masking areas

Hello. I only want to display the results over Arizona, New Mexico,
Utah, and Colorado in the attached plot / script. Could you recommend
a way to mask out the rest?
I tried to follow this example: http://www.ncl.ucar.edu/Applications/
Images/maponly_6_lg.png
but was unsuccessful.

Thanks, Mike

   wks = gsn_open_wks ("ps", "lilac_4states")
   plot=new(2,graphic)
   setvalues wks
     "wkColorMap" : "BlWhRe"
   end setvalues
   res = True
   res_at_gsnDraw = False
   res_at_gsnFrame = False
   res_at_cnFillOn = True
   res_at_cnRasterModeOn = True
   res_at_cnLinesOn = False
   res_at_cnLevelSelectionMode = "ExplicitLevels"
   res_at_cnLevels = (/60.,75.,90.,105.,120./)
   res_at_cnFillColors=(/95,79,59,46,31,2/)
   res_at_tiMainFontHeightF = 0.033
   res_at_tiMainString = "Lilac Leaf Onset"
   res_at_tmYROn = False
   res_at_tmXTOn = False
   res_at_mpOceanFillColor = -1
   res_at_pmLabelBarDisplayMode = True
   res_at_lbOrientation = "vertical"
   res_at_mpMinLatF = 30.75
   res_at_mpMaxLatF = 42.25
   res_at_mpMinLonF = -115.25
   res_at_mpMaxLonF = -101.75
   res_at_mpCenterLonF = -108.
   res_at_mpCenterLonF = avg(lon)
   res_at_mpOutlineBoundarySets = "AllBoundaries"
   res_at_mpGeophysicalLineThicknessF = 2.5
   res_at_gsnAddCyclic=False
   plot(0) = gsn_csm_contour_map_ce(wks,gridleaf, res)

   res_at_tiMainString = "Lilac Bloom Onset"
   res_at_cnLevels = (/80.,100.,120.,140.,160./)
   plot(1) = gsn_csm_contour_map_ce(wks,gridbloom, res)

   pres=True
   pres_at_gsnDraw=True
   pres_at_gsnFrame=True
   pres_at_gsnMaximize = True ; blow up plot
   pres_at_gsnPaperWidth=8.5
   pres_at_gsnPaperHeight=11.0
   pres_at_gsnPaperOrientation="portrait"
   pres_at_gsnPaperMargin=0.07
   pres_at_gsnPanelLeft = 0.04
   pres_at_gsnPanelRight = 0.96
   pres_at_gsnPanelTop = 0.96
   pres_at_gsnPanelBottom = 0.04
   pres_at_gsnPanelYWhiteSpacePercent = 3
   pres_at_gsnPanelRowSpec=True
   gsn_panel(wks,plot,(/1,1/),pres) ; create panel plot

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