surface and the 850-mb charts. The area of genesis will show progressively colder temperatures at the surface and aloft; however, the drop in the 850-mb temperatures does not occur at the same rate as at the surface. This is an indication that a very strong inversion is in the process of forming. The air in the source region must be relatively stagnant. . High-level anticyclogenesis is indicated when low-level  warm  air  advection  is  accompanied  by stratospheric cold air advection. This situation has primary  application  to  the  formation  of  blocks,  as high-level  anticyclogenesis  is  primarily  associated  with the formation of blocks and the intensification of the ridges  of  the  subtropical  highs. .  Blocks  should  normally  be  forecast  to  form  only over the eastern portion of the oceans in the middle and high latitudes. Warm air is normally present to the north and northwest. Formation Forecasting Principles (Lows) There  are  certain  conditions  required  in  the atmosphere, as well as certain atmospheric indicators, for cyclogenesis to occur. The greater the number of these indicators/conditions in agreement, the greater the success  in  forecasting  cyclogenesis.  Some  of  them  are listed  below: l  An  area  of  divergence  exists  aloft. .  A  jet  maximum  on  the  west  side  of  a  low indicates  deepening  and  southward  movement. .  Cold  air  advection  in  the  lower  troposphere  and warming in the lower stratosphere is associated with the formation  of  or  deepening  of  lows. Formation Forecasting Principles (Cutoff  Lows) Another  task  in  forecasting  is  that  of  the  formation of  cutoff  lows.  Some  of  the  indicators  are  as  follows: l They generally form only off the southwestern coast of the United States and the northwestern coast of Africa. .  The  upstream  ridge  intensifies  greatly.  This intensifying upstream ridge contains an increasing, strong,  southwesterly  flow. . Strong northerlies on the west side of the trough. . Height falls move south or southeastward. . Strong cold air advection occurs on the west side of the upper trough. Constructing Upper Level Prognostic Charts The constant pressure prognostic chart is about to take form. The forecasted position of the long wave troughs and ridges have been determined and depicted on the tentative prognostic chart. The position of the highs,  lows,  and  cutoff  centers  were  then  determined and  depicted  on  the  tentative  prognostic  chart.  Short waves were treated in a similar fashion. Contours are then depicted. The height values of the contours are determined  by  actual  changes  in  intensity  of  the systems.   The   pattern   of   the   contours   is   largely determined by the position of the long waves, short waves,  and  closed  pressure  systems.  Contours  are drawn in accordance with the following eight steps: 1. Outline the areas of warm and cold advection in the stratum between 500 and 200 hPa, and move the thickness  lines  at  approximately  50  percent  of  the indicated thickness gradient in the direction of the thermal  wind. 2. Tentatively note, at several points on the chart, the  areas  of  height  changes  on  the  constant  pressure surface  above  the  existing  height  values. 3. Move the areas of 24-hour height rises and falls at the speed of the short waves, and note at several key points  the  amount  and  direction  of  the  height  change from the current chart. 4. Adjust the advected height changes, and, in turn, adjust  these  for  positions  of  the  long  waves,  pressure systems,  and  short  waves. 5. Extrapolate heights for selected points at 500 hPa  on  the  basis  of  the  24-hour  time  differential indications  and  advection  considerations,  provided  that they  are  justified  by  the  indications  of  high-level convergence  and  divergence.  When  the  contributions from  advection  and  time  differentials  are  not  in agreement  with  convergence  and  divergence,  adjust  the contribution of each and use accordingly. 6.  Adjust  the  height  values  to  the  forecasted intensities of the systems. These adjustments can lead to  the  following: .  All  factors  point  toward  intensification (deepening of lows and filling of highs). .  One  factor  washes  away  the  contribution made by another, and the system remains at or near its present state of intensity. 2-9