Wave cyclones form most readily on stationary or slow moving fronts. A preferred position is along a decelerating cold front in the region of greatest deceleration. Normally, the 700-hPa winds are parallel to the front along this area.

Under conditions characteristic of the eastern Pacific, a secondary wave cyclone may rapidly develop (fig. 3-7, step 1). As the secondary wave forms on the

Figure 3-7.-Illustration of secondary cyclone development over the eastern Pacific.

retarded portion of the cold front, the original warm front associated with the primary cyclone tends to wash out, or become masked in the more or less parallel flow between the returning cold air from the high to the east and the original warm feeding current. In this stage of  development, the new secondary wave cyclone is rapidly moving, but with no appreciable deepening as it moves along the front and few, if any, indications of occluding (fig. 3-7, step 2). As this new low moves eastward along the front, the pressure gradients surrounding it will tighten and tend to intensify the old masked warm front (fig. 3-7, step 3). Later, as the wave moves rapidly eastward, it will pickup this intensified warm front and begin to occlude (fig. 3-7, step 4). This occlusion deepens as much as 10 to 15 hPa in 12 hours. The resultant rapid deepening and increase in cyclonic circulation results in a portion of the original polar front discontinuity between the new and the old cyclone being washed out (fig. 3-7, step 5).

INDICATIONS ALOFT FOR DEEPENING AND FILLING OF SURFACE LOWS

There are numerous atmospheric factors aloft that affect the central pressure of surface lows. The following text discusses a few of these factors.

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