Deriving fields not within grib file

[Dan Leins]

All,

I'm trying to figure out how to make some images of derived WRF model
data. I've got a series of grib files containing the traditional
fields one would expect to see from a model. That's all fine and good,
but I'd like to create some images of fields not contained within the
grib file. Specifically, I'd like to make a plot of CAPE above the
-20C isotherm. Also, maybe something like the difference between the
surface temp and the convective temperature? These are things that I
would usually find on a skew-t, but I'm aiming for a plan view
projection of these fields.

Does anyone have any idea how I might approach this?

Thanks!
Dan