Pentad 250 hPa Height and Anomaly Loop
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Data:  NCEP reanalysis daily 250 hPa geopotential height on a 2.5 x 2.5 deg. lat/lon grid.
Base Period:  1961-1990
Data Source:  NCEP

The plot indicates the pentad (5-day) average of the 250 hPa geopotential height as unlabelled grey contours (contour interval 30 gpm).   Shading indicates the corresponding geopotential height anomaly with blue indicating negative, and yellow shades positive, anomalies.  Shading begins at +/- 25 gpm.  NCEP is an acronym for the National Centers for Environmental Prediction, US.