A storm was defined for this study as a period of 

 precipitation, uninterrupted for a period exceeding 1 

 hour, delivering at least 0.10 inch (2.5 mm) of water. 

 Most of these storms were convective thunderstorms 

 and frontal thunderstorms aided through orographic 

 lifting. Summer convective cells, often associated with 

 lightning, usually approach from the south or southwest, 

 which is the direction of the prevailing wind of that 

 season. Some of the storms that delivered the greatest 

 intensity of rainfall were probably of a type that has been 

 termed orographic-convective. The primary source of 

 summer moisture aloft comes from the Pacific Ocean 

 (Hales 1972, 1973). A small proportion of the total 

 storms comes from large frontal systems. 



Summer convective storms delivering very high- 

 intensity rainfall have been the source of destructive 

 debris floods. Summer debris floods emanating from the 

 Wasatch Range were particularly destructive (Bailey 

 and others 1934; Bailey and other 1947). These 

 summer-flood flows took lives, destroyed property, and 

 disrupted communities. 



METHODS 



Machine methods were used to digitize the original 

 analog rainfall records. Compilation of the digitized 

 records was done by computer. The final computer 

 output for every storm consisted of both accumulated 

 precipitation depth and rainfall intensity for the fol- 

 lowing 1 2 time durations: 2, 5, 1 0, 1 5, 20, and 30 minutes 

 and 1 ,2,4, 6, 1 2, and 24 hours. The computer output also 



included the total precipitation depth for every month as 

 well as a yearly summary of maximum depth and 

 intensity. 



Record Consistency 



All of the records were checked for consistency by 

 double-mass plotting (Searcy and Hardison 1 960). This 

 technique was applied to the combined depth records 

 onlyfor Julyand August because all of the gages were in 

 operation during these months. 



Frequency Analysis 



A detailed annual seriesfrequencyanalysisof rainfall 

 intensity was made for every station. A separate an- 

 alysis was made for each of the 1 2 time durations. The 

 formula developed by Weibull was used to obtain plot- 

 ting positions (Chow 1964): 



T = 



n + 1 



m 



where 



T = recurrence interval, years 

 n = number of years of record 

 m = order number of the items arranged in des- 

 cending order. 



Thisformula has been found to be theoretically suitable 

 for plotting annual maximum series on extremal distri- 

 bution paper (Chow 1 953). 



Table 1 .—Listing of precipitation intensity stations, Straight Canyon barometer 

 watershed 



Location, 



Precipitation 



Period of 



No. of 



Station 



fig. 1 



zone number 



record 



years Elevation 











Feet 



Horn Mountain 



1 



1 



1967 - 1974 



8 9,275 



Bubs Meadow 



2 



1 



1967 - 1974 



8 8,150 



Wagon Road Ridge 



3 



2 



1967 - 1974 



8 10,100 



Seely Guard Station 



4 



1 



1967 - 1974 



8 8,990 



Swasey Ridge 



5 



2 



1967 - 1974 



8 10,030 



Skyline 



6 



1 



1967 - 1974 



8 10,400 



Lower Black Canyon 



7 



3 



1967 - 1974 



8 7,765 



Central Weather Station 



8 



1 



1967 - 1974 



8 9,020 



Orange Olsen 



9 



1 



1967 - 1974 



8 7,235 



Scad Valley 



10 



1 



1967- 1974 



8 9,160 



Table 2.— Average properties of the precipitation zones of Straight Canyon barometer 



watershed 











Precipitation Average 



Average Average 



Average Average Vegetation 



zone elevation 



•10/I2 



penetration 



R R10/R2 type 



Feet 





Miles 







1 8,890 



1.98 



29.1 



12.5 



3.8 Conifer-aspen 



2 10,065 



3.1 1 



26.7 



13.9 



8.1 Grass 



3 7,765 



2.98 



27.7 



16.7 



4.1 Grass-sage 



2 



