The Evolution from Squall Lines to Isolated Supercells

A crucial element of a severe weather forecast is determining how and when thunderstorms will develop, and how that development will evolve over time. This study seeks to examine how storm organization evolves from an initial squall line to intense, isolated supercells. The evolution is non-traditional in the sense that storms are more commonly observed to grow “upscale” from isolated storms into a squall line. This may make the evolution unexpected, making it difficult to predict and forecast. This study seeks to improve the understanding and identification of these events, making them easier for forecasters to anticipate.

Nine squall line-to-supercell evolution cases have been examined to address the following two objectives: (1) document the background environment and forcing associated with each case, and (2) develop a conceptual model to explain the storm-scale processes associated with the squall line-to-supercell evolution. It is hypothesized that most squall line-to-supercell cases occur in environments with large vertical wind shear and strong forcing, with squall lines characterized by discrete, cellular lifting rather than by an unbroken slab of ascent.

Archived surface observations, analyses from the Rapid Update Cycle (RUC) mesoscale forecast model, and NOAA Storm Prediction Center (SPC) mesoanalysis data were used to characterize the background environment for each case. Parameters, such as CAPE, helicity, and vertical wind shear, indicate that the environments for most of the cases fell within the climatological range favorable for tornadic supercells. The cases are also characterized by strong synoptic forcing with strong upper level short-wave troughs and jet streaks impinging on the regions of interest. At the surface, both a dryline and cold front were present in most cases, with the cold front generally overtaking the dryline during the course of the event. The initial squall line typically formed along the dryline, with the evolution to supercells beginning as the cold front overtook the dryline. This is likely a result of the increased vertical wind shear associated with the cold front (owing to thermal wind considerations), creating an even more favorable environment for supercells. This shift towards a favorable supercellular environment is what helped initiate the squall line-to-supercell breakup in each case.

A detailed analysis of the squall line-to-supercell breakup for each case was also conducted using archived WSR-88D data analyzed with the Gibson Ridge GR Level 2 Analyst software to create three-dimensional renderings of each event. These maps revealed that dominant cells within the initial squall line eventually evolved into the supercells while other portions of the line weakened. This lends credence to the hypothesis that these lines are characterized by more “cellular” lifting rather than broad slabs of ascent as discussed by James et al. (2005). These dominant cells also showed a tendency for supercell development along the right flank of the storm motion while each case was still in its squall line phase. This presentation will show the results of these analyses, and compare them with simulations of several of these cases that are being run using the Weather Research and Forecasting (WRF) model. The WRF simulations will provide a means of evaluating the cold pool evolution and other stormscale processes that are not immediately observable with the WSR-88D data.