A Retrospective Look at the CIMSS NearCast Model Performance during the May 2013 Tornado Events in Central Oklahoma

The Cooperative Institute for Meteorological Satellite Studies (CIMSS) NearCast system is a Lagrangian model that dynamically projects under-utilized Geostationary Operational Environmental Satellite (GOES) temperature and moisture retrieval data forward in time along isobaric surfaces. This provides detailed, hourly-updated information about the vertical structure of moisture and stability in the pre-convective environment up to nine hours in advance. The model can aid in forecasting when and where convection may be most (and least) likely to occur in the near future, and it is also helpful in assessing whether the downstream environment will support persistence or further growth of existing convective storms. Analysis and forecast products include low- and mid-level theta-e and precipitable water, as well as layer differences of these fields. An isentropic version of the model has been recently developed to more accurately project parcels forward in time in three-dimensions and to retain more information from the observations.

Central Oklahoma experienced three days of particularly extreme weather during the second half of May 2013. On May 19, rapidly developing convection produced large hail, damaging winds, and a few tornadoes in the central Oklahoma area. On the following day, May 20, convection initiated just west of Moore, quickly becoming severe and producing an EF5 tornado, which destroyed numerous structures in the city. The May 31 convective event resulted in the widest tornado (2.6 miles) in recorded history near El Reno, followed by widespread flooding in the downtown Oklahoma City area.

NearCast model output was able to depict the synoptic-scale pattern changes that occurred leading up to these events, and provide useful short-term predictions about the mesoscale pre-convective environment in the hours immediately preceding the initiation of each event. The multi-day evolution of low- and mid-level moisture was examined using an animation of NearCast model analyses. It was found that low-level flow shifted from a westerly to a southerly direction, resulting in warm and moist air returning northward from the Gulf of Mexico into the southern Plains, while a drier air mass in the middle/upper levels progressed toward the Plains from the west. NearCast model forecasts in the hours leading up to the May 19 and May 20 events were skillful in predicting the eastward movement of a well-defined low-level moisture boundary (dry line), along which severe convection developed. For the El Reno case, the model predicted the development and movement of a local maximum in low-level theta-e along the boundary where convection initiation would focus.