Nowcasts of Thunderstorm Initiation and Evolution

This paper reports on experimental space-specific 30-min nowcasts of thunderstorm initiation, evolution, and movement. The experiments were conducted near Denver, Colorado, with the purpose of providing weather information for planning purposes to air traffic control managers. The nowcasts were based primarily on Doppler weather radar observations of the clear-air boundary layer, storm reflectivity, storm Doppler velocity structure, and visual observations of clouds. The forecasters found that they could often anticipate thunderstorm initiation by monitoring radar-detected boundary-layer convergence lines together with monitoring visual observations of cloud development in the vicinity of the convergence lines. Nowcast procedures and nowcast results for experiments in 1989 and 1990 are presented. The procedures are based on research experiments and exploratory field tests conducted since 1984. The forecaster results were better than persistence or extrapolation forecasts because of the ability to nowcast storm initiation and dissipation. The results are applicable to weakly forced synoptic situations where storms are typically small and short lived. Forecasters often had difficulty in precisely timing and placing the location of storm initiation. They had even more difficulty in forecasting the evolution of existing storms. Three reasons for these difficulties are believed to be: 1) there are basic deficiencies in our knowledge of the details of storm initiation and evolution, 2) there is a need for detailed observations of boundary-layer thermodynamics and more detailed observations of cumulus cloud location and growth, and 3) many of the forecaster activities were manually intensive and prone to error. Plans for addressing these problems are presented.