Isallobaric Indications
Lows tend to move toward the center of the largest
3-hour pressure falls. This is normally the point where
the maximum warm air advection is occurring.
Figure 3-4 shows the movement of an occluding
wave cyclone through its stages of development in
relation to the surface pressure tendencies. This one
factor cannot be used alone, but, compared with other
indications, you may arrive at the final forecasted
position of the lows. Too, you should remember that the
process depicted in this illustration takes place over
several days, and many other factors enter into the
subsequent movement.
Circular, or nearly circular, cyclonic centers
generally move in the direction of the greatest pressure
falls. Anticyclone centers move in the direction of the
greatest pressure rises.
Troughs move in the direction of the greatest
pressure falls, and ridges move in the direction of the
greatest pressure rises. See figure 3-5.
Relative to Warm Sector Isobars
Warm, unoccluded lows move in the direction of the
warm sector isobars, if those isobars are straight. These
lows usually have straight paths (fig. 3-6, view A),
whereas old occluded cyclones usually have paths that
are curved northward (fig. 3-6, view B). The speed of
the cyclones approximates the speed of the warm air.
Whenever either of these rules is in conflict with
upper air rules, it is better to use the upper air rules.
Relative to Frontal Movement
The movement of the pressure systems must be
reconciled with the movement of the associated fronts
if the fronts are progged independently of the pressure
systems. Two general rules are in use regarding the
relationship of the movement of lows to the movement
of the associated fronts: First, warm core lows are
steered along the front if the front is stationary or nearly
so; and second, lows tend to move with approximately
the warm front speed and somewhat slower than the cold
front speed.
Figure 3-4.-Movement of occluding wave cyclone in relation to isallobaric centers.
3-4
