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PV on the 310, 330, and 350 K surfaces

Caption:
+Potential vorticity on the 310 K isentropic surface (blue contours at 1.5 and 3.0 PVU).
+Potential vorticity on the 330 K isentropic surface (shaded according to the color bar in PVU).
+Potential vorticity on the 350 K isentropic surface (black contours at 1.5 and 3.0 PVU).
+Image was generated from 1.0x1.0^o NCEP-GFS data.