library(lattice)

LON    <-      3.689e-02
LONSQR <-      2.273e-04
LAT    <-      8.273e-02
LATSQR <-     -1.108e-03
LONLAT <-      5.561e-05

first_time <- T

for (x in seq(-125,-67,2.0)) {
  for (y in seq(24,49,2.0)) {
    z <- LON*x + LONSQR*x*x + LAT*y + LATSQR*y*y + LONLAT*x*y
    if (first_time) { 
      X<-x
      Y<-y
      Z<-z
      first_time <- F
    } else {
      X <- rbind(X,x)
      Y <- rbind(Y,y)
      Z <- rbind(Z,z)
    }
  }
}

grid <- data.frame(lon=X,lat=Y,z=Z,row.names=NULL)

wf <- wireframe(z ~ lat*lon, grid, shade=T)

trellis.device(postscript,onefile=F,print.it=F,color=T,file="cty_wire.eps")

print(wf)

dev.off()