Practical and Intrinsic Predictability of Warm-Core Mesoscale Vortex Formation with the 8 May 2009 “Super Derecho” Event

On the morning of 8 May 2009, a particularly intense mesoscale convective system, or MCS, developed over southwestern Kansas. This MCS, colloquially known as the “Super Derecho,” spawned the development of an intense, warm-core mesoscale vortex on its northern flank and was responsible for straight-line wind gusts of up to 50 m s-1, twenty-six tornadoes, and approximately $115 million in damage as it traveled from southwestern Kansas to the southern Appalachians (Evans et al. 2014, J. Atmos. Sci.). A numerical simulation conducted in real-time at NCAR was able to successfully replicate the observed event (Weisman et al. 2013, Wea. Forecasting). This and other studies have demonstrated that numerical models are capable of providing skillful forecasts of MCSs and their associated hazards in spite of imperfect initial conditions and physical parameterization methods. However, it remains uncertain whether these forecast successes are primarily serendipitous or instead imply that MCSs and their associated hazards are to some extent predictable.

To investigate the practical and intrinsic predictability of the 8 May 2009 “Super Derecho” MCS and its warm-core mesoscale vortex, a fifty-member Ensemble Kalman filter (EnKF)-based cycled data assimilation and numerical simulation forecast system is developed. The EnKF-based data assimilation system utilized within this study is that implemented within the Data Assimilation Research Testbed (DART). Cycling of the data assimilation system is conducted using version 3.7 of the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) numerical forecast model. Technical details of the cycled data assimilation and numerical simulation forecast system closely resemble those of Romine et al. (2013, 2014, both Mon. Wea. Rev.) and Schwartz et al. (2015, Wea. Forecasting). Cycled data assimilation commences at 1200 UTC 2 May 2009 and proceeds every six hours until 1200 UTC 7 May 2009, at which time numerical simulations (extending out 36 h at a horizontal grid spacing of 3 km) are launched. Results from the cycled data assimilation and numerical simulation system will be presented. Particular emphasis will be given to quantification of the ensemble's ability to accurately forecast the development and structure of the “Super Derecho” event, including its accompanying warm-core mesoscale vortex, and evaluation of the effects of initial condition uncertainty upon forecast variability and skill.