|WHAT IS MEANT BY “WASHES OUT”?
METEOROLOGIST JEFF HABY
“Washes out” is a phrase that can show up in forecast discussions. While often a focus is put on the development and intensification
of a weather system, the opposite can occur also. The diminishment of a meteorological process is what is meant by washes out. The
strength of the system is reduced or it dissipates when it washes out. Below are several examples of a system or meteorological
process washing out:
Cold front washes out: As a cold front moves into tropical latitudes, it becomes weaker. Eventually it stalls and over time the air mass
on each side of the front becomes the same. All that may be left is a subtle wind shift boundary. This cold front has washed out.
Low pressure system washes out: A low pressure system tends to diminish after it reaches the mature stage. The fronts occlude and the
low pressure system loses access to the energy from temperature gradients near the low pressure system. The system weakens as it becomes
surrounded by colder air. If there is no process to regenerate the low then the low will wash out and dissipate.
Short wave washes out: Examples of how a shortwave can be created are by positive vorticity advection, a thermal boundary or jet streak
divergence. When the process responsible for the generated the shortwave starts coming to an end and there is no other process to
intensify the shortwave then the shortwave will being to dampen and wash out.