[gembud] Recent model run vs. previous

All:

I'm trying to write a script that makes a comparison of the most current
model run, say 24h forecast, versus the previous cycle's (say it is 6h
earlier) 30h forecast, effectively valid at the same time.  I'd like to
just be able to see changes in the model solution from previous runs.  I
have something like:

gdpfun=3D sub(hght^1200f012, hght^0600f018)

which actually works, but I'd like to be able to do something like this
with specifying the times, but a more general script in which it
compares the last run versus the previous one, something in the form of:

gdpfun=3D sub(hght^last, hght^'(last-1)')

I'd appreciate any suggestions as soon as possible.

Thanks,
Pedro

Pedro Mulero
Meteorologist, Wind Asset Management
1125 NW Couch, Suite 700
Portland, OR 97209
Office: 503.478.6302
Cell: 503.382.7827
Pedro.Mulero@xxxxxxxxxxxxx