This writing looks at the factors that can reduce or eliminate a precipitation event. Each of these factors will be listed and how it contributes to reducing or eliminating the precipitation threat. Many of these factors are interrelated with each other: Dry air: When the air is drier, more lifting will be required to saturate the air. If lifting is weak, then dry air can prevent precipitation from materializing. If precipitation does fall aloft, dry air can also evaporate the precipitation before it reaches the ground. Precipitation that evaporates before reaching the ground is termed virga. Downward vertical velocities: This occurs from dynamic mechanisms that cause the air to sink. Some of these include low level cold air advection, negative vorticity advection, downsloping wind, jet streak convergence, and low level divergence. Sinking mechanisms can reduce or eliminate any lifting mechanisms that are present if there are any lifting mechanisms present. Lack of low level convergence / lack of moisture convergence: These factors add in the generation of moist updrafts. This can happen when there is a lack of a convergence boundary (no fronts, no significant mesoscale boundaries). Cap: A strong cap can prevent lifting (especially deep convective lifting that originates from the surface). A cap is a stable layer that impedes lifting that tries to penetrate the cap. Ridging: The overall pattern of ridging leads to sinking air and drying air. This can impede lifting mechanisms from causing precipitation. Stability: When the air is stable, vertical motions will be immobilized from producing large vertical motions. Stability can be produced from an inversion, warm layer aloft, low level cold air advection, sinking air and dry air. Subsidence wind flow: This can occur from a down sloping wind over terrain, isentropic down glide, flow out of a high pressure system, and sinking mechanisms. A forecaster looks at these factors that are relevant to the forecast region when putting together a forecast. These factors can reduce or eliminate the chance for precipitation or the intensity of the precipitation event.  |