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- Author or Editor: Robert Hale x
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Abstract
A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of surface fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail reports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated severe storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and tornadoes from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).
Abstract
A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of surface fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail reports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated severe storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and tornadoes from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).
Abstract
During 9–11 November 1998 and 9–10 March 2002, two similar convective lines moved across the central and eastern United States. Both convective lines initiated over the southern plains along strong surface-based cold fronts in moderately unstable environments. Both lines were initially associated with cloud-to-ground (CG) lightning, as detected by the National Lightning Detection Network, and both events met the criteria to be classified as derechos, producing swaths of widespread damaging wind. After moving into areas of marginal, if any, instability over the upper Midwest, CG lightning production ceased or nearly ceased, although the damaging winds continued. The 9 March 2002 line experienced a second phase of frequent CG lightning farther east over the mid-Atlantic states. Analysis of these two events shows that the production of CG lightning was sensitive to the occurrence and vertical distribution of instability. Periods with frequent CG lightning were associated with sufficient instability within the lower mixed-phase region of the cloud (i.e., the temperature range approximately between −10° and −20°C), a lifting condensation level warmer than −10°C, and an equilibrium level colder than −20°C. Periods with little or no CG lightning possessed limited, if any, instability in the lower mixed-phase region. The current Storm Prediction Center guidelines for forecasting these convective lines are presented.
Abstract
During 9–11 November 1998 and 9–10 March 2002, two similar convective lines moved across the central and eastern United States. Both convective lines initiated over the southern plains along strong surface-based cold fronts in moderately unstable environments. Both lines were initially associated with cloud-to-ground (CG) lightning, as detected by the National Lightning Detection Network, and both events met the criteria to be classified as derechos, producing swaths of widespread damaging wind. After moving into areas of marginal, if any, instability over the upper Midwest, CG lightning production ceased or nearly ceased, although the damaging winds continued. The 9 March 2002 line experienced a second phase of frequent CG lightning farther east over the mid-Atlantic states. Analysis of these two events shows that the production of CG lightning was sensitive to the occurrence and vertical distribution of instability. Periods with frequent CG lightning were associated with sufficient instability within the lower mixed-phase region of the cloud (i.e., the temperature range approximately between −10° and −20°C), a lifting condensation level warmer than −10°C, and an equilibrium level colder than −20°C. Periods with little or no CG lightning possessed limited, if any, instability in the lower mixed-phase region. The current Storm Prediction Center guidelines for forecasting these convective lines are presented.
