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