The AG2 TRAMAN, volume 1, discusses the migration of the jet stream both northward and southward. Some general considerations can be made concerning this migration and the movement of waves in the troposphere:

. In a northward migrating jet stream, a west wind maximum emerges from the tropics and gradually moves through the lower midlatitudes. Another maximum, initially located in the upper midlatitudes, advances toward the Arctic Circle while weakening.

. Open progressive wave patterns with pronounced amplitude and a decrease in the number of waves due to cutoff centers exist. The jet is well organized and troughs extend into low latitudes.

. As the jet progresses northward, the amplitude of the long waves decrease and the cutoff lows south of the westerlies dissipate. By the time the jet reaches the midlatitudes, a classical high zonal index (AG2 TRAMAN, volume 1) situation exists. Too, we have weak, long waves of large wavelength and small amplitude, slowly progressive or stationary. Few extensions of troughs into the low latitudes are present, and in this situation, the jet stream is weak and disorganized.

. As the jet proceeds farther northward, there will often be a sharp break of high zonal index with rapidly increasing wave amplitudes aloft. Long waves retrograde. As the jet reaches the upper midlatitudes and into the sub-Arctic region, it is still the dominant feature, while a new jet of the westerlies gradually begins to form in the subtropical regions. Long waves now begin to increase in number, and there is a reappearance of troughs in the tropics. The cycle then begins again.

With a southward migrating jet, the processes are reversed from that of the northward moving jet. It should be noted that shortwaves are associated with the jet maximum and move with about the same speed as these jet maximums.

FORECASTING THE INTENSITY OF TROUGHS AND RIDGES

Forecasting the intensity of long wave troughs and ridges often yields nothing more than an indication of the expected intensity; that is, greater than or less than present intensity. For instance, if deepening or falling is indicated, but the extent of deepening or tilling is not definite, the forecaster is forced to rely on experience and intuition in order to arrive at the amount of deepening or tilling. FNMOC upper level charts forecast the intensity of upper waves with a great deal of success. If available, you should check your intensity and movement predictions against these prognoses.

Patterns on upper level charts are more persistent than those on the surface. Therefore, extrapolation gives better results on the upper air charts than on surface charts. When you use height changes aloft, the procedure is to extrapolate height change and add or subtract the change to the current height values.

The time differential chart is discussed in the AG2 TRAMAN, volume 1.

The time differential chart constructed for the 500-hPa level shows the history of changes that have taken place at the 500-hPa level at 24-hour intervals. In considering the information on the time differential chart, those centers with a well defined history of movement will be of greatest value. Take into consideration not only the amount of movement, but also the changes in intensity of the centers. Centers with no history should be treated with caution, especially with regard to their direction of movement which is usually downstream from the current position. Information derived from the time differential chart should be used to supplement information obtained from previous considerations, and when in agreement, used as a guide for the amount of change.

Normally, the 24-hour height rise areas can be moved with the speed of the associated short wave ridges, and the speed of the fall centers with the speed of the associated short wave troughs. It must be remembered that height change centers may be present due to convergence or divergence factors and may not have an associated short wave trough or ridge. Be cautious not to move a height change center with the contour flow if it is due primarily to convergence or divergence. However, with short wave indications, a change center will appear and move in the direction of the contour flow.

Once you have progged the movement of the height change centers and determined their magnitude, apply the change indicated to the height on the current 500-mb chart. You should use these points as guides in constructing prognostic contours.

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