LARGE-SCALE SYNOPTIC PROCESSES UNDERLYING PRECIPITATION 3 
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7 
4. al 
OBSERVED me 
MEAN 700-MILLIBAR CONTOURS” ! 
OCTOBER 1952 
OBSERVED 
PRECIPITATION 
OCTOBER 1952 
Fie. 3—Mean 700-mb contours, temperature anomaly and precipitation patterns for October 1952 
index, the climatologically expected increase in America. After an abrupt drop in zonal westerlies 
zonal westerlies from November to December (onset of an index cycle) in January, however, a 
(which occurred strikingly in 1958) required ex- rearrangement of waves with increased wave 
pansion of the wave troughs spanning North number occurred, representing nature’s way of 
