Figure 21. - Analysis of conditions at 0500 m.s.t. 

 September 2, with surface fronts shown. Warm 

 front is shown by round-toothed line. 



brought about by (a) further erosion of the 

 ridge over western Canada as the trough 

 pushed inland, and (b) a slight diurnal in- 

 crease in the strength of the ridge in the Great 

 Basin area. Southwesterly winds of about 35 

 knots are now indicated in northern Idaho, 

 whUe 45 to 50 knots are observed upwind in 

 northwestern Oregon. 



At the surface, the weak Pacific front has 

 moved inland across British Columbia and 

 western Washington. (Position 6 hours earlier 

 is given by dotted line.) However, by this 

 time, a new cold front has apparently devel- 

 oped east of the Cascades in Washington- 

 Oregon; this is related to increased juxtaposi- 

 tion of the cool and warm air masses (as indi- 

 cated by the increased temperature gradient 

 in figure 20). Also, a separate, more pro- 

 nounced, cold front has apparently developed 

 in the southeastern British Columbia area. 



Twelve hours later, at 0500 on September 

 2 (fig. 21), we find the trough has progressed 

 further eastward across western Canada and 

 has dug into the northern portion of the 

 western United States ridge. The offshore por- 



Figure 22. — Analysis of conditions at 1700 m.s.t. 

 September 2, with surface fronts — cold, warm, 

 and occluded — sho^/n. 



tion of the trough has flattened. Airflow over 

 the northwest has thus become more wester- 

 ly, and the zone of strongest pressure gradient 

 and related wind has moved southward of the 

 northern Idaho area. At the surface, the origi- 

 nal Pacific front has dissipated, while the two 

 newly defined cold fronts in Canada and the 

 United States continue to move eastward, 

 generally steered by the upper flow, retaining 

 separate identities. The southern portion of 

 the Canadian front has lagged behind that in 

 the United States. Some light showers and 

 thunderstorms have occurred with the Cana- 

 dian front, but there have been none thus far 

 with the United States front. Part of northern 

 Idaho appears to be in a region between these 

 fronts. Such frontal detail, possibly induced 

 by topography, is not reflected in the upper 

 air analysis, which is based on observations at 

 stations about 250 to 300 miles apart. 



The final map in this 700-mb. series (fig. 

 22) shows conditions 12 hours later at 1700 

 on September 2. The pressure gradient over 

 the northwestern United States has decreased 

 further, and flow is now westerly. Little re- 



27 



