Using the Current 500-hPa Chart In the deepening of lows there must be removal of air  at  high  levels  due  to  divergence  in  the  400-  to 200-hPa  stratum,  resulting  in  stratospheric  warming. Insufficient inflow at very high levels to compensate the subsidence  results  in  500-hPa  contour  falls. This  is  roughly  the  mechanism  thought  to  be responsible   for   the   development   of   low-pressure systems. The   high-level   decrease   in   mass overcompensates  the  low  tropospheric  increase  in density;  the  high-level  effect  thus  determines  the reduction of pressure at the surface when lows are intensifying. Stratospheric and Upper Tropospheric Decrease in Mass The  chief  cause  of  deepening  lows  is  the  decrease in  mass  in  the  upper  troposphere  and  the  lower stratosphere. With rapidly deepening lows, it is known that the change in mass in the stratosphere contributes as much to the local surface pressure change as do the tropospheric changes in density, if not more. Warming is   frequently   observed   in   the   stratosphere   over deepening  surface  lows,  pointing  to  subsidence  in  the lower stratosphere. This warming is accompanied by lowering  heights  of  constant  pressure  surfaces  in  the lower stratosphere, indicating a decrease in mass at high levels. See figure 3-8. Deepening, to a large extent is controlled by mass changes in the upper atmosphere. For example, it has been shown that the lower two-thirds (below about 300-hPa level) of the central column become colder and denser as the areas of low pressure deepen, while the upper one-third of the column becomes warmer. The upper  mass  decreases  by  an  amount  sufficient  to counteract  the  cooling  in  the  lower  layers,  plus  an additional amount to deepen the low. The preferred region for deepening of lows is in the top third of the atmospheric column or, roughly, the stratosphere. See figure 3-8. Using  Facsimile  and  NODDS  Products Facsimile  and  NODDS  products  currently  contain prognostic 500 mb, 1000- to 500-mb thickness, and 500-mb vorticity charts. These charts can be used in making predictions of advective changes, thickness patterns, and subsequent changes to the surface pattern. DEEPENING OF LOWS RELATIVE TO WEATHER TYPES Weather  types  were  discussed  previously  under  the section  Movement  of  Low-Pressure  Systems.  This method can also be used to forecast changes in intensity of pressure systems, as each system or type has its own average movement plus average deepening or filling. DEEPENING OF LOWS IN RELATION TO NORMAL STORM TRACK Lows whose tracks deviate to the left of the normal track frequently deepen. In general, the normal track of a low is parallel to the upper flow. If a low deviates to the left of normal, it crosses upper contours (assuming an   undisturbed   upper   current)   and   becomes superimposed by less mass aloft, resulting in deepening of the low. As long as this crossing of upper contours is unaccompanied  by  sufficient  compensatory  cooling  at the  surface  low  center,  the  system  will  deepen. RELATION BETWEEN DEEPENING LOWS AND MOVEMENT There is little basis for the rule that deepening storms move slowly and tilling storms move rapidly. The speed of movement of a low, whatever its intensity, is dependent upon the isallobaric gradient and other factors.  The  magnitude  of  the  surface  isallobaric gradients depends upon the low-level advection, the magnitude  of  the  upper-level  height  changes,  and  the phase relation between the two levels. FORECASTING THE INTENSITY OF SURFACE HIGHS The following section will deal with atmospheric factors   aloft   and   how   they   affect   surface anticyclogenesis.  This  section  will  also  discuss  rules  for forecasting  the  intensity  of  surface  highs. Anticyclogenesis Indicators In the case of developing dynamic anticyclones, cooling takes place at about 200 hPa and above. This cooling  is  due  to  the  ascent  of  air,  resulting  from convergence   in   the   400-   to   200-hPa   stratum. Incomplete  outflow  at  very  high  levels  causes  piling  up of  air  above  fixed  upper  levels,  resulting  in  high-level pressure rises. At the same time, warming occurs in the lower   troposphere.    This warming sometimes occurs very rapidly in the lower troposphere above the surface 3-12