the dry line, but during the afternoon hours high cirrus clouds overspread the region, resulting in filtered sunshine in some areas that caused atmospheric destabilization. The sunshine and heating, combined with abundant low-level moisture, combined to produce a very unstable air mass. Upper air balloon soundings observed strong directional wind shear, cooling temperatures at high atmospheric levels, and the increased potential of CAPE values potentially exceeding 4000 J/kg, levels that are considered favorable for supercells and tornadoes. As observations and forecasts began to indicate an increasing likelihood of widespread severe weather conditions even more favorable for strong tornadoes, the SPC issued a moderate risk of severe weather at 11:15 a.m. CDT for portions of Kansas, Oklahoma and Texas along and near the Interstate 40 corridor.〔(Severe Weather Outlook at 11:15 a.m. CDT on May 3, 1999 )〕 By 3:00 p.m. CDT, it had become evident that a widespread severe weather event was imminent; the Storm Prediction Center upgraded locations within the moderate risk area to a high risk of severe weather around 4:00 p.m. CDT as wind shear profiles, combined with volatile atmospheric conditions, had made conditions highly conducive for a significant tornadic event across most of Oklahoma, southern Kansas and north Texas, including the likelihood of violent, damaging tornadoes.〔 The SPC issued a tornado watch by mid-afternoon as conditions gathered together for what would be a historic tornado outbreak. By the time thunderstorms began developing in the late-afternoon hours, CAPE values over the region had reached to near 6,000 J/kg. Large supercell thunderstorms developed and in the late afternoon through the mid-evening hours of that Monday, tornadoes began to break out across the state. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1999 Oklahoma tornado outbreak」の詳細全文を読む スポンサード リンク
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