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