2011-09-08_REVISION - M1977342 (35)

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Seasonality The PMF potential is constrained to the warm portion of the year due to the high elevation of the drainage basin. Extreme precipitation events occurring during the cold months add to the snowpack and thereby contribute to the snowmelt runoff during the ensuing spring months. The factors listed from a previous study by the Electric Power Research Institute all apply to the Henderson location. They are as follows: 1. General storms produce snow during much of the year 2. Peak streamflows are typically produced by snowmelt runoff 3. Potential PMP events are 1) a rain event coinciding with the maximum snowmelt period (typically the latter half of June) and 2) a rain event when soil and rock are most exposed, and when snow cover is at its minimum. The window for PMP events to occur includes the four -month period from mid -May through mid- September, considering storms which could be expected to occur over /near the Henderson Mill drainage. Figure 5 shows the distribution of the largest historic storm events throughout the year. Warm Season Storm Types By mid -may the cold season prevailing westerly flow transitions to the southwest, ushering in the so -called summer monsoon as the northward branch of the jet stream and the general cyclonic storm track shift northward. Moisture of Pacific Ocean origin is circulated northeastward across Mexico into the southwestern states as the monsoon flow becomes better established beginning in June. Rains can be relatively frequent, but are generally light during the period of June through August as localized showers and thunderstorms affect the region. Small area precipitation totals infrequently exceed two inches, with an upper limit on totals at or near the latitude of the Williams Fork of about 2.5 inches (Climatological Consulting Corporation, 1991). Dying tropical storms (sometimes hurricanes), exemplified by Norma in 1970, become a factor during the period from mid- August to about mid- September, but the northern limit of sufficient moisture for production of significant rains is generally accepted as being approximated by the San Juan Range in southwestern Colorado. After mid- September, the end of the Williams Fork warm season, the likelihood of significant tropical moisture intrusion, even as far north as the San Juans, is greatly decreased. A return to westerly flow and general storms in October is accompanied by sufficient cooling for a return to the predominance of snow above elevations of about 9,000 feet. Page 8