FORECASTING THE INTENSITY OF TROUGHS AND RIDGES

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. Extrapolation 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. Use of Time Differentials 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. Isotherm-Contour Relationship In long waves, deepening of troughs is associated with cold air advection on the west side of the trough and filling of troughs with warm air advection on the west side of the trough. The converse is true for ridges. Warm air advection on the western side of a ridge indicates intensification, and cold air advection indicates weakening. This rule is least applicable immediate yeast of the Continental Divide in the United States, and probably east of any high mountain range where westerly winds prevail aloft. In short waves, deepening of troughs is associated with cold air advection on the west side of the trough and falling of troughs with warm air advection, particularly if a jet maximum is in the northerly current of the trough and tilling is indicated by warm air advection on the western side. In reference to the above paragraph, the advection is not the cause of the intensity changes, but rather is a “sign” of what is occurring. High level convergence/divergence is the cause. Effect of Super Gradient Winds Figure 2-1, views (A) through (D), shows the effect of the location of maximum winds on the intensity of troughs and ridges. Explanation of figure 2-1 is as follows: l When the strongest winds aloft are the westerlies on the western side of the trough, the trough deepens [fig. 2-1, view (A)]. . When the strongest winds aloft are the westerlies at the base of the trough, the trough moves rapidly eastward and does not change in intensity [fig. 2-1, view (B)]. . When the strongest winds are on the east side of the trough, the trough fills [fig. 2-1, view (C)]. 2-4