Your definitions look fine to me, but test your definition of
wintermonth, maybe by plotting just a line plot of that variable,
"plot wintermonth".
What version of Ferret are you running? Try this little test
script of the ribbon/missing=blank qualifier, using a mask similar
to yours.
let/title="xpts"/units=degrees_east xpts = {\
151.0,153.0,155.0,157.0,159.0,161.0,163.0,165.0,167.0,169.0,171.0,173.0,175.0,\
177.0,179.0,181.0,183.0,185.0,187.0,189.0}
let/title="ypts"/units="degrees_north" ypts = {\
29.42,29.43,29.33,29.19,29.22,29.12,29.36,29.22,29.04,28.92,29.00,28.59,28.31,\
28.69,28.20,28.86,27.98,27.80,28.29,27.94}
let/title="VAR" var = {\
14.16,14.31,13.34,11.90,12.19,11.20,13.57,12.25,9.3,11.12,4.95, 5.91, \
3.13, 6.88, 2.02, -8.63,-0.23,-1.97, 7.94,9.65}
let mask = if var gt 0 then 1
let var2 = mask*var
go basemap x=120e:150w/y=20:50
plot/vs/over/ribbon/missing=blank xpts,ypts,var2