PMAX 

     PMAX defines the maximum possible pressure from which data may 
     be interpolated to the output vertical coordinate.

     PMAX originated as a way to exclude low-level PV when interpolating
     to the tropopause level from a THTA-coordinate input file.  A good
     value of PMAX could be 650 or 700 (mb).

     When interpolating from pressure coordinates or any other coordinate
     system, make sure that PMAX does not conflict with the bounds of
     the search as specified by STARTL and STOPL.