White squall

Abstract: Three cases of squall lines composed of strong to severe thunderstorms that formed over Arizona, and Sonora (Mexico) on July, 16-17 and 17-18, 1984, and August 2-3, 1986, are examined. Data, which included satellite imagery, VISSR-derived fields, surface data, and records or cloud-to-ground lightning strikes, indicate that the initiation, growth, and dissipation of all three squall lines were very similar. Results indicate that these mesoscale convective systems developed in an environment of relatively strong low-level shear with very weak shear aloft and that they possessed almost all the properties of a typical tropical squall line.

Abstract: From 8 to 9 March 1992 cold frontogenesis aloft (CFA), which was associated with the development of a vigorous baroclinic wave, triggered a series of squall lines that produced large hail and several tornadoes as they moved across the central United States. The air lifted by the CFA, which produced the squall lines, was made potentially unstable as a result of the circulation associated with a surface drytrough. This study provides further support for the view that in winter and early spring CFA plays an important role in triggering severe weather in the central United States.

Abstract: A case study was conducted of the development and early evolution of a severe squall line that occurred 9 April 1991. The squall line formed near the border between Arkansas and Tennessee, then raced toward the northeast during the next 14 hours. Damaging winds were widespread with the squall line; thus, the 9 April 1991 event fits the definition of a derecho. Radar observations of the evolving squall line show signatures often correlated with damaging surface winds, including a bow echo, strong reflectivity gradients, and weak echo channels. Synoptic conditions under which the 9 April 1991 event occurred were significantly different in many respects from those associated with warm-season derechos. Differences include absence of important low-altitude signatures and more vigorous weather systems. There were similarities to the warm-season pattern, however, including moderately strong winds aloft, a very unstable thermodynamic profile, and cool dry air at midlevels. The similarities suggest refine...

Abstract: Severe thunderstorms are an important and relatively common component of the annual weather across the State of Oklahoma. Such weather brings hazardous features such as, large hail, damaging winds, and tornadoes, while also providing beneficial precipitation vital to the state's agricultural and hydrological needs. In any given year, severe weather activity is dictated by seasonal and monthly changes in atmospheric conditions, which frequently determine the type and severity of subsequent storms. This study focuses on a 10-year period between 1994 and 2003 to quantify the spatial and temporal characteristics of severe squall line storms across Oklahoma. A squall line is a linearly organized set of storms that has a sharp radar reflectivity gradient at its leading edge typically followed by a less intense stratiform region. Squall line storms are one of the most significant storm modes observed in Oklahoma due to their large aerial extent, long durations, high winds, heavy rains, and hail. For this study, geographic information systems (GIS) were used to store geo-referenced storm data and spatially analyse storm events across varying timescales. The analysis revealed that squall lines were most predominant across eastern Oklahoma with a decreasing westward gradient. The annual averaged storm track was from the west to east, while the monthly mean squall line tracks were oriented from southwest to northeast from January through April, from west to east during May, from northwest to southeast from June through September, and from southwest to northeast through the end of the year. In addition, high-resolution analyses of squall line initiation and termination locations revealed important geographical variability in typical storm lifecycle. Copyright © 2007 Royal Meteorological Society