Outflow Convergence


Outflow is a boundary that results from a thunderstorm or group of thunderstorms. This gust front has different temperature and moisture characteristics from the surrounding environmental air. Thus, it has a different density than the surrounding air. This is important since the outflow can act as a new lifting mechanism to develop more thunderstorms or to sustain a thunderstorm that has already developed. An outflow boundary could develop new convection right away or the boundary could sit around and even help develop new convection a day later. The convergence between two different outflow boundaries can be a significant lifting mechanism since at their intersection there will be a near doubling of the convergence. This can be a prime focal point for a new burst point to occur.

Before a storm hits, often a gusty, more humid and cooler wind will be felt. This is the gust front. The coolness of the air is due to a combination of evaporative cooling and the air that was brought from aloft ending up being cooler than the environmental air. The density difference helps accelerate the air toward the surface thus the wind can be very gusty and at times can produce damaging straight line wind. This gust front continues to move and mixes with environmental air. The mixing is gradual and the time it takes for the gust front to thoroughly mix with the environmental air is variable. Air with different densities has trouble mixing thus the influences of the outflow can be sustained for many hours.

Outflow can be as significant of a lifting mechanism as a front, upper level divergence and low level warm air advection. Thus, the development of one storm can signal the development of several storms or a grouping of storms since the lifting from the outflow combines with preexisting lifting mechanisms already in place. These outflows are difficult to predict since it is difficult to pin point exactly where the first storm will form. Once a storm does develop, then monitoring outflow is a major player in mesoscale forecasting since it leads to more burst points.