Comparison of Two Forecasts for Tornadoes Associated with Cold Core 500-mb Lows: Surprise and Bust

Tornadoes associated with closed, cold core 500mb lows can often be difficult to forecast. This is because these cold core tornadoes are typically associated with weaker magnitudes of instability and shear in comparison to other tornadic environments. For example, cold core tornado environments tend to have much lower surface based CAPE or mixed layer CAPE, as well as weaker vertical shear values than typical tornado environments, making derived parameters such as the Significant Tornado Parameter (STP) much less useful for forecasting cold core tornadoes. Previous studies have determined the values of meteorological parameters that discriminate between ordinary supercell tornadoes and cold core tornadoes, as well as between multiple cases of cold core tornadic systems and cold core null events. In the case of cold core events, both tornadic and null, significant overlap exists between the 25-75 percentile ranges of several of the discriminating parameters. This indicates that each event is different, and can prove to be a challenge to both the forecaster and the models. To date, studies have not been done that examine the NWP s ability to forecast such events. Two cold core events have been chosen for analysis. One event exhibits an environment favorable for tornado development, but no tornadoes occur. A second event exhibits an environment not favorable for tornado development, but tornadoes do occur. This study examines NAM and SPC forecasts in an attempt to determine the extent of forecast accuracy for each case. RUC model forecasts initialized closer to the events will be utilized to determine if the accuracy of the NWP guidance improved as forecast lead time decreased.