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Petterssen Frontogenesis and EPV*

Caption:
+850 (upper left), 700 (upper right), 600 (lower left), and 500 (lower right) hPa multipanel plot.
+Petterssen frontogenesis [solid contours every 0.2 K (100 km)-1 (3 h)-1; positive only].
+Saturation equivalent potential vorticity (EPV*; shaded according to the color bar in PVU).
+Fields were computed from the 0.5x0.5^o NCEP-GFS data. Frontogenesis was smoothed by 5 passes from a 9-point local smoother. The EPV* fields were not smoothed.