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-13- long-term rainfall records have not been kept in the Gore Creek Valley. Consequently, it was necessary to use data from one or more of the available stations for developing rainstorm data for the Gore Creek Valley. Elevation and both orographic and geograph1corien- tation of these gaging stations are very important since the elevation and temperature limits the amount of moisture available to a thunder- storm. The orographic character helps determine the potential uplift and the geographic orientation determines the degree of protection from moist air masses. The Dillon precipitation data was determined to be the most nearly representative of the records available, con- sidering the above-enumerated factors. Analyses have been made of the Dillon rainfall data for storms that could be identified as thunderstorms (Figure 1II-2). These analyses show the one-percent probability, 24-hour precipitation to be 1.85 inches and the ten-percent, 24-hour value to be 1.07 inches. A portion of this precipitation usually falls as hail pellets or g~aupcl, c~pccial1y at elevations above 11,000 fppt.1,2 Examination of sixteen precipitation records taken from the Gore Creek and Eagle River Valley stations for summer and fall months indi- cates that 68% of the daily or 24-hour precipitation would occur during a thunderstorm. The remaining 32% of the rain typically falls in light showers after the cell dissipates and later in the day. Comparisons of the 68% value and the Weather Bureau time distribution in TP_4021 of 80%-90% resulted in using a compromise value of ~1%. Study of the records also indicates the areal distribution of the ..,." . , , .i ., ~! '" , . . -14- typical thunderstorm in and around the Gore Creek Valley is limited to approximately ten square miles.l Using this data, the 10 and 100 year thunderstorm rainfall values are: TlMEFROM 10 YEAR 100 YEAR 8EG. OF STORM THUNOERSTORM THUNDERSTORM (cum. ins.) (cum. ins.) 15 min. .41 .72 30 min. .61 1.07 45 min. .76 1.32 60 min. .87 1.50 24 hrs. 1.07 l.8S The infrequent general rainstorm, which occurs primarily in September and lasts several hours to over 48 hours in some ~ases. has also been developed from Di110n rainfall records. This general storm may cover a 1,000 square mile or larger area. The one-percent probability, 24-hour rainfall from a general storm is 1.89 inches and the corresponding 48-hour value is 2.17 inches. Once again, the cool autumn temperatures will cause muCh of this precipitat\on to fall as snow or hail at the higher elevations. The adopted distribution of the general type stonnisas follows: HOURS FROM BEG.. OF STORM 100 YEAR GENERAL STORM PRECIPITATION (cum. ins.) 10 YEAR GENERAL STORM PRECIPITATION (cum. ins.) 1 2 3 4 5 5 8 10 11 24 " .21 .28 .33 ,37 .40 ,44 .51 .59 .67 .91 1.05 .44 .58 .58 ,76 ," ,go 1.05 1.22 1.38 1.89 2.17