Frequently neither of these two situations exist, and both the change in movement and the height center change occur at a proportional rate. This is illustrated in figure 2-2, view (C). From a sequence of charts 24 hours apart, it is shown that the low is filling at a decreasing  rate  and  also  moving  at  a  decreasing  rate. The height change value is 50 percent of the value 24 hours previously on the successive charts, and the rate of  movement  is  75  percent.  We  then  assume  this constant percentage rate to continue for the next 24 hours, so the low is forecast to move 225 nautical miles and fill only 15 meters. Accelerations may be handled in a similar manner as  the  decelerations  shown  in  figure  2-2.  Also,  a sequence of 12-hour charts could be used in lieu of 24-hour charts to determine past trends. CRITICAL   ECCENTRICITY.—   When  a migratory  system  is  unusually  intense,  the  system  may extend  vertically  beyond  the  300-hPa  level.  Advection considerations,    contour-isotherm    relationships, convergence and divergence considerations, and the location of the jet max will yield the movement vector. These principles are applied in the same manner as when the  movement  of  long  waves  are  determined.  The eccentricity  formula  may  be  applied  to  derive  a movement  vector,  but  only  when  a  nearly  straight eastward   or   westward   movement   is   apparent. Migratory  lows  also  follow  the  steering  principle  and the  mean  climatological  tracks.  The  climatological tracks  must  be  used  cautiously  for  the  obvious  reasons. The rise and fall centers of the time differential charts are  of  great  aid  in  determining  an  extrapolated movement vector, and extrapolation is the primary method  by  which  the  movement  of  a  closed  low  is determined. Certain cutoff lows and migratory dynamic cold lows  lend  themselves  to  movement  calculation  by  the eccentricity  formula.  The  conditions  under  which  this formula may be applied are: .  The  low  must  have  one  or  more  closed  contours (nearly circular in shape). . The strongest winds must be directly north or south of the center. The location of the max winds determines  the  direction  of  movement.  When  the strongest winds are the easterlies north of the low, the low  moves  westward;  when  the  strongest  winds  are  the westerlies south of the low, the low will move eastward. The  low  will  also  move  toward  the  weakest  diverging cyclonic gradient and parallel to the strongest current. Systems moving eastward must have a greater speed in order  to  overcome  convergence  upstream-there  is normally convergence east of a low system. The eccentricity formula is written: E c = V - V ´ - 2 C or 2 C = V - V ´ - E C where Ec is the critical eccentricity y value. V is the wind speed south of the closed low. V´ is the wind speed north of the closed low. C is the speed of the closed low (in knots). To obtain the value of C, it is necessary to determine the latitude of the center of the low and the spread (in degrees latitude) between the strongest winds in the low and the center of the low. Apply these values to table 2-1 to determine the tabular value. Apply the tabular value to the critical eccentricity formula to obtain 2C, thus C. In determining the critical eccentricity of a system,  it  is  necessary  to  interpolate  both  for  latitude  and the spread. A negative value for C indicates westward movement;  a  positive  value  indicates  eastward movement. LOCATION OF THE JET STREAM.—  As  long as a jet maximum is situated, or moves to the western side of a low, this low will not move. When the jet center has rounded the southern periphery of the low, and is not followed by another center upstream, the low will move rapidly  and  fill. Table 2-1.-Critical Eccentricity Value Latitude Spread  (degrees  latitude) (degrees) 1° 3° 5° 10° 20° 80 .1 .9 2.5 -- -- 70 .2 1.8 4.9 19.5 80.0 60 .3 2.6 7.1 27.0 115.0 50 .4 3.3 9.1 37.0 150.0 40 .4 4.0 10.9 43.5 175.0 30 .5 4.5 12.3 50.0 200.0 20 .5 4.9 13.3 53.0 -- 10 .6 5.2 14.0 56.0 -- 2-7