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Squall lines (lines of active thunderstorms) are a common type of storm system east of the Rocky Mountains.  Of particular concern are large, destructive prefrontal squall lines that extend across many states and can contain strong  winds, damaging hail, and multiple tornadoes.  Recent research has shown a connection between large prefrontal squall lines east of the Rocky Mountains and deep tropospheric cold fronts that are (non-conventionally) tipped forward in the lower troposphere.  That is, the elevated leading edge of the cold air moves forward in advance of the surface position of the cold air.  The most significant vertical velocities and weather in these storms are associated with the elevated leading edge of the cold air, which is referred to as a cold front aloft (CFA).  There is increasing evidence that many, perhaps most, large pre-frontal squall lines are caused by CFAs.. 

 

Squall lines associated with CFAs can range from a line of non-damaging thunderstorms to lines of numerous tornadic storms.  In the largest outbreak of tornadoes ever recorded, the Super Outbreak of 3-4 April 1974, one of the squall lines was associated with a CFA.  Prior to 1950, several research reports by U.S. Weather Bureau forecasters attributed some severe weather east of the Rocky Mountains to CFAs.  This idea fell out of favor in the 1950's with the advent of the paradigm that squall lines are organized and maintained by their own precipitation-induced cold pool and outflow boundary.  Only a few studies after the 1950's continued to suggest that cold fronts aloft might be important in producing severe weather east of the Rocky Mountains.  Since the late 1980's, research on the nature of CFAs and their relationship to prefrontal squall lines has been revived. These  studies , based on both observations and high-resolution computer model simulations, demonstrate how CFAs form and how they can trigger, organize, and maintain squall lines.

 

This website contains an animation of the Super Tuesday storm of 8-11 March 1992.  This storm contained a CFA squall line and is a good example of the evolution of this type of storm.  A companion tutorial describes the formation and evolution of CFA squall lines  using the Super Tuesday storm as an example.  For anyone not familiar with  standard weather map symbols, a short explanation should make the animation and tutorial more meaningful. An extensive bibliography on cold fronts aloft is also included.

 

This website is designed and maintained by: John Locatelli, Mark Stoelinga, and Peter Hobbs

Maintenance of this website, and much of the research that underpins the ideas presented here, is made possible by grants from the National Science Foundation Mesoscale Dynamic Meteorology Program, Atmospheric Sciences Division. However, any opinions, findings, recommendations and conclusions expressed on this website are those of the authors, and do not necessarily reflect the views of the National Science Foundation. 

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