A Synoptic Climatology of Winter Precipitation from 700-mb Lows for Intermountain Areas of the West

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  • 1 Weather Bureau, ESSA, Silver Spring, Md.
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Abstract

A synoptic climatology of precipitation resulting from upper closed lows is developed for the intermountain area of the West for the winter months of December, January and February. Precipitation amounts at about 280 stations for 12-hr periods, centered at upper-air observation times and expressed as percentages of 7-day normal station amounts, are related to the positions of low centers at the 700-mb level through the use of a moving grid system. The upper lows are classified into three intensity categories according to the departure from normal of the central height. For each category, the average precipitation amount, extent, and frequency of occurrence over the area of the grid system are presented. The area of maximum precipitation is found just southeast of the centers of intense 700-mb lows. As the intensity of the low center decreases, not only does the frequency of precipitation diminish, but the area of maximum frequency also progressively shifts to positions south and west of the center.

Abstract

A synoptic climatology of precipitation resulting from upper closed lows is developed for the intermountain area of the West for the winter months of December, January and February. Precipitation amounts at about 280 stations for 12-hr periods, centered at upper-air observation times and expressed as percentages of 7-day normal station amounts, are related to the positions of low centers at the 700-mb level through the use of a moving grid system. The upper lows are classified into three intensity categories according to the departure from normal of the central height. For each category, the average precipitation amount, extent, and frequency of occurrence over the area of the grid system are presented. The area of maximum precipitation is found just southeast of the centers of intense 700-mb lows. As the intensity of the low center decreases, not only does the frequency of precipitation diminish, but the area of maximum frequency also progressively shifts to positions south and west of the center.

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