LARGE-SCALE SYNOPTIC PROCESSES UNDERLYING PRECIPITATION 37 
Under 
50 Percent 
—— an 
FF 15 100 over 150 
Percentage of Normal Precipitation for January 1959 
150 
10075 50__ 50 75 
Under 
50 Percent 
= 
FA rs 100 Somes 
Fic. 5—Precipitation anomalies for December 1958 and January 1959 
over North America to those of October 1952) descending motion behind the trough, and the 
were responsible for the abnormally dry weather consequent lack of access to moisture from the 
observed in the Midwest (Fig. 5). This dryness Gulf of Mexico. During the last half of January, 
resulted from the prevailing northwest flow and however, the new trough over the Midwest was 
