Short-Range Forecasting and Nowcasting Using a Simple, Isentropic Prediction Model

Abstract

The recent advancement of mini- and microcomputers into the local-work environment can provide local forecast offices with the capability to run simple numerical models for specific nowcasting and short-term forecast needs. While the capabilities of sophisticated, centralized modeling systems continue to progress, short-term forecasts are hampered by a number of inadequacies in this guidance, ranging from numerical deficiencies of the models to the inability to access the gridded model output in a timely manner and view it at frequent enough time intervals or to enhance particular local aspects of the initial analyses. In many instances, observations and extrapolations are still considered to be more important than numerical guidance in the very short range. In this paper, an isentropic forecast technique is used to provide temporally and spatially detailed short-term advective forecasts of temperature, moisture, and wind changes. While geostrophic potential vorticity conservation is used to simplify the model, the advective computations can use both geostrophic and ageostrophic wind data. The inclusion of pronounced baroclinic structures in the detailed initial isentropic analyses allows the model to preserve large observed vertical and horizontal wind shears, to monitor local stability changes, and to detect areas of strong subsidence and overrunning. A series of short-range forecasts of the preconvective environment associated with severe convection is discussed. High time frequency fields of midtropospheric static stability, low-level moisture and temperature, tropopause height, and conventional stability indices are presented and combined with observed surface data to provide updated stability and buoyancy guidance. In addition, forecasted winds are used to advect VAS mid- and low-level moisture imagery, which can be used to predict areas of changing convective potential and extend the lead time required to issue forecasts for severe convective storms.