9 mi^xi^^mm 



THE BUILDUP 



The fire story begins in the months before 

 the outbreak itself, in the buildup of fire 

 danger during the summer of 1967. The fire 

 season in northern Idaho developed quite nor- 

 mally, and in the first 3 months showed char- 

 acteristics of an average fire season. The fire 

 weather records of the Priest River Experi- 

 mental Forest in northern Idaho, interpreted 

 according to the National Fire-Danger Rating 

 System, illustrate the seasonal buildup of fire 

 danger. The record presented in figure 3 

 shows the buildup index remaining below the 

 previous record high for the period of 1954 to 

 1966 until August 11, 1967. The continued 

 buildup in 1967 from late June until mid- 

 September differs from the historical pattern, 

 which has shown one or more breaks in build- 

 up during this period. This year, day by day 

 the situation became more serious, and re- 

 strictions were imposed to minimize the 

 hazard of man-caused fires. 



1967 Buildup index — — 

 P Previous high buildup index — — — 



Early in August, a high pressure zone had 

 become deeply entrenched over the north- 

 western United States and was effectively 

 blocking the influx of moist maritime air 

 from the west. No significant amount of pre- 

 cipitation occurred between August 11 and 

 September 1. In the 62 days from July 10 to 

 September 10, only 0.10 inch of rain was 

 measured at Priest River Experimental Forest 

 from four different showers. From August 11 

 to August 31, the 10-day averages showed the 

 temperatures were the highest, the humidities 

 the lowest, and the fine fuel moisture content 

 the lowest for the season. Relative humidity is 

 estimated to have been below 35 percent con- 

 tinually for at least 72 hours prior to the fire 

 run. 



On August 31, aU stations in northern 

 Idaho except those at high elevations had 

 buildup indexes rated "extreme." Late on 

 that day, a Pacific weather system was 

 approaching the. Washington coast. A warning 



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