Assessing the utility of several analysis schemes for diagnosing precursor signals for convective initiation and non-supercell tornadogenesis along boundaries

Although numerical models continue to advance with increasing horizontal grid resolution sufficient to begin to resolve convective storms, convective initiation remains a difficult short-term forecast problem. And determining whether the convection that does initiate will produce a severe storm with the development of a non-supercell tornado is an even more tenuous proposition. Surface-based boundaries are typically key areas for forecasters to monitor for both convective initiation and storms that produce non-supercell tornadoes. Studies dating back to the 1980s (Szoke et al., 1984) revealed that a concentration of low-level convergence and cyclonic vorticity occurred along a quasi-stationary boundary known as the Denver Convergence-Vorticity Zone (DCVZ, or “Denver Cyclone”) prior to non-supercell tornado formation. Studies in other areas (for example, by the National Weather Service (NWS) Weather Forecast Office (WFO) in LaCrosse, Wisconsin, see Baumgardt 2006) have also documented the importance of monitoring surface boundaries as potential areas for non-supercell tornado formation. The WFO LaCrosse study even developed a parameter known as the Non-Supercell Tornado parameter (NST) that could be calculated using their LAPS (Local Analysis and Prediction System) on AWIPS. The focus of this study is to examine both LAPS and a number of other analysis schemes as they relate to their utility to help diagnose convective initiation along boundaries, and the potential for that convection to produce a non-supercell tornado.

Forecasters typically use calculated fields such as CAPE and CIN to monitor convective potential. Analyses from LAPS and RUC (Rapid Update Cycle) are two of the more widely used schemes that have been available on AWIPS for a number of years to examine convective potential. Several more recently developed analyses will be used in this study and compared, including LAPS at both 5 km (LAPS currently on AWIPS is at a horizontal resolution varying from 10 to 5 km), and a new 1 km horizontal resolution, a scheme known as STMAS (for Space and Time Multiscale Analysis System) at 5 km horizontal resolution, and analyses from the HRRR (High-Resolution Rapid Refresh model, a 3 km horizontal resolution variant of the RUC Rapid Refresh model that is run hourly out to 15 h (only analyses will be used in this study)). All of these will be compared to the NCEP RTMA (Real-Time Mesoscale Analysis) that is now available on AWIPS at 5 km resolution.

We will focus on examining cases with well-defined boundaries, such as the DCVZ, but also other stationary and moving boundaries. The two main fields that will be subjectively compared will be surface convergence and vorticity, since these have been related to the potential for non-supercell tornado development in previous studies. The main question to be addressed will be whether trends in these fields using the analysis schemes noted above that are available in real-time are useful aids to forecasting convective initiation and non-supercell tornado potential. With the variety of schemes used we can examine issues such as the importance of horizontal resolution as well as different analysis methods. The various schemes are now available online in real-time and some are available on AWIPS. We intend to make all of them available on AWIPS so that forecasters at the Boulder WFO will be able to evaluate them in real-time during the summer of 2010.

Szoke, E.J., M.L. Weisman, J.M. Brown, F. Caracena and T.W. Schlatter, 1984: A subsynoptic analysis of the Denver tornadoes of 3 June 1981. Mon. Wea. Rev., 112, 790-808.

Baumgardt, D.A., and K. Cook, 2006: Preliminary evaluation of a parameter to forecast environments conducive to non-mesocyclone tornadogenesis. 23rd Conference on Severe Local Storms, St. Louis, Paper 12.1.