MOVEMENT OF HGH-PRESSURE SYSTEMS In  general,  the  methods  for  extrapolation  of low-pressure  areas  are  applicable  to  the  movement  of high-pressure  areas. The  following  are  general  considerations  in forecasting the movement of high-pressure systems: . A surface high, or that portion situated under a blocking  high  aloft,  remains  very  nearly  stationary. . A high situated under or very near a jetstream is steered by the current aloft. l Cold, shallow highs are steered more easily than the larger ones. The Canadian and Siberian highs move little when there is no jet max in their vicinity or above them, and they move rapidly when the jet max is present. .  Progressive  warm  highs  move  with  a  speed consistent with that of the major ridges aloft. . With straight westerly currents aloft, surface highs  are  displaced  equatorward. . Highs tend to move in the direction of, and with the speed of, the isallobaric centers; however, this rule is  not  very  reliable  because  the  isallobaric  rises  often follow the low rather than lead the high. Steering  is  not  used  for  high-pressure  systems  as widely as for lows because high-pressure cells do not have as great a vertical extent as low-pressure systems. However,  steering  seems  to  work  about  75  percent  of the time for cold highs. FORECASTING THE INTENSITY OF SURFACE PRESSURE SYSTEMS LEARNING  OBJECTIVES:  Forecast   the intensity  of  surface  low-  and  high-pressure centers  by  using  extrapolation,  isallobaric indications,  relation  to  frontal  movement,  aloft indications, weather type, and in relation to normal storm tracks. The changes in intensity of pressure systems at the surface are determined, to a large extent, by events occurring above the system. EXTRAPOLATION The  3-hour  pressure  tendencies  reported  in  a synoptic  plot  indicate  the  sum  of  the  pressure  change due  to  movement  of  the  system,  plus  that  due  to deepening and filling. If the exact amount of pressure change due to movement could be determined, it could be assumed that the system would continue to deepen or fill at that rate. However, it is not normally prudent to assume that the current rate of change will continue, nor just how much of the pressure change is due to movement. ISALLOBARIC  INDICATIONS Isallobaric  analyses  at  the  surface  show  the following relationships between the isallobars and the change  in  intensity  of  pressure  systems: . When the 3-hour pressure falls extend to the rear of the low, the low is deepening. l When the 3-hour pressure rises extend ahead of the low, the low tends to fill. . When the 3-hour pressure rises extend to the rear of the high or ridge, the high or ridge tends to fill. l Since low-pressure systems usually move in a direction parallel to the isobars in the warm sector, and since the air mass in the warm sector is homogeneous, it is possible to assume that the pressure tendencies in the warm sector are an indication of the deepening or filling  of  the  system.    The  effects  of  frontal  passages must be removed. Therefore, if a low moves parallel to warm sector isobars, the 3-hour pressure tendency in the warm sector is equal to the deepening or falling of the system. Remember  that  when  you  use  the  present  3-hour pressure tendency values for any of the above rules, they arc merely an indication of what has been happening, and not necessarily what will be taking place in the future. Consequently, if you use the tendencies for indication of deepening or filling, you will need to study the past trend of the tendencies. RELATIVE TO FRONTAL MOVEMENT Wave cyclones form most readily on stationary or slow moving fronts.    A preferred position is along a decelerating  cold  front  in  the  region  of  greatest deceleration.  Normally,  the  700-hPa  winds  are  parallel to the front along this area. Under  conditions  characteristic  of  the  eastern Pacific, a secondary wave cyclone may rapidly develop (fig. 3-7, step 1). As the secondary wave forms on the 3-8