The forecaster should always examine the long waves for the isotherm-contour relationships, and then apply the rules for the movement of long waves. These rules are covered in the AG2 TRAMAN, volume 1. These rules are indicators only, but if they confirm or parallel other applied techniques, they have served their purpose. A number of observations and rules are stated regarding the progression, stationary characteristics, or retrogression of long waves. These rules are discussed in the following text.

PROGRESSION OF LONG WAVES. Progression (eastward movement) of long waves is usually found in association with relatively short wave lengths and well defined major troughs and ridges. At the surface, there are usually only one or two prominent cyclones associated with each major trough aloft. Beneath the forward portion of each major ridge there is usually a well developed surface anticyclone moving toward the east or southeast. The 24-hour height changes at upper levels usually have a one-to-one association with major troughs and ridges; that is, motion of maximum height fall and rise areas associated with major trough and ridge motion. The tracks of the height change centers depend on the movement and changes in intensity of the long waves.

STATIONARY LONG WAVE PATTERNS. Once established, stationary long wave patterns usually persist for a number of days. The upper airflow associated with the long wave pattern constitutes a steering pattern for the smaller scale disturbances (short waves). These short waves, with their associated height change patterns and weak surface systems, move along in the flow of the large scale, long wave pattern. Short wave troughs deepen as they move through the troughs of the long waves, and fill as they move through the ridges of the long waves. The same changes in intensity occur in sea level troughs or pressure centers that are associated with minor troughs aloft. Partly as a result of the presence of these smaller scale systems, the troughs and ridges of the stationary long waves are often spread out and hard to locate exactly.

RETROGRESSION OF LONG WAVES. A continuous retrogression of long wave troughs is a rare event. The usual type of retrogression takes place in a discontinuous manner; a major trough weakens, accelerates eastward, and becomes a minor trough, while a major wave trough forms to the west of the former position of the old long wave trough. New major troughs are generally formed by the deepening of minor troughs into deep, cold troughs.

Retrogression is seldom a localized phenomenon, but appears to occur as a series of retrogressions in several long waves. Retrogression generally begins in a quasi-stationary long wave train when the stationary wavelength shows a significant decrease. This can happen as a result of a decrease in zonal wind speed, or of a southward shift in the zonal westerlies. Some characteristics of retrogression are as follows: 

. Trajectories of 24-hour height change patterns at 500-hPa deviate from the band of maximum wind.

. New centers appear, or existing ones rapidly increase in intensity.

. Rapid intensification of surface cyclones occurs to the west of existing major trough positions.

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