36-hour prognostic charts for these levels. In this way, changes both in space and time can be considered. For a direct application for a short period of time, transfer the position of the low center to the concurrent 700-mb chart. For direction, move the center in the direction of the contours downstream and slightly inclined to the left for  low-pressure  areas. Experience  with  moving systems  of  this  type  will  soon  tell  you  how  much deviation should be made. For speed of the surface cyclone, average the basic current downstream over which the cyclone will pass (take into consideration changes indirection and speed of flow over the forecast period). Take 70 percent of this value for the mean speed for 24 hours.    Move the low center along the contours, as described above, for this speed for 24 hours. This should be your position at that time. For  the  500-mb  chart,  follow  the  same  procedure, except use 50 percent of the wind speed for movement. If  these  two  are  not  in  agreement,  take  a  mean of  the  two.  There  may  be  cases  where  the  500-mb chart is the only one used. In this case, you will not be able to check the movement against the 700-mb chart. FORECASTING   PRINCIPLES.—   T he following  empirical  relationships  and  rules  should  be taken into account when you use the steering technique: .  Warm,  unoccluded  lows  are  steered  by  the current at the level to which the closed low does not extend. When so steered, lows tend to move slightly to the left of the steering current. l Warm lows (unoccluded) are steered with the upper flow if a well-defined jet is over the surface center or if there is no appreciable fanning of the contours aloft. Low-pressure  systems,  especially  when  large,  tend  to move slightly to the left of the steering current. .  Rises  and  falls  follow  downstream  along  the 500-hPa  contours;  the  speed  is  roughly  half  of  the 500-hPa  gradient. The  3-hour  pressure  rises  and pressure falls seem to move in the direction of the 700-hPa flow; while 24-hour pressure rises and pressure falls move with the 500-hPa flow. .  Cold  lows,  with  newly  vertical  axes,  are  steered with the upper low (in the direction of upper height falls), parallel to the strongest winds in the upper low, and toward the weakest contour gradient. . Occluded lows, the axes of which are not vertical, are steered partly in the direction of the warm air advection  area. . A surface low that is becoming associated with a cyclone aloft will slow down, become more regular, and follow  a  strongly  cyclonic  trajectory. . Surface lows are steered by jet maximums above them, and deviate to the left as they are so steered. They move at a slower rate than the jet maximum, and are soon left behind as the jet progresses. . During periods of northwesterly flow at 700 hPa from Western Canada to the Eastern United States, surface lows move rapidly from the northwest to the southeast,  bringing  cold  air  outbreaks  east  of  the Continental  Divide. . If the upper height fall center (24 hour) is found in the direction in which the surface cyclone will move, the cyclone will move into the region or just west of it in 24 hours. Direction of Mean Isotherms (Thickness Lines) A number of rules have been compiled regarding the movement of low-pressure systems in relation to the mean isotherms or thickness lines. These rules are outlined  as  follows: . Unoccluded lows tend to move along the edge of the cold air mass associated with the frontal system that precedes the low; that is, it tends to move along the path of the concentrated thickness lines. When using this method, you should remember that the thickness lines will change position during the forecast period. If there is  no  concentration  of  thickness  lines,  this  method cannot be used. l When the thickness gradient (thermal wind) and the  mean  windflow  are  equal,  the  low  moves  in  a direction  midway  between  the  two.  This  rule  is  more reliable when both the thermal wind and the mean windflow  are  strong. l When the menu windflow gradient is stronger than the thickness gradient, the low will move more in the direction of the mean windflow. l When the thickness gradient is stronger than the mean windflow gradient, the low will move more in the direction  of  the  thickness  lines. l  With  warm  lows,  the  mean  isotherms  show  the highest temperature directly over the surface low, which is about halfway between the 700-hPa trough and ridge line. This indicates the mean isotherm and 700-hPa isoheights are 90 degrees out of phase. Since warm 3-6