Figure 3-13.-Infrared, local midnight, second day.
When high or middle clouds completely surround
the vortex center, the cyclone has reached maturity
and can be expected to fill. This generally indicates the
advection of cold, dry air into the cyclone has ceased.
FORECASTING THE MOVEMENTS AND
INTENSITY OF FRONTS
movement and intensity of fronts by using
extrapolation and the geostrophic wind method.
The following text discusses various rules for the
movement of fronts, as well as the effects upper air
features have on fronts.
MOVEMENT OF FRONTS
Fronts are ordinarily forecasted after the pressure
systems have been forecasted. However, there may be
cases for short-range forecasting where movement of
all of the systems is unnecessary. Extrapolation is
perhaps the simplest and most widely used method of
moving fronts for short periods. When moving fronts
for longer periods, other considerations must be taken
The following text discusses a simple extrapolation
method, as well as the geostrophic wind method, and
considerations based on upper air influences.
When fronts are moved by extrapolation, they are
merely moved based on past motion. Of course such
factors as occluding, frontogensis, frontolysis, change
in position, intensity of air masses and cyclones, and
orographic influences must be taken into account.
Adjustments to the extrapolation frontal positions are
made on the basis of the above considerations.
You should keep in mind that the adjacent air
masses and associated cyclones are the mechanisms
that drive the fronts.
You should also keep in mind the upper air
influences on fronts; for example, the role the 700-hPa
winds play in the movement and modification of fronts.
Finally, you should remember that past motion is not a
guarantee of future movement, and the emphasis
should be on considering the changes indicated, and
incorporating these changes into an extrapolated
Geostrophic Wind Method
Frontal movement is forecasted by the geostrophic
wind at the surface, and at the 700-hPa level at several
points along the front. The basic idea is to determine
the component of the wind at the surface and aloft,
which is normal (perpendicular) to the front and,
therefore, drives the front. Determination of the
component normal to the front is made by
THE PROCEDURE. The steps are as follows: