Conceptually, forecasting thunderstorms is rather simple. All that is necessary is for one to diagnose air masses in which air parcels may become moist buoyant and for a mechanism to cause that moist buoyancy. Today's techniques for forecasting thunderstorms are some of the most advanced in a meteorologist's arsenal. Although the techniques are advanced, they are still quite subjective.
VVSTORM is an attempt to objectify the ingredients-based techniques. Using numerical model output from today's numerical models, VVSTORM computes the vertical velocity determined by the maximum of one of three methods, divergence of frontogenesis vectors, Eckman pumping, or model-based omega. Those vertical velocities are about 10-20 cm/sec along strong boundaries on 40 km resolution grids. Those are almost never enough to overcome convective inhibition. Maximum vertical velocities computed by very high resolution numerical models along strong boundaries are actually about 5-10 m/sec which are high enough to penetrate caps. Therefore, in order to find a parameterized maximum vertical velocity, the model's vertical velocity is multiplied by a function of the grid resolution and the height above ground. VVSTORM also includes the additional vertical velocity resulting from vertical wind shear.
It then lifts the most unstable parcel with the parameterized vertical velocity at the same level. If that parcel reaches its Level of Free Convection (LFC) still with a upward vertical velocity, then buoyancy takes over and accelerates the parcel upward through the troposphere until it reaches the Equilibrium Level (EL). Buoyancy is reduced by a bulk entrainment function. At the EL the parcel begins deceleration. The vertical velocity at the EL is a measure of a thunderstorm's intensity.
Maps of VVSTORM maximum vertical velocity are where numerical model environmental conditions favor thunderstorms. With the temporal resolution of a numerical model, VVSTORM output now gives temporal guidance to thunderstorm outlook forecasts that heretofore have been broad-brushed in time and space
The 8th Conference on Aviation, Range, and Aerospace Meteorology