Figure 19. - Analysis of conditions at 0500 m.s.t. 

 September 1, with concurrent surface position 

 of cold front shown by sharp-toothed line. 



In comparison, the "normal" 700-mb. flow 

 over the Pacific Northwest at this time of year 

 is rather zonal (west-to-east), with a slight 

 trough along the Washington-Oregon coast 

 and only a slight ridge over the Canadian 

 Yukon. The "normal" refers to conditions 

 averaged, and thus smoothed, over the peri- 

 od of a month. Nevertheless, the anomaly in 

 the pattern on August 30, 1967, is out- 

 standing. 



Twenty-four hours later (map not shown), 

 eastward movement of both the upper pres- 

 sure trough and ridge had resumed but the 

 area of highest temperature remained sta- 

 tionary over the West. The pressure gradient 

 and observed winds near the British 

 Columbia- Alaskan coast weakened somewhat, 

 while a slight increase appeared over the 

 Washington-Oregon area. A tendency for slow 

 northward movement of warmer air con- 

 tinued over the interior Northwest. Resump- 

 tion of the trough's movement was in re- 

 sponse to a sequence of larger scale. Northern 

 Hemispheric circulation features, whose com- 

 plexity is beyond the scope of this paper. 



The second map (fig. 19), an additional 24 

 hours later at 0500 m.s.t. (P.d.t.) September 

 1, shows continued eastward movement of 

 the upper trough. At this time, a portion of 

 the western ridge persisted over the Great 

 Basin area. These two factors brought an in- 

 creased pressure gradient in the Washington- 

 Oregon area; there were southwesterly winds 

 of about 40 knots near the coast. The speeds 

 approached 25 knots in northern Idaho. 

 Warm air persisted over the interior North- 

 west, but colder air associated with the trough 

 had increased the temperature gradient across 

 western Washington and Oregon. 



Superimposed on this map as the standard 

 sharp-toothed line, is the current (0500) sur- 

 face map position of a cold front (or possibly 

 an occluded front) that had been moving 

 slowly eastward toward the Pacific Coast. 

 This front is weak, and no pronounced tem- 

 perature contrast corresponding to it can be 

 found at the 700-mb. level. 



By 1700 on September 1 (fig. 20), we see 

 that the strongest pressure gradient has 

 shifted further southeastward. This has been 



Figure 20. — Analysis of conditions at 1700 m.s.t. 

 September 1, with surface cold fronts shown as 

 in figure 19, except that dissipating Pacific front 

 is shown in dashed form. The frontal position 6 

 hours eariier is shown by dotted line. 



26 



