Isallobaric Indications

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